Quantum absorption spectroscopy system and quantum absorption spectroscopy method

Abstract

A quantum absorption spectroscopy system (100) comprises a laser light source (1), a quantum optical system (201), a photodetector (31), and a controller (4). The laser light source (1) emits pump light. The quantum optical system (201): comprises a non-linear optical crystal (23) that, upon having pump light emitted thereon, generates quantum entangled photon pairs of signal photons and idler photons, and a moving mirror (25) that is capable of changing the phase of the idler photons; and is configured such that quantum interference occurs between the plurality of physical processes in which the quantum entangled photon pairs are generated. The photodetector (31) detects the signal photons when a sample is disposed on the optical path of the idler photons and the phase of the idler photons is changed by the non-linear optical crystal (23) and outputs a quantum interference signal corresponding to the detected number of photons. The controller (4) calculates an absorption spectrum characteristic for the sample by subjecting the quantum interference signal to a Fourier transform.

Description

[Technical field]

[0001] The present disclosure relates to quantum absorption spectroscopy systems and methods. [Background technology]

[0002] In general, in infrared absorption spectroscopy, infrared light is irradiated onto a sample. The change in the intensity of the infrared light due to absorption by the sample is then obtained as the infrared absorption spectrum of the sample. Among infrared absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR) in particular is widely used to identify molecular structures (such as the type of functional groups or three-dimensional structures) in the fields of chemistry, biology, and pharmacology. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent No. 10,648,908 [Non-patent literature]

[0004] [Non-Patent Document 1] Anna Paterova, Hongzhi Yang, Chengwu An, Dmitry Kalashnikov and Leonid Krivitsky, "Measurement of infrared optical constants with visible photons", New Journal of Physics 20(2018)043015 [Non-Patent Document 2] Masayuki Okano, Hwan Hong Lim, Ryo Okamoto, Norihiko Nishizawa, Sunao Kurimura and Shigeki Takeuchi, "0.54 μm resolution two-photon interference with dispersion cancellation for quantum optical coherence tomography", Scientific Reports volume 5, Article number: 18042 (2015) Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, in the field of quantum technology such as quantum metrology, quantum communication, and quantum computing, research has been conducted to realize new functions by using photon pairs in which two photons are "quantum entangled" and have a quantum mechanical correlation. Hereinafter, such photon pairs are also referred to as "quantum entangled photon pairs." The present inventors have focused on the application of quantum entangled photon pairs to spectroscopy systems and spectroscopy methods. The present inventors have found that it is possible to widen the wavelength range that can be spectroscopically analyzed by performing appropriate arithmetic processing on the detection signal from a photodetector.

[0006] The present disclosure has been made to solve such problems, and an object of the present disclosure is to provide a technology capable of performing spectroscopy over a wide wavelength range in a spectroscopy system or spectroscopy method (quantum absorption spectroscopy system or quantum absorption spectroscopy method) that applies quantum entangled photon pairs. [Means for solving the problem]

[0007] (1) A quantum absorption spectroscopy system according to a first aspect of the present disclosure includes a light source, a quantum optical system, a photodetector, and a computing device. The light source emits pump light. The quantum optical system includes a nonlinear optical element that generates a quantum entangled photon pair of a signal photon and an idler photon by irradiation with the pump light, and a phase conversion unit configured to be able to change the phase of one of the signal photon and the idler photon. The quantum optical system is configured to cause quantum interference between a plurality of physical processes that generate the quantum entangled photon pair. The photodetector outputs a quantum interference signal corresponding to the number of detected signal photons when the phase of the one photon is changed by the phase conversion unit in a state where the sample is placed in the optical path of the idler photon. The computing device calculates the absorption spectroscopic characteristics of the sample by Fourier transform of the quantum interference signal.

[0008] (2) In an embodiment, the calculation device may calculate a Fourier spectrum by Fourier transform of the quantum interference signal in a state where the sample is placed in the optical path of the idler photons, and may also calculate a reference Fourier spectrum by Fourier transform of the quantum interference signal in a state where the sample is not placed in the optical path of the idler photons. The calculation device may calculate a complex transmittance spectrum of the sample based on a ratio between the Fourier spectrum and the reference Fourier spectrum.

[0009] (3) In an embodiment, the computing device may calculate the absorption spectrum of the sample by squaring the absolute value of the complex transmittance spectrum of the sample.

[0010] (4) In an embodiment, the computing device may calculate the absorption spectroscopic characteristics of the sample by Fourier transforming a quantum interference signal obtained without wavelength sweeping of the quantum entangled photon pairs or wavelength resolution of the signal photons in the quantum optical system.

[0011] (5) In one embodiment, the nonlinear optical element may be a chirp-type or fan-type quasi-phase matching element.

[0012] (6) In another embodiment, the nonlinear optical element may be a quasi-phase matching element including a nonlinear optical crystal. The quasi-phase matching element may be configured such that, when quantum entangled photon pairs are generated multiple times, the wavelengths of the idler photon group including the idler photon are distributed over a wide wavelength range determined according to the material of the nonlinear optical crystal and the poling period.

[0013] (7) In an embodiment, the material of the nonlinear optical crystal may include lithium niobate. The polarization inversion period of the nonlinear optical crystal may be determined so that the idler photon group includes a plurality of photons having different wavelengths from each other within the wavelength range of 0.4 μm to 5.2 μm.

[0014] (8) In another embodiment, the material of the nonlinear optical crystal may include gallium phosphide. The polarization inversion period of the nonlinear optical crystal may be determined so that the idler photon group includes a plurality of photons having different wavelengths from each other within the wavelength range from 0.7 μm to 12 μm.

[0015] (9) In still another embodiment, the material of the nonlinear optical crystal may include gallium arsenide. The polarization inversion period of the nonlinear optical crystal may be determined so that the idler photon group includes a plurality of photons having different wavelengths from each other within the wavelength range from 1 μm to 18 μm.

[0016] (10) In still another embodiment, the material of the nonlinear optical crystal may include lithium tantalate. The polarization inversion period of the nonlinear optical crystal may be determined so that the idler photon group includes a plurality of photons having different wavelengths from each other within the wavelength range from 0.3 μm to 5.5 μm.

[0017] (11) In still another embodiment, the material of the nonlinear optical crystal may include zinc selenide. The polarization inversion period of the nonlinear optical crystal may be determined so that the idler photon group includes a plurality of photons having different wavelengths from each other within the wavelength range from 0.4 μm to 22 μm.

[0018] (12) In some embodiments, the photodetector may be a single pixel photodetector.

[0019] (13) In one embodiment, the signal photons are photons in the visible range, and the photodetector may be a silicon-based photodetector.

[0020] (14) In an embodiment, the quantum optical system may further include a total reflection measurement device configured to enable total reflection measurement of the sample.

[0021] (15) In an embodiment, the phase conversion unit may include a first movable mirror configured to be movable along the propagation direction of the signal photon and a second movable mirror configured to be movable along the propagation direction of the idler photon. The quantum absorption spectroscopy system further includes a control device. The control device may be configured to selectively move one of the first and second movable mirrors.

[0022] (16) In an embodiment, the phase converter may include a movable mirror configured to be movable along the propagation direction of the signal photon.

[0023] (17) In an embodiment, the nonlinear optical element may be configured to generate idler photons in the ultraviolet region. The computing device may calculate ultraviolet absorption spectroscopic characteristics of the sample.

[0024] (18) A quantum absorption spectroscopy method (QAS) according to a second aspect of the present disclosure includes first to third steps. The first step is a step of generating quantum entangled photon pairs by irradiating a nonlinear optical element with pump light in a quantum optical system configured to cause quantum interference between a plurality of physical processes in which quantum entangled photon pairs of a signal photon and an idler photon are generated. The second step is a step of detecting a signal photon by a photodetector when the phase of one of the signal photon and the idler photon is changed by a phase conversion unit in a state in which a sample is placed in the optical path of the idler photon, thereby acquiring a quantum interference signal according to the number of detected photons. The third step is a step of calculating the absorption spectroscopic characteristics of the sample by Fourier transforming the quantum interference signal of the photodetector. Effect of the Invention

[0025] According to the present disclosure, it is possible to perform spectroscopy in a wide wavelength range in a quantum absorption spectroscopy system or a quantum absorption spectroscopy method. [Brief description of the drawings]

[0026] [Figure 1] 1 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a first embodiment of the present disclosure. [Diagram 2] FIG. 1 is a conceptual diagram for explaining the principle of quantum absorption spectroscopy. [Diagram 3] FIG. 2 is a functional block diagram for illustrating an outline of a calculation process performed by a calculation device. [Figure 4] 4 is a flowchart showing a processing procedure of quantum absorption spectroscopy in the first embodiment. [Diagram 5] FIG. 2 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a modified example of the first embodiment. [Figure 6] FIG. 11 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a second embodiment. [Figure 7] FIG. 1 is a diagram illustrating a configuration example of a quasi-phase matching device. [Figure 8]FIG. 11 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a third embodiment. [Figure 9] FIG. 13 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a first modification of the third embodiment. [Figure 10] 1A and 1B are conceptual diagrams for explaining differences in detection principles depending on the type of photodetector. [Figure 11] FIG. 13 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a second modification of the third embodiment. [Figure 12] FIG. 13 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a fourth embodiment. [Figure 13] FIG. 13 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a modified example of the fourth embodiment. [Figure 14] FIG. 2 is a diagram showing an example of the configuration of a total reflectance measurement unit. [Figure 15] FIG. 13 is a diagram showing an example of a measurement result of a quantum interference signal. [Figure 16] FIG. 16 is a diagram showing a Fourier spectrum obtained by Fourier transform of the quantum interference signal shown in FIG. 15. [Figure 17] FIG. 13 is a diagram showing the difference in signal photon spectrum due to quantum interference effect. [Figure 18] FIG. 13 is a diagram showing an example of a measurement result of a quantum interference signal regarding quartz glass. [Figure 19] 19 is a diagram showing a Fourier spectrum obtained by Fourier transform of the quantum interference signal shown in FIG. 18. FIG. [Figure 20] FIG. 1 is a diagram showing the absolute value of transmittance and phase difference obtained by measuring quartz glass. [Figure 21] FIG. 2 is a diagram showing the polarization inversion period at each position in a nonlinear optical crystal. [Figure 22] FIG. 13 is a diagram showing an example of a simulation result of a spectrum of a quantum entangled photon pair generated using a quasi-phase matching device. [Figure 23] 23 is a diagram showing a simulation result of a quantum interference signal obtained when a quantum entangled photon pair having the spectrum shown in FIG. 22 is used. FIG. [Figure 24]FIG. 1 is a diagram showing the relationship between the polarization inversion period and the generation wavelength range of idler light for each material of a nonlinear optical crystal. [Diagram 25] FIG. 1 is a diagram summarizing the characteristics of various materials that can be used for nonlinear optical crystals. [Figure 26] FIG. 13 is a diagram showing an example of a simulation result in which a complex transmittance spectrum over a wide wavelength range is obtained. [Figure 27] FIG. 27 is a diagram showing the results of a simulation in which the complex refractive index of a sample is calculated from the complex transmittance spectrum shown in FIG. 26. [Figure 28] FIG. 1 shows an example of a measurement result of a quantum interference signal over a wide wavelength range. [Figure 29] FIG. 29 is a diagram showing the complex transmittance spectrum of a sample obtained from the quantum interference signal shown in FIG. 28. [Diagram 30] FIG. 13 shows quantum interference signals obtained by simulation of sweeping the idler or signal path. [Diagram 31] FIG. 31 is a diagram showing a Fourier spectrum obtained by Fourier transform of the quantum interference signal shown in FIG. 30. [Diagram 32] FIG. 13 is a diagram showing a Fourier spectrum obtained by a simulation in which the idler optical path or the signal optical path is swept with respect to a state in which a sample that is an infrared absorber is placed in the idler optical path. [Diagram 33] FIG. 13 is a diagram showing a quantum interference signal obtained by a simulation in which an idler optical path or a signal optical path is swept in a quantum absorption spectroscopy system for measuring absorption spectroscopic characteristics in the ultraviolet region. [Diagram 34] FIG. 13 is a diagram showing a Fourier spectrum obtained when the signal optical path is swept. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference characters, and the description thereof will not be repeated.

[0028] In the present disclosure and its embodiments, the ultraviolet range refers to a wavelength range of 10 nm to 360 nm. The visible range refers to a wavelength range of 360 nm to 1050 nm. The near-infrared range refers to a wavelength range of 1050 nm to 2 μm. The mid-infrared range refers to a wavelength range of 2 μm to 5 μm. The far-infrared range refers to a wavelength range of 5 μm to 20 μm. The ultra-far-infrared range (Terahertz range) refers to a wavelength range of 20 μm to 1 mm. The infrared range may include all of the near-infrared range, the mid-infrared range, the far-infrared range, and the ultra-far-infrared range.

[0029] [Embodiment 1] In the first embodiment (and in the second to fourth embodiments described below), a configuration for measuring the absorption spectroscopic characteristics of a sample in the infrared range using a quantum absorption spectroscopy system according to the present disclosure will be described. However, as will be described in detail later, the wavelength range that can be measured by the quantum absorption spectroscopy system according to the present disclosure is not limited to the infrared range.

[0030] <System configuration> 1 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a first embodiment of the present disclosure. The quantum absorption spectroscopy system 100 is configured to be capable of measuring infrared absorption spectroscopic characteristics (such as infrared absorption spectrum) of a sample by applying quantum entangled photon pairs to infrared absorption spectroscopy. The quantum absorption spectroscopy system 100 includes a laser light source 1, a quantum optical system 201, a photodetector 31, a controller 4, and a monitor 5.

[0031] The laser light source 1 emits pump light for exciting a nonlinear optical crystal 23 (described later). In the figure, the pump light is indicated by Lp. In the first embodiment, the laser light source 1 emits continuous wave (CW) laser light included in the visible range. Specifically, for example, a semiconductor laser that emits green laser light with a wavelength of 532 nm can be used as the laser light source 1.

[0032] The quantum optical system 201 is configured to generate quantum interference between multiple physical processes that generate quantum entangled photon pairs of a signal photon and an idler photon. As will be described in detail later in FIG. 2, it should be noted that quantum interference is different from normal light interference. In the figure, the optical path of the signal light (light consisting of signal photons) is indicated by Ls, and the optical path of the idler light (light consisting of idler photons) is indicated by Li. In the following, the optical path of the signal light may be abbreviated as the "signal optical path," and the optical path of the idler light may be abbreviated as the "idler optical path."

[0033] In this embodiment, quantum optical system 201 is an optical system that applies a configuration similar to that of a Michelson interferometer. Quantum optical system 201 includes lenses 211 to 214, dichroic mirrors 221 and 222, nonlinear optical crystal 23, sample holder 24, movable mirror 25, and fixed mirror 26.

[0034] The lens 211 is disposed between the laser light source 1 and the dichroic mirror 221. The lens 211 is configured to collect the pump light from the laser light source 1 and to focus the collected pump light on the nonlinear optical crystal 23.

[0035] Dichroic mirror 221 is disposed between lens 211 and nonlinear optical crystal 23. Dichroic mirror 221 transmits light in a wavelength range that includes the wavelength of the signal light, while reflecting light outside the above wavelength range.

[0036] In this example, the wavelength of the pump light is 532 nm. The wavelength of the signal light is, for example, within the wavelength range of 603 nm or more and 725 nm or less. The wavelength of the idler light is, for example, within the wavelength range of 2 μm or more and 4.5 μm or less. Therefore, dichroic mirror 221 transmits the signal light while reflecting the pump light and the idler light. The pump light is reflected by dichroic mirror 221 and irradiated onto nonlinear optical crystal 23.

[0037] The nonlinear optical crystal 23 generates signal light and idler light from the pump light focused by the lens 211. More specifically, the nonlinear optical crystal 23 generates photon pairs of a signal photon and an idler photon by spontaneous parametric down-conversion (SPDC) of the pump light. The nonlinear optical crystal 23 is, for example, a lithium niobate (LiNbO3) crystal. In this case, the signal light is visible light, and the idler light is infrared light (more specifically, near-infrared light or mid-infrared light). However, the type of the nonlinear optical crystal 23 is not particularly limited. Other types of nonlinear optical crystals, such as a silver gallium sulfide (AgGaS2) crystal, may also be used. The principle of quantum absorption spectroscopy using the nonlinear optical crystal 23 will be described with reference to FIG. 2.

[0038] In this specification, when a compound is expressed by a stoichiometric composition formula, the stoichiometric composition formula is merely a representative example. The composition ratio may be non-stoichiometric. For example, when lithium niobate is expressed as "LiNbO3", unless otherwise specified, lithium niobate is not limited to the composition ratio of "Li / Nb / O=1 / 1 / 3" and may contain Li, Nb and O in any composition ratio. The same applies to the other compounds exemplified below.

[0039] Dichroic mirror 222 is disposed between nonlinear optical crystal 23 and movable mirror 25, and between nonlinear optical crystal 23 and fixed mirror 26. In this embodiment, dichroic mirror 222 transmits visible light and reflects infrared light. Signal light in the visible range transmits through dichroic mirror 222 together with the pump light toward fixed mirror 26. Meanwhile, idler light in the infrared range is reflected by dichroic mirror 222 toward movable mirror 25. Note that dichroic mirror 222 may transmit infrared light and reflect visible light.

[0040] The lens 212 is disposed between the dichroic mirror 222 and the sample holder 24. The lens 212 collimates the idler light reflected by the dichroic mirror 222. The lens 213 is disposed between the dichroic mirror 222 and the fixed mirror 26. The lens 213 collimates the pump light and the signal light transmitted through the dichroic mirror 222.

[0041] Sample holder 24 is placed between nonlinear optical crystal 23 and movable mirror 25. Sample holder 24 holds a sample (indicated as SP in the figure). A material that is transparent to idler light (infrared light in this example) is used for sample holder 24. The idler light is irradiated onto the sample, and the transmitted light is directed toward movable mirror 25.

[0042] The movable mirror 25 is configured to be movable along the propagation direction of the idler light. Specifically, the movable mirror 25 is provided with a driving device 250. The driving device 250 is an electric actuator controlled by the controller 4, and is, for example, a motor driving device (such as a servo motor or a stepping motor) that is mechanically displaced according to a control command from the controller 4. The driving device 250 may be a piezoelectric element that is displaced according to an applied voltage from the controller 4. The position of the movable mirror 25 is periodically changed using the driving device 250 (moving the movable mirror 25 back and forth), thereby making it possible to sweep the idler light path (the state of the sweep is represented by ΔX in the figure).

[0043] Movable mirror 25 is preferably a flat mirror, and reflects the idler light that has passed through the sample. The reflected idler light is further reflected by dichroic mirror 222 and returns to nonlinear optical crystal 23. The idler light passes through nonlinear optical crystal 23, but does not reach photodetector 31 because it is reflected by dichroic mirror 221.

[0044] The movable mirror 25 is an example of a phase conversion unit according to the present disclosure. The phase conversion unit according to the present disclosure may include a phase modulator (not shown) such as an electro-optic modulator (EOM) instead of or in addition to the movable mirror 25. For example, the optical path length can be changed relatively coarsely using the movable mirror 25 provided with the driving device 250, and then a fine change in the optical path length can be realized using the phase modulator.

[0045] Fixed mirror 26 is, for example, a flat mirror, and reflects the pump light and the signal light that have passed through dichroic mirror 222. The reflected pump light and the reflected signal light pass through dichroic mirror 222 again and return to nonlinear optical crystal 23. The pump light passes through nonlinear optical crystal 23 but is reflected by dichroic mirror 221. On the other hand, the signal light passes through nonlinear optical crystal 23 and is also transmitted through dichroic mirror 221. Fixed mirror 26 may be a concave mirror. In this case, lens 213 can be omitted.

[0046] The lens 214 is disposed between the dichroic mirror 221 and the photodetector 31. The lens 214 collects the signal light transmitted through the dichroic mirror 221, and outputs the collected signal light to the photodetector 31.

[0047] The photodetector 31 is a silicon-based photodetector and has optical characteristics capable of separating visible light (and a part of near-infrared light). In the first embodiment, the photodetector 31 is a multi-pixel type photodetector including a plurality of pixels arranged in a two-dimensional array. Specifically, the photodetector 31 is a CCD (Charged-Coupled Device) image sensor or a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor. The photodetector 31 detects signal light in response to a control command from the controller 4, and outputs the detection signal to the controller 4. The intensity of the detection signal of the signal light is an intensity according to the number of signal photons detected by the photodetector 31 (more specifically, a signal intensity directly proportional to the number of photons). Therefore, it can be said that the photodetector 31 outputs a detection signal (a quantum interference signal to be described later) according to the number of detected photons.

[0048] The controller 4 is, for example, a microcomputer. The controller 4 includes a processor 41 such as a CPU (Central Processing Unit), a memory 42 such as a ROM (Read Only Memory) and a RAM (Random Access Memory), and an input / output port 43. The controller 4 controls the devices (laser light source 1 and driving device 250) in the quantum absorption spectroscopy system 100.

[0049] Moreover, the controller 4 executes various arithmetic processing for realizing quantum absorption spectroscopy. More specifically, the controller 4 calculates the infrared absorption spectroscopic characteristics of the sample based on the detection signal from the photodetector 31. The infrared absorption spectroscopic characteristics of the sample may include the Fourier spectrum, complex transmittance spectrum, and infrared absorption spectrum of the sample. The arithmetic processing by the controller 4 will be described in detail later, but the controller 4 first calculates the Fourier spectrum of the sample. Then, the controller 4 calculates the complex transmittance spectrum and infrared absorption spectrum of the sample by performing a predetermined calculation using the Fourier spectrum. The controller 4 is an example of the arithmetic device and the control device according to the present disclosure. The arithmetic device and the control device according to the present disclosure may be configured separately.

[0050] The monitor 5 is, for example, a liquid crystal display, and displays the results of the calculation processing by the controller 4. This enables the operator to check the infrared absorption spectroscopic characteristics, such as the Fourier spectrum, of the sample.

[0051] Dichroic mirror 221 may transmit light of the wavelength of the idler light (or a longer wavelength). In that case, a short-pass filter (not shown) may be installed between dichroic mirror 221 and lens 214. The short-pass filter transmits signal light while reflecting (or absorbing) idler light. This makes it possible to prevent the idler light from reaching photodetector 31. However, if photodetector 31 has low sensitivity in the wavelength range of the idler light, the installation of the short-pass filter may be omitted and the idler light may be allowed to reach photodetector 31.

[0052] The nonlinear optical crystal 23 is an example of a nonlinear optical element according to the present disclosure. The nonlinear optical element according to the present disclosure is not limited to this, and may be, for example, a ring resonator or an optical waveguide formed of silicon (Si) and / or silicon nitride (SiN) (neither of which is shown).

[0053] 1 employs a quantum interferometer similar to a Michelson interferometer. However, the quantum optical system according to the present disclosure may have a configuration similar to a Mach-Zehnder interferometer including multiple nonlinear optical elements.

[0054] In a typical existing quantum absorption spectroscopy system, a light source that emits infrared light is combined with a photodetector that is highly sensitive to infrared light, but there may be room for improving the performance of the light source and the photodetector. To explain by taking a specific example, many light sources that emit infrared light (such as tungsten lamps) use blackbody radiation, and therefore have low energy conversion efficiency. In addition, the sensitivity of infrared photodetectors based on compound semiconductors such as indium gallium arsenide (InGaAs) is generally lower than that of visible light detectors (such as silicon-based photodetectors). On the other hand, in recent years, visible light laser light sources such as semiconductor lasers have become smaller and less expensive. In addition, as digital cameras and smartphones become more widespread, silicon-based photodetectors have become more sensitive and less expensive. By adopting such a laser light source and photodetector, it is possible to achieve high sensitivity and low cost in the quantum absorption spectroscopy system 100.

[0055] <Measurement principle> Fig. 2 is a conceptual diagram for explaining the principle of quantum absorption spectroscopy. In Fig. 1, a configuration in which only one nonlinear optical crystal 23 is arranged in the optical path of the pump light is explained. In Fig. 2, in order to facilitate understanding of the measurement principle, a configuration in which two nonlinear optical crystals are arranged in the optical path of the pump light is explained as an example. The two nonlinear optical crystals are referred to as a first crystal 231 and a second crystal 232.

[0056] When the first crystal 231 is irradiated with pump light from the laser light source 1, one photon with a relatively large energy is split into two photons with smaller energy by SPDC in the first crystal 231 while satisfying the law of conservation of energy. In the example shown in FIG. 2, a quantum entangled photon pair of one visible photon (signal photon) and one infrared photon (idler photon) is generated from one visible photon (pump photon). Similarly, a quantum entangled photon pair of one visible photon and one infrared photon is generated by irradiating the second crystal 232 with pump light. In this example, a photodetector 31 is disposed in the traveling direction of the visible photon of the quantum entangled photon pair.

[0057] Quantum interference occurs between an event in which a quantum entangled photon pair is generated by the first crystal 231 (hereinafter referred to as a "first physical process") and an event in which a quantum entangled photon pair is generated by the second crystal 232 (hereinafter referred to as a "second physical process"). More specifically, when a probability amplitude representing the first physical process and a probability amplitude representing the second physical process are added together, if the two probability amplitudes are in phase, the first physical process and the second physical process constructively interact with each other, whereas if the two probability amplitudes are in phase opposite to each other, the first physical process and the second physical process cancel each other out (quantum interference effect). In the following, an example of interference in which the first physical process and the second physical process cancel each other out (destructive interference) will be described. However, the quantum optical system 201 may be configured to cause constructive interference between the first physical process and the second physical process.

[0058] If the infrared absorbing sample is not placed in the idler optical path, the first physical process and the second physical process cannot be distinguished from each other, and the first physical process and the second physical process cause quantum interference (in this example, they cancel each other out). In this case, it is observed that no quantum entangled photon pairs are generated in the stage subsequent to the second crystal 232. In other words, the signal light (visible photons) is not detected by the photodetector 31.

[0059] On the other hand, when the sample is placed in the idler optical path, the idler light is absorbed by the sample. Then, the first physical process and the second physical process are distinguished from each other, and the quantum interference between the first physical process and the second physical process is incomplete. As a result, the signal light is detected by the photodetector 31.

[0060] Thus, in quantum absorption spectroscopy, by detecting one visible photon (signal photon) of a quantum entangled photon pair by photodetector 31, it is possible to determine that the other infrared photon (idler photon) has been absorbed by the sample.

[0061] 1, the nonlinear optical crystal 23 serves as both the first crystal 231 and the second crystal 232. By making the pump light go back and forth through the nonlinear optical crystal 23, the number of nonlinear optical crystals to be installed can be reduced.

[0062] 2, an example has been described in which quantum optical system 201 causes quantum interference between two physical processes (first and second physical processes). However, the quantum optical system according to the present disclosure may cause quantum interference between three or more physical processes. In other words, the quantum optical system according to the present disclosure may be configured to cause quantum interference between at least two physical processes.

[0063] <Calculation processing by controller> 3 is a functional block diagram for illustrating an outline of the calculation process by the controller 4. The controller 4 includes a light source control unit 401, a mirror control unit 402, a count rate calculation unit 403, a Fourier transform unit 404, a first storage unit 405, a second storage unit 406, a transmittance calculation unit 407, an absorption spectrum calculation unit 408, and a monitor control unit 409. First, an overview of the function of each block will be described.

[0064] The light source control unit 401 controls the optical output (laser power) of the laser light source 1. The laser power during measurement by the quantum absorption spectroscopy system 100 is basically kept constant.

[0065] Mirror control unit 402 controls the movement (reciprocating motion) of movable mirror 25. When the position of movable mirror 25 changes, the idler optical path length changes, and therefore the propagation time of the idler light (hereinafter abbreviated as "idler propagation time") t0 changes. Therefore, by causing movable mirror 25 to reciprocate, the detection signal of the signal light from photodetector 31 is acquired as a time waveform of the idler propagation time t0.

[0066] The count rate calculation unit 403 calculates the signal photon count rate P s Calculate the signal photon count rate P as a function of the idler propagation time t0. s (t0) is the number of signal photon counts per unit time. Signal photon count rate P s The calculation result of (t0) is output to the Fourier transform unit 404.

[0067] The Fourier transform unit 404 calculates the signal photon count rate P s In the quantum absorption spectroscopy system 100, the signal photon count rate P s (t0) is acquired. The signal photon count rate P s The Fourier spectrum obtained by the Fourier transform of (t0) is called "A s On the other hand, the signal photon count rate P s The Fourier spectrum obtained by the Fourier transform of (t0) is called "A s 0 The Fourier transform unit 404 converts the Fourier spectrum A s (ω) is output to the first storage unit 405 and the monitor control unit 409, and the Fourier spectrum A s 0 (ω) is output to the second storage unit 406.s 0 (ω) corresponds to the "reference Fourier spectrum" according to the present disclosure.

[0068] The first storage unit 405 stores the Fourier spectrum A s The second storage unit 406 stores the Fourier spectrum A (ω) in a non-volatile manner when a sample is not placed on the sample holder 24. s 0 (ω) is stored in a non-volatile manner. The stored Fourier spectrum (A s (ω) or A s 0 (ω) is appropriately read out by the transmittance calculation unit 407.

[0069] The transmittance calculation unit 407 calculates the Fourier spectrum A s (ω) and Fourier spectrum A s 0 The transmittance calculation unit 407 calculates the complex transmittance spectrum τ(ω) of the sample based on (ω). The transmittance calculation unit 407 outputs the calculation result of the complex transmittance spectrum τ(ω) to the absorption spectrum calculation unit 408 and the monitor control unit 409.

[0070] The absorption spectrum calculation unit 408 calculates the infrared absorption spectrum of the sample based on the complex transmittance spectrum τ(ω) of the sample. The absorption spectrum calculation unit 408 outputs the calculation result of the infrared absorption spectrum to the monitor control unit 409.

[0071] The monitor control unit 409 detects the results of calculations by the controller 4 (Fourier spectrum A of the sample). s The complex transmittance spectrum τ(ω), the complex transmittance spectrum τ(ω), and the infrared absorption spectrum are displayed on the monitor 5.

[0072] <Calculation process details> Next, the functions of some blocks are explained in detail. In the following, the parameters marked "signal" are parameters related to the signal photons. The parameters marked "idler" are parameters related to the idler photons.

[0073] First, a description will be given of the calculation process performed by the count rate calculation unit 403. A state vector |Ψ> obtained by superposing two events (the first and second physical processes described above) in which quantum entangled photon pairs are generated by the nonlinear optical crystal 23 is expressed as in the following formula (1).

[0074]

number

[0075] In equation (1), the vacuum state vector is represented by |vac>. The SPDC generation efficiency is represented by η. The signal frequency is represented by ω. s and the idler frequency is ω i The two-photon field amplitude is expressed as F(ω s ,ω i ) The signal generation operators in the first and second physical processes are expressed as a + s1 and a + s2 The idler generation operator in the first physical process is expressed as a + i1,in or a + i1,out The idler light generated in the first physical process passes through a sample placed on the idler light path, so the idler light before and after passing through the sample are distinguished by adding the subscript in to the generation operator corresponding to the idler light before passing through the sample and the subscript out to the generation operator corresponding to the idler light after passing through the sample. The idler generation operator in the second physical process is expressed as a + i2 The phase delay that the pump light acquires corresponding to the optical path difference of the pump light between the first physical process and the second physical process is represented as φ p It is expressed as:

[0076] When the optical loss due to the sample placed in the idler optical path is evaluated according to the beam splitter model, the idler annihilation operator a i1,out is expressed as the following equation (2).

[0077]

number

[0078] In equation (2), the complex transmittance (complex transmission amplitude) of the sample is represented by τ, and the complex reflectance (complex reflection amplitude) of the sample is represented by r. In the beam splitter model representing the j-th (j = 1, 2) transmission through the sample, the vacuum field incident from a port different from the port through which the idler photon is input is represented by a vj It is expressed as:

[0079] The mode of the signal light generated in the first physical process is adjusted so as to spatially match the mode of the signal light generated in the second physical process. In addition, the mode of the idler light generated in the first physical process is adjusted so as to spatially match the mode of the idler light generated in the second physical process. These modes can be expressed by the same creation and annihilation operators, except for the phase change due to propagation, as shown in the following equations (3) and (4).

[0080]

number

[0081] In equations (3) and (4), t1 denotes the propagation time until the signal light generated in the first physical process reaches the nonlinear optical crystal 23 again. Similarly, t0 denotes the propagation time until the idler light generated in the first physical process reaches the nonlinear optical crystal 23 again in a situation where the sample is not placed in the idler light path.

[0082] Next, the electric field E of the signal light in the photodetector 31 s (+) (t) is expressed as the following equation (5).

[0083]

number

[0084] Signal photon count rate Ps is expressed as the following equation (6) using the state vector |Ψ> shown in equation (1).

[0085]

number

[0086] By substituting equations (1) to (4) into equation (5), the following equation (7) is derived. Equation (7) represents the time waveform of quantum interference (quantum beat) between the first and second physical processes that generate quantum entangled photon pairs.

[0087]

number

[0088] where the signal frequency ω s the center frequency ω s0 We redefine it by using the detuning Ω from ω s =ω s0 This is expressed as -Ω. The idler frequency ω i Similarly, for the center frequency ω i0 Using the detuning Ω from ω i =ω i0 +Ω. Then, from the above equations (6) and (7), the signal photon count rate P s is transformed into the following equation (8).

[0089]

number

[0090] In the experimental apparatus disclosed in Non-Patent Document 1, a spectrometer (monochromator) is placed in front of an avalanche photodiode (APD) (see FIG. 2 in Non-Patent Document 1). The installation of the spectrometer represents the restriction of signal photons reaching the APD to narrowband photons, thereby narrowing the infrared absorption spectrum. This is because Non-Patent Document 1 restricts the wavelength of the signal photon to a narrow wavelength range on the order of the resolution of the spectrometer, and estimates the absorption at the wavelength of the idler photon corresponding to the signal photon by measuring the light intensity of each wavelength range of the signal photon under this restriction.

[0091] Thus, in Non-Patent Document 1, it is essentially necessary to use a spectrometer to disperse the signal light (measure the wavelength) and to measure the light intensity of the signal light for each dispersed wavelength. In general, a device including a spectrometer can be large and expensive. In addition, wavelength scanning by a spectrometer takes time, which can be an obstacle in terms of reducing the measurement time.

[0092] Also, in the optical system disclosed in Patent Document 1, a filter system 524 is disposed in front of the photodetector 510 (see FIG. 5A of Patent Document 1 and the second paragraph from the bottom in column 11). The filter system 524 performs wavelength selection for the signal light in addition to removing unnecessary pump light. Thus, Patent Document 1 also considers only limiting in advance so that only signal photons in a specific narrow wavelength range reach the photodetector 510 and measuring the signal photons under this limitation.

[0093] In contrast, in the present embodiment, the signal photons of the entire frequency band generated by the nonlinear optical crystal 23 are detected by the photodetector 31 without passing through a spectrometer or an unnecessary filter. This is because the frequency (signal frequency) ω of the signal photon detected by the photodetector 31 is s Regardless of which frequency band is included, the signal photon count rate P sThis is based on the idea of ​​including all possibilities when calculating. This idea is also reflected in the fact that the frequency component (detuning Ω) is integrated in the above equation (8).

[0094] Furthermore, in this embodiment, taking into consideration the frequency dependence of the complex transmittance τ and complex reflectance r of the sample, the complex transmittance τ and the complex reflectance r are both calculated at an idler frequency ω i Let (τ→τ(ω i ),r→r(ω i )) In this case, the following formula (9) is obtained from formula (8).

[0095]

number

[0096] In equation (9), the first term (the constant term of 2) is the signal photon count rate P s The second term (integral term) and the third term (complex conjugate term) represent the offset component of the signal photon count rate P s represents the quantum interference component of the signal light. Since the optical path length of the signal light in the two physical processes that generate quantum entangled photon pairs is constant, the propagation time t1 is a fixed value. In addition, the following equation (10) that represents the normalization condition of the two-photon field amplitude is established. Therefore, from equation (9), the signal photon count rate P s It can be seen that the value depends on the idler propagation time t0. Therefore, by periodically changing the idler optical path length by moving the movable mirror 25 back and forth (by sweeping the idler optical path), the signal photon count rate P s (t0) can be measured. In this way, the signal photon count rate P s The signal related to this is also called a "quantum interference signal" (or quantum interference waveform).

[0097]

number

[0098] Next, the calculation process by the Fourier transform unit 404 will be described. The Fourier transform unit 404 performs a Fourier transform on the quantum interference signal (see the above formula (9)) measured while changing the idler optical path length. As a result, a Fourier spectrum A s (ω) is obtained, and the information for each wavelength of infrared light absorbed by the sample is reproduced. Note that in the Fourier integral shown in equation (11), the constant integral term that gives the DC component and the complex conjugate term that gives the -ω component are omitted.

[0099]

number

[0100] Next, the calculation process by the transmittance calculation unit 407 will be described. As described above, the Fourier spectrum when the sample is placed in the idler optical path is expressed as A s (ω), and the Fourier spectrum when the sample is not placed in the idler path is A s 0 The two are distinguished by writing (ω). Fourier spectrum A s 0 (ω) corresponds to the complex transmittance τ of the sample being 1 in the above formula (11). Therefore, the difference in the Fourier spectrum with and without the sample, more specifically, the Fourier spectrum A s 0 Fourier spectrum A for (ω) s When the amplitude ratio (ω) is calculated, the following equation (12) is derived.

[0101]

number

[0102] From equation (12), it can be seen that by performing two measurements, one with and one without a sample, and taking the ratio of the two Fourier spectra, the frequency dependence of the complex transmittance τ of the sample (i.e., the complex transmittance spectrum τ(ω)) can be obtained.

[0103] Finally, a description will be given of the calculation process performed by the absorption spectrum calculation unit 408. The absorption spectrum calculation unit 408 calculates the (intensity) absorption spectrum of the sample in the infrared range by calculating the square of the absolute value of the complex transmittance spectrum τ(ω).

[0104] In this way, the quantum interference signal in this embodiment is not generated based on light in a specific wavelength range obtained by wavelength resolution of the signal light using a spectroscope. The quantum interference signal in this embodiment is generated based on the signal light in all wavelength ranges that is generated by the nonlinear optical crystal 23 and passes through the quantum optical system 201 and then enters the photodetector 31. Therefore, according to this embodiment, it is possible to realize infrared absorption spectroscopy in a wide wavelength range compared to the case where a spectroscope is used. In other words, according to this embodiment, it is possible to obtain the complex transmittance spectrum of a sample in a wide wavelength range by one photodetection without the need for photodetection for each wavelength range in principle (more specifically, it is not necessary to sweep the wavelength of the quantum entangled photon pair using a MEMS (Micro Electro Mechanical Systems)-based wavelength tunable resonator or the like, or to resolve the wavelength of the signal light using a spectroscope or the like).

[0105] In addition, the signal photon count rate P s (quantum interference signal) is calculated as a function of the idler propagation time t0 (see equation (9) above). The quantum interference signal in the time domain is then Fourier transformed into the frequency domain to obtain the Fourier spectrum A s (ω) is obtained (see equation (11) above). Fourier spectrum A s By analyzing (ω), the absolute value of the transmittance of the sample can be calculated from the change in intensity of the light transmitted through the sample. In addition, in this embodiment, it is possible to determine how the phase of the transmitted light is shifted depending on whether or not the sample is present. More specifically, the Fourier spectrum A when the sample is placed in the idler optical path is s (ω) and Fourier spectrum A when no sample is placed in the idler path s 0(ω) is calculated (see formula (11) above). This calculation makes it possible to calculate the complex transmittance spectrum τ(ω) of the sample over a wide wavelength range. Furthermore, based on the complex transmittance spectrum τ(ω) of the sample, the infrared absorption spectrum of the sample over a wide wavelength range can also be calculated.

[0106] <Measurement flow> 4 is a flowchart showing the processing procedure of quantum absorption spectroscopy in the first embodiment. This flowchart is called from the main routine and executed, for example, when an input device (not shown) such as an operation button accepts an operation by a measurer. Each step is basically realized by software processing by the controller 4, but may also be realized by hardware processing by an electronic circuit fabricated in the controller 4. Hereinafter, steps are abbreviated as "S".

[0107] In S1, a sample is placed on the sample holder 24 arranged in the idler optical path. The sample is usually placed by an operator. However, it is also possible to automate the process by providing a feed device (not shown) for transporting the sample.

[0108] In S2, the controller 4 controls the laser light source 1 to start outputting the pump light.

[0109] In S3, the controller 4 controls the driver 250 provided on the moving mirror 25 to start or continue a reciprocating motion at high speed. In general, the signal-to-noise ratio of the infrared absorption spectrum can be improved by integrating the light detection results. Therefore, in S3, the moving mirror 25 reciprocates several tens of times per second in a range of sub-mm (e.g., several tens of μm) to several cm (e.g., up to 5 cm). However, when integration is not required or when high speed is required, such as when acquiring a time change in the infrared absorption spectrum, it is sufficient to perform the reciprocating motion only once.

[0110] In order to calculate the complex transmittance spectrum of the sample with high accuracy, it is necessary to measure the phase change of the transmitted light depending on the presence or absence of the sample with high accuracy. For this purpose, it is desirable to stabilize the optical path length of each of the signal light and the pump light on a sub-wavelength scale of, for example, 25 nm or less. It is also desirable for the driver 250 that drives the movable mirror 25 to have positioning accuracy on the sub-wavelength scale. However, the accuracy of the optical path length and the positioning accuracy of the driver 250 can be appropriately determined depending on the performance (resolution and / or stability) required for the quantum absorption spectroscopy system 100.

[0111] In S4, the controller 4 calculates the signal photon count rate P based on the detection signal from the photodetector 31. s Calculate (t0).

[0112] In S5, the controller 4 determines whether a condition (termination condition) for terminating the reciprocating motion of the movable mirror 25 is satisfied. For example, the controller 4 may determine whether a signal photon count rate P s When (t0) is calculated, it can be determined that the termination condition is met. When the termination condition is not met (NO in S5), the controller 4 returns the process to S3. As a result, the processes of S3 and S4 are repeated until data is acquired a specified number of times or for a specified period of time. When the termination condition is met (YES in S5), the controller 4 advances the process to S6.

[0113] In S6, the controller 4 controls the laser light source 1 to stop outputting the pump light. The controller 4 also controls the driver 250 of the movable mirror 25 to stop the reciprocating motion of the movable mirror 25.

[0114] In S7, the controller 4 determines the signal photon count rate P s The Fourier spectrum A is obtained by Fourier transforming (t0) (quantum interference signal). s Calculate (ω).

[0115] Although not shown in the figure, before executing the series of processes S1 to S7, a Fourier spectrum A is obtained by performing a similar process (so-called background measurement) in a state where the sample is not placed in the idler optical path. s 0 In S8, the controller 4 calculates the Fourier spectrum A s (ω) and the previously acquired Fourier spectrum A s 0 (ω) is used to calculate the complex transmittance spectrum τ(ω) of the sample. If background measurement has not been performed, the Fourier spectrum A is obtained by performing the same processes as those in S1 to S7 without placing a sample. s 0 (ω) can be obtained.

[0116] In S8, the controller 4 determines the Fourier spectrum A when the sample is not placed in the idler optical path. s 0 (ω) (reference Fourier spectrum) and Fourier spectrum A when the sample is placed in the idler path s The complex transmittance spectrum τ(ω) of the sample is calculated by calculating the ratio with τ(ω).

[0117] In S9, the controller 4 calculates the square of the absolute value of the complex transmittance spectrum τ(ω) of the sample, thereby calculating the infrared absorption spectrum of the sample.

[0118] In S10, the controller 4 controls the monitor 5 to display the measurement results of the infrared absorption spectroscopic characteristics of the sample obtained by the processes of S1 to S9. In detail, the controller 4 outputs the result of the process of S7 and displays the Fourier spectrum A s The controller 4 outputs the result of the process of S8 and causes the monitor 5 to display the complex transmittance spectrum τ(ω) of the sample. The controller 4 also outputs the result of the process of S9 and causes the monitor 5 to display the infrared absorption spectrum of the sample.

[0119] As described above, in the first embodiment, all the signal light passing through the quantum optical system 201 is detected by the photodetector 31 without being dispersed (wavelength-resolved or frequency-resolved) by a spectrometer or removed by a filter. Then, the quantum interference signal is Fourier-transformed to obtain a Fourier spectrum A s (ω) is calculated. Fourier spectrum A s Since (ω) reflects information in all wavelength ranges, infrared absorption spectroscopy in a wide wavelength range is possible compared to a configuration equipped with a spectrometer that reflects only information in a specific wavelength range (see Non-Patent Document 1). Furthermore, phase information of the sample can be obtained from the difference in the Fourier spectrum depending on whether or not a sample is placed in the idler optical path.

[0120] In general Fourier transform infrared spectroscopy (FTIR) using classical optics, the intensity of light incident on the sample is relatively high, which can cause the sample to become very hot. In contrast, in the first embodiment using quantum optics, the intensity of light incident on the sample is significantly reduced. To give a specific numerical value, in the first example described later, the intensity of the idler light is 100 fW (=1×10 -13 W), which is about 10 W light intensity compared to a general FTIR. 9 Therefore, according to the first embodiment, it is possible to measure the infrared absorption spectroscopic characteristics of a sample while suppressing changes in the characteristics of the sample (which may include deterioration and damage to the sample) that are caused by heating due to infrared light irradiation.

[0121] Moreover, in the first embodiment, a visible laser light source 1 and a visible light detector 31 are used instead of a conventional infrared light source and infrared light detector. By using a visible light source, heat countermeasures become easier and the system can be made smaller. In addition, by using a light detector with high sensitivity in the visible range, cooling with liquid nitrogen to reduce thermal noise is no longer necessary, which also makes it possible to make the system smaller. As a result, according to the first embodiment, it is possible to realize measurement at the site where the sample is collected (so-called on-site measurement).

[0122] In the first embodiment, infrared absorption spectroscopy in the near-infrared region is described as an example, but the infrared light used in infrared absorption spectroscopy is not limited to near-infrared light. The quantum absorption spectroscopy according to the present disclosure is also applicable to mid-infrared light, far-infrared light, or terahertz light. When the infrared region is the measurement target, the quantum absorption spectroscopy (QAS) according to the present disclosure can be called quantum Fourier transform infrared spectroscopy (Q-FTIR). In addition, the light irradiated to the nonlinear optical crystal 23 is not limited to visible light, and may be ultraviolet light or infrared light.

[0123] Furthermore, although the SPDC of the pump light is used to generate quantum entangled photon pairs, a four-wave mixing process may be used instead of the SPDC. When the four-wave mixing process is used, signal photons and / or idler photons having a shorter wavelength than the wavelength of the pump light can be generated. Therefore, it is also possible to measure the absorption spectroscopic characteristics of a sample in the ultraviolet or visible range by using the quantum absorption spectroscopy system according to the present disclosure (see the eighth embodiment described later).

[0124] Various variations of the quantum absorption spectroscopy system according to the present disclosure will be described below in order. For the sake of clarity, the features of each embodiment and the modified examples will be briefly summarized below.

[0125] First embodiment: Basic configuration Modification of the first embodiment: Basic configuration + dispersion element Second embodiment: Quasi-phase matching device Third embodiment: Single-pixel type photodetector First modification of the third embodiment: Single pixel type photodetector+quasi-phase matching device Modification 2 of the third embodiment: Single pixel type photodetector + circular polarization measurement Fourth embodiment: Idler optical path sweep + signal optical path sweep Modification of the fourth embodiment: Signal light path sweep + total reflection measurement [Modification of the first embodiment] 5 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a modified example of embodiment 1. The quantum absorption spectroscopy system 100A differs from the quantum absorption spectroscopy system 100 according to embodiment 1 (see FIG. 1) in that it further includes a dispersive optical element 6 between the output side (lens 214) of the quantum optical system 201 and the photodetector 31.

[0126] The dispersion optical element 6 is, for example, a diffraction grating or a prism, and disperses the signal light output from the quantum optical system 201 in different directions according to the wavelength. As a result, the multi-pixel photodetector 31 is configured to measure the light intensity of the signal light of different wavelengths for each of a plurality of pixels arranged in an array. Fourier spectrum A in the quantum absorption spectroscopy system 100A s The features of the calculation method for (ω) will be explained later with reference to FIG.

[0127] [Embodiment 2] In the second embodiment, a configuration will be described in which the wavelength range of the idler light is widened to realize infrared absorption spectroscopy in an even wider wavelength range.

[0128] 6 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to embodiment 2. The quantum absorption spectroscopy system 200 differs from the quantum absorption spectroscopy system 100 according to embodiment 1 (see FIG. 1) in that it includes a quantum optical system 202 instead of the quantum optical system 201. The quantum optical system 202 includes a quasi-phase-matched (QPM) device 7 instead of the nonlinear optical crystal 23.

[0129] 7 is a diagram showing an example of the configuration of the QPM device 7. The QPM device 7 includes a lens 71, a nonlinear optical crystal 72, a long-pass filter 73, a lens 74, and a sharp-cut filter 75.

[0130] The nonlinear optical crystal 72 has a periodic polarization inversion structure. The arrows in the figure indicate the spontaneous polarization direction. The material of the nonlinear optical crystal 72 is, for example, magnesium doped stoichiometric lithium tantalate, also written as Mg:SLT. The nonlinear optical crystal 72 has a rectangular parallelepiped shape. When pump light is incident on one end (first end) 721 of the mutually facing end faces of the nonlinear optical crystal 72, signal photons and idler photons are emitted from the other end (second end) 722. The nonlinear optical crystal 72 has, for example, five polarization inversion structures (also called sections or segments) between the first end 721 and the second end 722. The typical number of divisions of the sections provided in the nonlinear optical crystal 72 is about several tens. The width of the section (polarization inversion period Λ) gradually increases from the first end 721 to the second end 722, as shown in FIG. 7. By appropriately designing the number of sections and the polarization inversion period Λ, it is possible to generate quantum entangled photon pairs over a wide wavelength range (see Figures 21 and 22 for details).

[0131] The nonlinear optical crystal 72 can be manufactured using nanofabrication poling technology. In the nanofabrication poling technology, a resist pattern is first drawn by electron beam lithography. The resist pattern is then transferred to aluminum by dry etching. After that, a strong electric field is applied to the aluminum electrodes in a vacuum. For details of the structure and manufacturing method of the QPM device 7, see Non-Patent Document 2.

[0132] Each of the long-pass filter 73 and the sharp-cut filter 75 cuts out pump light having a wavelength shorter than a specific wavelength, although the long-pass filter 73 and the sharp-cut filter 75 may be omitted.

[0133] The QPM device 7 corresponds to the "quasi-phase matching element" according to the present disclosure. The QPM device 7 shown in Fig. 7 is a chirp-type element in which the polarization inversion period changes along the optical path. However, the quasi-phase matching element according to the present disclosure may be a fan-type (fan-out structure) element in which the polarization inversion period changes in a fan shape.

[0134] As described above, in the second embodiment, idler photons are generated using the QPM device 7 arranged in the optical path of the pump light. As will be described in detail later with reference to Figs. 21 to 23, by selecting an appropriate material for the nonlinear optical crystal 72 and appropriately designing the polarization inversion period Λ, it is possible to generate idler light having a flat intensity distribution over a wide wavelength range. Therefore, according to the second embodiment, the wavelength range in which infrared absorption spectroscopy is possible can be further expanded compared to the first embodiment.

[0135] In the present disclosure, the method of widening the wavelength range in which infrared absorption spectroscopy is possible is not limited to the method of using a "quasi-phase matching element". Even when using a bulk-type nonlinear optical crystal 23 as in the first embodiment, it is possible to widen the wavelength range in which infrared absorption spectroscopy is possible. Specifically, the sample holder 24 holding the nonlinear optical crystal 23 is placed on a rotation stage (not shown), and the rotation stage is rotated by a predetermined angle each time the signal photon count rate Ps is measured. By changing the orientation of the nonlinear optical crystal 23 (the inclination of the optical axis of the nonlinear optical crystal 23 with respect to the incident direction of the pump light) by rotating the rotation stage, idler photons of various wavelengths can be generated.

[0136] However, in the above case, the measurement time may be long because the procedure of measuring the signal photon count rate Ps by rotating the rotating stage must be repeated. In addition, the calculation process for calculating the complex transmittance spectrum of the sample may become complicated. In other words, the complex transmittance is calculated using the measurement results of the signal photon count rate Ps for each angle of the rotating stage, and then the complex transmittance spectrum can be obtained by plotting the frequency dependence of the calculated complex transmittance. In contrast, when a "quasi-phase matching element" is used, it is possible to calculate the complex transmittance spectrum from only the measurement results of the signal photon count rate Ps once.

[0137] [Embodiment 3 (+Modification 1)] In the first and second embodiments, a configuration has been described in which a multi-pixel type photodetector 31 in which a plurality of pixels are arranged in a two-dimensional array is employed. However, when using the computational processing of quantum absorption spectroscopy according to the present disclosure, a multi-pixel type photodetector (multi-channel detector) is not necessarily required. In the third embodiment (and a modified example), a configuration in which a single-pixel type photodetector (single-channel detector) is employed will be described.

[0138] 8 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to embodiment 3. The quantum absorption spectroscopy system 300 differs from the quantum absorption spectroscopy system 100 according to embodiment 1 (see FIG. 1) in that a single-pixel type photodetector 32 is provided instead of the multi-pixel type photodetector 31.

[0139] 9 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to Modification 1 of Embodiment 3. Quantum absorption spectroscopy system 300A differs from quantum absorption spectroscopy system 100 according to Embodiment 1 (see FIG. 1) in that it includes a single-pixel type photodetector 32 and, in addition, it includes a quantum optics 202 including a QPM device 7 (see FIG. 7) instead of quantum optics 201.

[0140] 8 and 9, the single pixel type photodetector 32 is, for example, a photodiode such as a PIN photodiode or an APD. However, a phototube or a photomultiplier tube may be used instead of the photodiode. Also, in order to obtain a high signal-to-noise ratio, a superconducting single photon detector (SSPD) may be adopted. The photodetector 32 detects signal light in response to a control command from the controller 4, and outputs the detection signal to the controller 4.

[0141] The configuration of the quantum absorption spectroscopy system 300 other than the photodetector 32 is common to the corresponding configuration of the quantum absorption spectroscopy system 100 according to the first embodiment. Moreover, the configuration of the quantum absorption spectroscopy system 300A other than the photodetector 32 is common to the corresponding configuration of the quantum absorption spectroscopy system 200 according to the second embodiment (see FIG. 6). Furthermore, the processing procedure of the quantum absorption spectroscopy in the third embodiment and its modified example is similar to the processing procedure of the quantum absorption spectroscopy in the first embodiment (see FIG. 4). Therefore, detailed description will not be repeated.

[0142] FIG. 10 is a conceptual diagram for explaining the difference in detection principle depending on the type of photodetector. Two configurations will be compared and explained here. The first configuration (see the top) is a configuration in which a typical nonlinear optical crystal 23 is combined with a multi-pixel type photodetector 31. Although not shown in FIG. 10, a dispersive optical element 6 (see FIG. 5) may be disposed between the nonlinear optical crystal 23 and the photodetector 31. The second configuration (see the bottom) is a configuration in which a QPM device 7 is combined with a single-pixel type photodetector 32.

[0143] First, referring to the first configuration, the emission direction of the signal photons generated by SPDC in the nonlinear optical crystal 23 has wavelength dependency. Therefore, when a multi-pixel type photodetector 31 is used, signal light of a different frequency is detected for each pixel. In other words, each pixel of the photodetector 31 detects signal light that has been split into monochromatic colors. Therefore, in order to broaden the bandwidth of infrared absorption spectroscopy, the signal photon count rate P s The Fourier spectrum A is obtained by Fourier transform of (t0). s However, to realize infrared absorption spectroscopy in a wide wavelength range, it is necessary to calculate the Fourier spectrum A obtained from each pixel. s It is necessary to synthesize (ω).

[0144] In contrast, in the second configuration, the signal photons generated by SPDC in the QPM device 7 are guided to the single-pixel type photodetector 32 without being spatially divided. Then, the signal light that may include all frequency components is detected by the single single-pixel type photodetector 32. Therefore, the single signal photon count rate P s (t0) (quantum interference signal) is Fourier transformed to obtain Fourier spectrum A s By simply calculating (ω), infrared absorption spectroscopy over a wide wavelength range can be achieved.

[0145] As described above, in the third embodiment, the signal photon count rate P s (t0) is calculated, and the Fourier spectrum A s In the first configuration, the signal light split into monochromatic colors must be aligned for each pixel of the multi-pixel photodetector 31. In contrast, in the third embodiment, which corresponds to the second configuration, the signal light only needs to be captured by the single-pixel photodetector 32, making it easy to adjust the optical system. Also, in the third embodiment, the Fourier spectrum A s(ω) is not required. Furthermore, the Fourier spectrum A from multiple pixels is s There is also no need to combine (ω). This reduces the computational load on the controller 4. These effects are particularly noticeable when a broadband QPM device 7 is used, as shown in the lower part of FIG.

[0146] [Modification 2 of the third embodiment] The idler photons irradiated to the sample are linearly polarized. In this case, when the sample has optical rotation and / or circular dichroism, the polarization state of the idler photons changes as they pass through the sample, and the idler photons become elliptically polarized after passing through the sample. Optical rotation refers to the property of rotating the polarization axis of the photon. Circular dichroism refers to the property of having different absorbances between left-handed circularly polarized light and right-handed circularly polarized light. Such a change in the polarization state of the idler photons reduces the clarity of quantum interference, which can reduce the measurement accuracy of the infrared absorption spectroscopic characteristics of the sample.

[0147] 11 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to Modification 2 of Embodiment 3. The quantum absorption spectroscopy system 300B differs from the quantum absorption spectroscopy system 100 according to Embodiment 1 (see FIG. 1) in that it includes a single-pixel type photodetector 32 and in that it includes a quantum optics 203 instead of the quantum optics 201. The quantum optics 203 differs from the quantum optics 201 in that it further includes a Faraday rotator 271 and a λ / 4 wave plate 272.

[0148] The Faraday rotator 271 is disposed between the lens 212 and the λ / 4 wave plate 272. The Faraday rotator 271 compensates for the rotation of the polarization axis of the idler photons by the λ / 4 wave plate 272.

[0149] The λ / 4 wave plate 272 is disposed between the λ / 4 wave plate 272 and the sample holder 24. The λ / 4 wave plate 272 converts the idler photons traveling from the dichroic mirror 222 toward the sample from linearly polarized light to circularly polarized light. The λ / 4 wave plate 272 also converts the idler photons traveling from the movable mirror 25 through the sample and returning to the dichroic mirror 222 from circularly polarized light to linearly polarized light.

[0150] As described above, in the second modification of the third embodiment, the Faraday rotator 271 and the λ / 4 wave plate 272 are added in front of the sample, and the idler photons irradiated to the sample are converted into circularly polarized light. When the incident light is circularly polarized, the polarization state of the incident light is not disturbed. Therefore, according to the second modification of the third embodiment, it is possible to suppress the decrease in clarity of quantum interference caused by the optical rotation and / or circular dichroism of the sample. As a result, it becomes possible to measure the infrared absorption spectroscopic characteristics of the sample with high accuracy.

[0151] [Embodiment 4] In the quantum optical systems 201 to 203 described so far, the movable mirror 25 is disposed in the idler optical path. However, in order to cause quantum interference between the signal photon and the idler photon, it is only necessary to change the phase (more specifically, the effective optical path length) of one of the signal photon and the idler photon with respect to the phase of the other photon. Therefore, a movable mirror may be disposed in the signal optical path.

[0152] 12 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to embodiment 4. Quantum absorption spectroscopy system 400 differs from quantum absorption spectroscopy system 100 according to embodiment 1 (see FIG. 1) in that quantum absorption spectroscopy system 400 includes quantum optical system 204 instead of quantum optical system 201. Quantum optical system 204 differs from quantum optical system 201 in that it includes movable mirror 28 arranged in the signal optical path in addition to movable mirror 25 arranged in the idler optical path. Movable mirror 28 is provided with a driving device 280.

[0153] The controller 4 is configured to be capable of sweeping the idler optical path by reciprocating the movable mirror 25, and also capable of sweeping the signal optical path by reciprocating the movable mirror 28. However, the sweeping of the idler optical path and the sweeping of the signal optical path are selectively executed.

[0154] In this example, a dichroic mirror 223 is added between the lens 213 and the movable mirror 28. Of the pump light and signal light transmitted through the dichroic mirror 222, the signal light transmits through the dichroic mirror 223 and heads toward the movable mirror 28. On the other hand, the pump light is reflected by the dichroic mirror 223. Although not essential, it is desirable to separate the signal light from the pump light in this manner.

[0155] 12, a single-pixel type photodetector 32 is used as in the second embodiment, but a multi-pixel type photodetector 31 may also be used (see FIG. 1). Also, a QPM device 7 may be used instead of the nonlinear optical crystal 23 (see FIG. 6).

[0156] First, in the state where a sample is not placed in the idler optical path, a comparison is made between the case where the idler optical path is swept by moving the movable mirror 25 back and forth and the case where the signal optical path is swept by moving the movable mirror 28 back and forth. The sweep width of the idler optical path (the moving distance of the movable mirror 25) is ΔL idl and the sweep width of the signal light path (the moving distance of the movable mirror 28) is written as ΔL sig It is written as follows.

[0157] As already explained in the first embodiment using the formula (9), the signal photon count rate P s depends on the idler propagation time t0. For convenience, equation (9) is restated below.

[0158]

number

[0159] Observing equation (9), the signal photon count rate P s It can be seen that equation (9) indicates that depends on the signal propagation time t1 as well as the idler propagation time t0. The signal propagation time t1 is the time it takes for the signal light generated by the nonlinear optical crystal 23 to be reflected by the movable mirror 28 and return to the nonlinear optical crystal 23.

[0160] Here, it is assumed that the spectral shape of the two-photon field amplitude F(Ω) is a Gaussian function, as shown in the following formula (13). In formula (13), the spectral width of the idler photon generated by the nonlinear optical crystal 23 is written as σ.

[0161]

number

[0162] For further simplification, the optical loss inside the quantum optical system 204 is ignored, and the complex transmittance τ of the sample not placed on the idler optical path is approximated as 1. Then, the above formula (9) can be transformed into the following formula (14).

[0163]

number

[0164] Equation (14) expresses the signal photon count rate P s This is an equation that expresses the dependence of the idler propagation time t0 and the signal propagation time t1 of the signal photon count rate P s Idler distance L idl and signal distance L sig Equation (14) can be transformed into an equation that expresses the dependence on L idl = ct0 and L sig According to the relational expression =ct1, the idler propagation time t0 is the idler distance L idl and convert the signal propagation time t1 into the signal distance L sigAlso, the central frequency of the idler photon is ω i0 ω i0 =ck i0 Using the dispersion relation with the central wave number k i0 and the central frequency of the signal photon ω s0 ω s0 =ck s0 Using the dispersion relation with the central wave number k s0 Then, the following equation (15) is derived from equation (14). By using equation (15), it is possible to simulate what kind of quantum interference signal is obtained when the idler path or the signal path is swept.

[0165]

number

[0166] Next, we will explain the Fourier spectrum obtained when an infrared absorbing sample is placed in the idler optical path. For distinction, the Fourier spectrum obtained by sweeping the idler optical path is called A s idl The Fourier spectrum obtained by sweeping the signal path is written as A s sig The Fourier spectrum A when sweeping the idler path is s idl is derived as shown in the following equation (16) following the above equation (11).

[0167]

number

[0168] The following equation (17) is obtained from equation (16).

[0169]

number

[0170] Idler frequency ω iis the central frequency ω of the idler photon i0 Using the detuning Ω from ω i =ω i0 +Ω. By substituting this relational expression into equation (17), equation (17) can also be written as the following equation (18). From equation (18), the amplitude |A s idl (ω i0 +Ω)| is the square of the two-photon field amplitude F(Ω) and the complex transmittance τ(ω i0 +Ω) squared.

[0171]

number

[0172] Similarly, the Fourier spectrum A when the signal path is swept is s sig is derived as shown in the following equation (19).

[0173]

number

[0174] The following equation (20) is obtained from equation (19).

[0175]

number

[0176] Signal frequency ω s is the central frequency ω of the signal photon s0 Using the detuning Ω from ω s =ω s0 -Ω. Therefore, equation (20) can be written as equation (21) below. Equation (21) expresses the amplitude |A s sig (ω s0+Ω)| is the square of the two-photon field amplitude F(-Ω) and the complex transmittance τ(ω i0 -Ω) squared.

[0177]

number

[0178] As described above, in the fourth embodiment, the quantum absorption spectroscopy system 400 is configured to be able to sweep both the idler optical path and the signal optical path. As will be shown in the seventh embodiment later, the same wavenumber resolution can be achieved whether the idler optical path is swept or the signal optical path is swept. Therefore, depending on the absorption spectroscopic characteristics of the sample, it is only necessary to sweep either the idler optical path or the signal optical path, whichever is more convenient.

[0179] When the idler optical path is swept by moving movable mirror 25 back and forth, movable mirror 25 corresponds to the phase conversion unit according to the present disclosure, and the idler photon corresponds to the "one photon" according to the present disclosure. On the other hand, when the signal optical path is swept by moving movable mirror 28 back and forth, movable mirror 28 corresponds to the phase conversion unit according to the present disclosure, and the signal photon corresponds to the "one photon" according to the present disclosure.

[0180] In the fourth embodiment, a configuration capable of sweeping both the idler optical path and the signal optical path has been described. However, a system configuration capable of sweeping only the signal optical path may be adopted. This is because by using the signal optical path as the optical path to be swept, advantages in terms of implementation of the quantum absorption spectroscopy system can be obtained, as in the following modified example.

[0181] [Modification of the fourth embodiment] It is also possible to use a reflection method instead of a transmission method in which transmitted light from a sample is detected by a photodetector. For example, an attenuated total reflection (ATR) measurement method may be applied to the quantum absorption spectroscopy system according to the present disclosure.

[0182] 13 is a diagram showing an overall configuration of a quantum absorption spectroscopy system according to a modification of the fourth embodiment. A quantum absorption spectroscopy system 400A includes a quantum optical system 205 instead of the quantum optical system 204. In , and is different from the quantum absorption spectroscopy system 400 (see FIG. 12) according to the fourth embodiment. The quantum optical system 205 is different from the quantum optical system 204 in that it includes an ATR unit 8 instead of the sample holder 24, and in that it includes a fixed mirror 29 instead of the movable mirror 25 and the driver 250 in the idler optical path.

[0183] 14 is a diagram showing an example of the configuration of the ATR unit 8. The ATR unit 8 includes a lens 81, a prism 82, and a lens 83. The lens 81, the prism 82, and the lens 83 are arranged in this order along the propagation direction of the idler photons. The ATR unit 8 may include a mirror for adjusting the optical axis instead of or in addition to the lenses 81 and 83.

[0184] Prism 82 has a high refractive index and is configured to come into contact with the sample surface. Idler photons entering prism 82 from lens 81 are totally reflected at the interface between prism 82 and the sample. At this time, idler photons (evanescent waves) leaking out to the sample side are absorbed by the sample surface, so that the infrared absorption spectroscopic characteristics of the sample surface can be measured by detecting the totally reflected light.

[0185] In the ATR, it is necessary to adjust the optical axis of the idler light so that the idler light enters one end of the prism and is emitted from the other end of the prism toward a subsequent mirror (fixed mirror 29 in FIG. 14). When the idler light path is swept by the reciprocation of movable mirror 25 arranged in the idler light path as described in the first embodiment (see FIG. 1), the optical axis of the idler light may change periodically as movable mirror 25 reciprocates. For this reason, quantum optical system 201 must be constructed so that the optical axis of the idler light is always maintained at an appropriate position even during the sweeping of the idler light path. However, adjusting such an optical system is not easy.

[0186] Furthermore, the ATR unit 8 may be larger than the sample holder 24 used in the transmission method or other reflection methods because it includes the prism 82 (and the optical system including the lenses 81, 82 before and after the prism 82). With a limited size of the device, it may be difficult to ensure a space behind the sample that allows the movable mirror 25 to reciprocate while avoiding mechanical contact between the sample and the movable mirror 25.

[0187] In contrast, in the modification of the fourth embodiment, the signal optical path is swept by moving the movable mirror 28 arranged in the signal optical path back and forth. In this way, the optical axis of the idler light does not change over time as the movable mirror 28 moves back and forth. Therefore, according to the modification of the fourth embodiment, the difficulty of constructing the quantum optical system 205 can be reduced. In addition, since there is no mechanical contact between the sample and the movable mirror, the problem of securing space can be avoided. Therefore, the implementation of a quantum absorption spectroscopy system can be easily realized.

[0188] Also, there may be cases where minute irregularities exist on the sample surface, in other words, the sample surface is microscopically rough. In such cases, in a typical transmission method, multiple modes may occur due to the idler photons being scattered and / or reflected on the sample surface. As a result, the measurement accuracy of the infrared absorption spectroscopic characteristics of the sample may decrease due to a decrease in signal intensity and an increase in noise. On the other hand, in the ATR, since the prism 82 is configured to contact the sample surface, the above-mentioned problem is unlikely to occur even if minute irregularities exist on the sample surface. Therefore, the infrared absorption spectroscopic characteristics of the sample can be measured with high accuracy.

[0189] The first to fourth embodiments and the modified examples of the respective embodiments may be appropriately combined. For example, the second, third, and fourth embodiments may be combined to adopt the QPM device 7 and the single-pixel type photodetector 32, and both the idler path and the signal path may be swept. The modified example of the first embodiment may be combined to adopt the dispersive optical element 6 in front of the multi-pixel type photodetector 31, and both the idler path and the signal path may be swept. The second and fourth embodiments may be combined to adopt the QPM device 7 and perform measurement by the total reflection measurement method.

[0190] [Example] Hereinafter, the results of simulating or measuring the Fourier spectrum and the absorption spectroscopic characteristics of a sample using the quantum absorption spectroscopy system according to any one of the first to fourth embodiments (and the modifications) will be described.

[0191] <First Example> In the first embodiment, the Fourier spectrum A is obtained by Fourier transforming the quantum interference signal acquired without placing the sample in the idler optical path. s 0 The results of calculating the reference Fourier spectrum (reference Fourier spectrum) are described below. This measurement is a so-called background measurement, and can also be said to measure the absorption spectroscopic characteristics of the air on the idler optical path. The measurement was performed using the system configuration of a quantum absorption spectroscopy system 200 (see FIG. 6) according to the second embodiment. A single crystal of LiNbO3 was used as the nonlinear optical crystal 23 (the same applies to the second and third examples described later).

[0192] 15 is a diagram showing an example of a measurement result of a quantum interference signal. The horizontal axis of FIG. 15 represents the sweep width ΔL of the idler optical path. idl The vertical axis represents the signal photon count rate P s FIG. 16 shows a Fourier spectrum A obtained by Fourier transform of the quantum interference signal shown in FIG. s 016, the horizontal axis represents wavelength, and the vertical axis represents light intensity.

[0193] The central wavelength λ of the signal photon generated by the nonlinear optical crystal 23 s is 816 nm, and the central wavelength of the idler photon, λ i The measurement revealed that the Fourier spectrum A had a peak near 1530 nm, which is the design wavelength of the idler light. s 0 It was confirmed that the Fourier spectrum A was obtained. s 0 The bandwidth (full width at half maximum) of the signal photon count rate P s The appropriate Fourier spectrum A is obtained by Fourier transform of s 0 It was confirmed that the following results could be obtained.

[0194] <Second Example> In the second example, the results of an experiment to confirm how the quantum interference effect is reflected in the signal photon spectrum will be described. This measurement was performed using the system configuration of a quantum absorption spectroscopy system 100A (see FIG. 5) according to a modification of the first embodiment. The central wavelength λ of the signal photon s is 810 nm, and the central wavelength of the idler photon, λ i The wavelength was 1550 nm. The sample was not placed in the idler path.

[0195] 17 is a diagram showing the difference in the signal photon spectrum due to the quantum interference effect, where the horizontal axis represents the wavelength and the vertical axis represents the light intensity of the signal photon.

[0196] FIG. 17 shows a signal photon spectrum in constructive interference where two processes (the first and second physical processes described above) that generate quantum entangled photon pairs in the nonlinear optical crystal 23 constructively interfere with each other, and a signal photon spectrum in destructive interference where the two processes cancel each other out. For comparison, the signal photon spectrum in the case where the quantum interference effect is completely eliminated by blocking the idler light is also shown. As shown in FIG. 17, it was confirmed that, compared to the case where the quantum interference effect is not generated, the intensity of the signal photon spectrum increases due to constructive interference, while the intensity of the signal photon spectrum decreases due to destructive interference.

[0197] <Third Example> In the third example, a sample is placed in the idler optical path, and the refractive index of the sample is calculated from the complex transmittance spectrum of the sample. This measurement was performed using the system configuration of the quantum absorption spectroscopy system 200 (see FIG. 6) according to the second embodiment.

[0198] As a sample, silica glass with a known refractive index (specifically, a silica glass substrate with a thickness of 140 μm) was used. The literature value of the refractive index of silica glass is 1.506 (wavelength 1550 nm). The sweep width ΔL of the idler optical path was calculated for both the case where silica glass was placed in the idler optical path and the case where silica glass was not placed. idl The step width of the idler path was set to 100 nm, and the signal photons detected at each step were integrated for 50 ms.

[0199] 18 is a diagram showing an example of the measurement results of the quantum interference signal related to the silica glass. Due to the phase delay caused by the silica glass, the peak of the quantum interference signal shifted by about 150 μm when the silica glass was placed compared to when the silica glass was not placed.

[0200] FIG. 19 shows the Fourier spectrum A obtained by Fourier transform of the quantum interference signal shown in FIG. s ,A s 0However, in order to obtain the measurement results of the entirety of these two Fourier spectra, the sweep width of the idler optical path ΔL idl has been expanded from 400 μm to 680 μm.

[0201] As explained in equation (12), the Fourier spectrum A when the silica glass is not placed in the idler optical path is s 0 (ω) and Fourier spectrum A when silica glass is placed in the idler path. s The complex transmittance spectrum τ(ω) of quartz glass can be obtained by calculating the amplitude ratio with (ω). Since it is difficult to plot the complex transmittance spectrum τ(ω) on a two-dimensional plane, the complex transmittance spectrum τ(ω) is plotted separately into the absolute value of transmittance T and the phase difference φ between the two Fourier spectra based on the relationship τ(ω)=Texp(iφ(ω)).

[0202] 20 is a diagram showing the absolute value T of transmittance and phase difference φ obtained by measuring quartz glass. The horizontal axis represents wavelength. The vertical axis represents transmittance T (absolute value) on the top and phase difference φ on the bottom.

[0203] Two Fourier spectra A s ,A s 0 The amplitude ratio of (|A s | / |A s 0 As a result of calculating the complex transmittance τ from |), the refractive index of quartz glass was calculated to be 1.53. This value is sufficiently close to the literature value (about 1.51). Therefore, it was confirmed that the quantum absorption spectroscopy system 200 can calculate the complex transmittance spectrum τ(ω) with high accuracy.

[0204] <Fourth Example> In the fourth to sixth examples, various results of simulating infrared absorption spectroscopy over a wide wavelength range will be described. In the fourth example, the system configuration of the quantum absorption spectroscopy system 300A (see FIG. 9) according to the first modification of the third embodiment was assumed.

[0205] 21 to 23, the material of the nonlinear optical crystal 72 of the QPM device 7 is a single crystal of LiNbO3. The length of the nonlinear optical crystal 72 in the light propagation direction is 2 cm. The wavelength of the pump light is 532 nm.

[0206] Fig. 21 is a diagram showing the polarization inversion period Λ at each position in the nonlinear optical crystal 72. The horizontal axis represents the position in the direction from the first end 721 (incident end) to the second end 722 (output end) of the nonlinear optical crystal 72 (see Fig. 7). The vertical axis represents the polarization inversion period Λ at each position.

[0207] It was assumed that the number of sections of the polarization inversion structure of the nonlinear optical crystal 72 was sufficiently large, and that the spatial frequency of the nonlinear optical crystal 72 (the inverse of the polarization inversion period Λ) changed linearly and continuously (chirped) between the first end 721 and the second end 722.

[0208] The polarization inversion period Λ at the first end 721 of the nonlinear optical crystal 72 was set to 7 μm, and the polarization inversion period Λ at the second end 722 of the nonlinear optical crystal 72 was set to 11.9 μm. This polarization inversion period Λ was designed so that the following two phase matching conditions were satisfied for both ends of the nonlinear optical crystal 72. The first phase matching condition is a phase matching condition for SPDC in which a signal photon with a wavelength of 900 nm and an idler photon with a wavelength of 1300 nm are generated at the first end 721 of the nonlinear optical crystal 72. The second phase matching condition is a phase matching condition for SPDC in which a signal photon with a wavelength of 600 nm and an idler photon with a wavelength of 4700 nm are generated at the second end 722 of the nonlinear optical crystal 72.

[0209] FIG. 22 is a diagram showing an example of a simulation result of the spectrum of a quantum entangled photon pair generated using the QPM device 7. The left side shows the spectrum of the signal photon, and the right side shows the spectrum of the idler photon. As shown in FIG. 22, the simulation result showed that it is possible to generate idler photons over a wide infrared range of about 1.5 μm to 4.5 μm. Note that absorption by the nonlinear optical crystal 72 on the longer wavelength side than 4.7 μm was not taken into consideration.

[0210] Fig. 23 is a diagram showing the simulation results of a quantum interference signal obtained when using a quantum entangled photon pair having the spectrum shown in Fig. 22. The left side shows an overall view of the quantum interference signal, and the right side shows an enlarged view of the vicinity of the peak of the quantum interference signal. The upper side shows the case where no sample was placed, and the lower side shows the case where a sample was placed. For details of the method for calculating the quantum interference signal, see the above formula (9), formula (14), or formula (15), etc.

[0211] In this simulation, the complex dielectric constant ε of the sample was assumed to be as shown in the following formula (22). The complex dielectric constant ε is the square of the complex refractive index N (ε = N 2 ) where εb is the background dielectric constant, Ki is the resonance wave number, and f i represents the oscillator strength, and γ i The effect of multiple reflections within the sample was assumed to be negligible.

[0212]

number

[0213] From Figure 23, when comparing the quantum interference signal when a sample is placed with the quantum interference signal when no sample is placed, it can be seen that although the oscillation period (the interval between interference fringes) is the same, a peak shift occurs due to infrared absorption by the sample.

[0214] Although lithium niobate (LiNbO3) has been described as an example of the material of the nonlinear optical crystal 72, other materials can also be used. For example, gallium phosphide (GaP), gallium arsenide (GaAs), lithium tantalate (LiTaO3), zinc selenide (ZnSe), etc. can be used. The material of the nonlinear optical crystal 72 may be an organic material. The material of the nonlinear optical crystal 72 may be, for example, DAST (4-N,N-Dimethylamino-4'-N' methylstilbazolium tosylate), DLD164, or a compound in which some functional groups of these compounds are replaced with other atoms or atomic groups.

[0215] FIG. 24 is a diagram showing the relationship between the polarization inversion period Λ and the generation wavelength range of the idler light for each material of the nonlinear optical crystal 72. The horizontal axis represents the polarization inversion period Λ of the nonlinear optical crystal 72, and the vertical axis represents the wavelength of the idler photon. FIG. 24 shows the simulation results for each case in which the material of the nonlinear optical crystal 72 is lithium niobate (pump light wavelength 532 nm, room temperature), gallium phosphide (pump light wavelength 800 nm, 55° C.), gallium arsenide (pump light wavelength 1064 nm, 55° C.), lithium tantalate (pump light wavelength 532 nm, room temperature), or zinc selenide (pump light wavelength 633 nm, room temperature). For zinc selenide, it is assumed that the Sellmeier equation, which has been confirmed to be effective up to a wavelength of 16 μm, can be applied up to a wavelength of 22 μm.

[0216] In any material, the wavelength range of the idler photons generated by the nonlinear optical crystal 72 can be adjusted by appropriately designing the range of the polarization inversion period Λ. Specifically, in lithium niobate, the wavelength range of the idler photons can be adjusted to a range of 1 μm to 5 μm by designing the polarization inversion period Λ to change in a range of 7 μm to 12 μm. In gallium phosphide, the wavelength range of the idler photons can be adjusted to a range of 1.6 μm to 12 μm by designing the polarization inversion period Λ to change in a range of 6.5 μm to 21 μm. In gallium arsenide, the wavelength range of the idler photons can be adjusted to a range of 2 μm to 18 μm by designing the polarization inversion period Λ to change in a range of 7.5 μm to 27.5 μm. In lithium tantalate, the wavelength range of the idler photons can be adjusted to a range of 1 μm to 5 μm by designing the polarization inversion period Λ to change in a range of 8 μm to 13.5 μm. In zinc selenide, the wavelength range of the idler photons can be adjusted to the range of 1.3 μm to 22 μm by designing the polarization inversion period Λ to change in the range of 5 μm to 31.5 μm. The characteristics of each material that can be used for the nonlinear optical crystal 72 are summarized in FIG. 25.

[0217] <Fifth Example> In the fifth example, a simulation result of a complex transmittance spectrum in the case where idler photons distributed over a wide wavelength range are generated using a QPM device 7 will be described. The true value of the complex refractive index of the sample is assumed to be known. More specifically, the true value of the complex refractive index of the sample is assumed to be determined according to the complex dielectric constant ε in the above formula (12). The simulation result and the true value are compared. In this simulation, the system configuration of the quantum absorption spectroscopy system 300A (see FIG. 9) according to the first modification of the third embodiment is also assumed.

[0218] FIG. 26 is a diagram showing an example of a simulation result in which a complex transmittance spectrum (a spectrum of transmittance T, which is an absolute value) over a wide wavelength range is obtained. As described above, this simulation result is obtained by performing measurements in two cases, with and without a sample placed, and obtaining two Fourier spectra A s ,A s 0The amplitude ratio of the idler path is given by the sweep width ΔL idl The Fourier spectrum wavenumber resolution was set to 5 cm. -1 It was decided.

[0219] Fig. 27 is a diagram showing the results of a simulation in which the complex refractive index N of a sample was determined from the complex transmittance spectrum shown in Fig. 26. The real part Re(N) of the complex refractive index N is shown at the top, and the imaginary part Im(N) of the complex refractive index N is shown at the bottom. The simulation results of the complex refractive index are shown on the left, and the true value of the complex refractive index is shown on the right. The complex refractive index obtained by simulation coincided with the true value to a considerable extent.

[0220] <Sixth Example> In the sixth embodiment, the results of measuring the complex transmittance over a wide wavelength range will be described. The sample has a bandgap of 12 nm width around 1550 nm. P A filter (FBH1550-12 manufactured by Thorlabs) was used. The quantum interference signal was measured both when the sample was placed in the idler light path and when it was not placed. The sweep width ΔL of the idler light path when the sample was placed was idl is set to 2000 μm, and the sweep width ΔL of the idler light path when no sample is placed is idl was set to 200 μm. In both cases, the step width of the idler optical path was set to 100 nm, and the detected signal photons were integrated for 50 milliseconds at each step. This measurement was performed using the system configuration of the quantum absorption spectroscopy system 300 (see FIG. 8) according to the third embodiment.

[0221] Figure 28 shows an example of the measurement results of the quantum interference signal over a wide wavelength range. The figure shows the quantum interference signal when a sample is placed above, and the quantum interference signal when no sample is placed below. When a sample is placed, the peak of the quantum interference signal shifted by about 2000 μm compared to when no sample is placed.

[0222] Fig. 29 is a diagram showing the results of determining the complex transmittance spectrum (spectrum of transmittance T) of the sample from the quantum interference signal shown in Fig. 28. Specifically, Fourier spectrum A with and without the sample s ,A s 0 The amplitude ratio of was calculated (see formula (12) above). Figure 29 also shows the measurement results of the transmittance spectrum using a commercially available conventional spectrometer (Shimadzu UV3600-Plus) for comparison. It can be seen that the complex transmittance spectrum obtained from the quantum interference signal is in good agreement with the complex transmittance spectrum obtained using the conventional spectrometer in terms of intensity and bandwidth.

[0223] <Seventh Example> In the seventh example, a simulation result when the signal optical path is swept will be described. This simulation is performed assuming the system configuration of the quantum absorption spectroscopy system 400 (see FIG. 12) according to the fourth embodiment.

[0224] 30 is a diagram showing a quantum interference signal obtained by a simulation of sweeping the idler optical path or the signal optical path. The horizontal axis represents the sweep width of the idler optical path (amount of change in the idler optical path length) ΔL idl or the sweep width of the signal optical path (change in signal optical path length) ΔL sig The vertical axis represents the signal photon count rate P s The overall view of the quantum interference signal is shown on the left, and an enlarged view of the quantum interference signal near the sweep width=0 is shown on the right.

[0225] In this simulation, the wavelength is 532 nm (wave number 18797 cm -1 ) pump light, the wavelength of 810 nm (wave number 12346 cm -1 ) signal photon and wavelength 1550 nm (wave number 6451 cm -1 ) is generated. The spectral width of the idler photon is σ / 2πc=40cm -1 The idler path sweep width ΔL was set to idland the sweep width ΔL of the signal light path sig The maximum value of was set to 500 μm in all cases. The step width when sweeping each optical path (the interval between plots shown by circles in the enlarged view) was set to 100 nm. In this case, 5000 points were plotted for each simulation.

[0226] As shown in the overall view on the left, the width of the quantum interference signal when the idler path was swept was equal to that when the signal path was swept. Also, as shown in the enlarged view on the right, the quantum interference signal was sparse when the idler path was swept, whereas it was dense when the signal path was swept. It can be seen that the oscillation period (spacing between interference fringes) of the quantum interference signal when the idler path was swept corresponds to the wave number of the idler photon, and the oscillation period of the quantum interference signal when the signal path was swept corresponds to the wave number of the signal photon.

[0227] FIG. 31 is a diagram showing a Fourier spectrum obtained by Fourier transform of the quantum interference signal shown in FIG. 30. It can be seen from FIG. 31 that the same shape of Fourier spectrum can be obtained whether the idler optical path or the signal optical path is swept, and that the wavenumber resolution (interval between plots) of the Fourier spectrum is the same. In this way, the sweep width (ΔL idl ,ΔL sig If the sweep width of both optical paths is set to 500 μm, the wavenumber resolution of the Fourier spectrum is 20 cm. -1 It was.

[0228] Furthermore, from FIG. 31, it can be seen that the central wave number of each Fourier spectrum corresponds to the wave number of the photon propagating through the swept optical path. That is, the central wave number k of the Fourier spectrum when the idler optical path is swept is i0 corresponds to the wave number of the idler photon, and in this example, k i0 =6452cm -1The central wave number k of the Fourier spectrum when the signal path is swept is s0 corresponds to the wave number of the signal photon, and in this example, k s0 =12346cm -1 It was.

[0229] FIG. 32 shows the Fourier transform obtained by a simulation in which the idler path or the signal path is swept with respect to a state in which an infrared absorbing sample is placed in the idler path. S The horizontal axis represents the wave number, and the vertical axis represents the amplitude (|A s idl |,|A s sig |).

[0230] In this simulation, the wavelength is 532 nm (wave number 18797 cm -1 ) pump light was irradiated, resulting in a wavelength of 810 nm (wave number 12346 cm -1 ) signal photon and wavelength 1550 nm (wave number 6451 cm -1 ) and an entangled photon pair with an idler photon of σ / 2πc=1114cm -1 The absorption band of the sample was set at a wavelength of 1750 nm (wave number 5714 cm -1 ) was set.

[0231] Fourier spectrum A when the idler path is swept s idl and the Fourier spectrum A when the signal path is swept. s sig In both cases, a dip indicating absorption by the sample was confirmed. s idl and Fourier spectrum A s sig It can be seen that the two are linearly symmetric (mirror symmetric) with respect to the axis of symmetry. The wave number of this axis of symmetry (shown by the dashed line) is 9396 cm, half the wave number of the pump light. -1 It was.

[0232] <Eighth Example> In the eighth example, a configuration for measuring absorption spectroscopic characteristics in the ultraviolet region will be described. In this measurement, a system configuration basically equivalent to that of the quantum absorption spectroscopy system 400 according to the fourth embodiment (see FIG. 12) was used. However, while the quantum absorption spectroscopy system 400 generates a quantum entangled photon pair of a signal light in the visible region and an idler light in the infrared region, the quantum absorption spectroscopy system according to this example generates a quantum entangled photon pair of a signal light in the visible region and an idler light in the ultraviolet region.

[0233] To generate idler light in the ultraviolet region, four-wave mixing can be used as described below. Alternatively, the type of nonlinear optical crystal 23 may be changed. For example, CsB3OP5 (CBO), CsLiB6O 10 Known crystals such as (CLBO) can be used.

[0234] FIG. 33 is a diagram showing quantum interference signals obtained by a simulation in which an idler optical path or a signal optical path is swept in a quantum absorption spectroscopy system for measuring absorption spectroscopic characteristics in the ultraviolet region.

[0235] In this simulation, we assumed the use of four-wave mixing, a process in which two excitation photons are converted into a photon pair consisting of a signal photon and an idler photon. In detail, by irradiating the pump light (two pump photons for excitation) with a wavelength of 400 nm, a wavelength of 927 nm (wave number 10788 cm -1 ) signal photon and wavelength 255 nm (wave number 392157 cm -1 ) and an entangled photon pair with an idler photon of σ / 2πc=400cm -1 The sample was not installed.

[0236] Sweep width of idler path ΔL idl and the sweep width ΔL of the signal light path sigThe maximum value of was set to 30 μm in all cases. The step width for sweeping each optical path was set to two values: 10 nm and 125 nm. In the figure, the left side shows the simulation results when the step width is 10 nm, and the right side shows the simulation results when the step width is 125 nm.

[0237] When the idler path was swept, a normal quantum interference signal was obtained when the step width was 10 nm, whereas distortion of the quantum interference signal was confirmed when the step width was 125 nm. On the other hand, when the signal path was swept, a normal quantum interference signal was obtained when the step width was both 10 nm and 125 nm. The reason for this is explained as follows.

[0238] Applying the sampling theorem to this embodiment, when sweeping the idler optical path, the step width Δ is set to half the wavelength of the idler photon (λ i / 2) (Δ<λ i When sweeping the signal path, the step width Δ is set to half the wavelength of the signal photon (λ s / 2) (Δ<λ s / 2). More specifically, the wavelengths of the idler photons and the signal photons are distributed in a wavelength range of a given width including the central wavelength. It is required to set the step width Δ to a value smaller than 1 / 2 of the lower limit of this wavelength range.

[0239] In this embodiment, the idler photons are ultraviolet photons. Therefore, the shorter wavelength of the idler photons (center wavelength 255 nm) makes it more difficult to satisfy the sampling theorem compared to when the idler photons are infrared photons. In this example, when the step width Δ=10 nm, 10 nm << 255 nm / 2, so the sampling theorem is satisfied. On the other hand, when the step width Δ=125 nm, 125 nm ≒ 255 / 2, so considering that the wavelength distribution of the idler photons is typically on the order of several tens of nm, it is considered that the sampling theorem is not satisfied (undersampling).

[0240] In this way, when the idler photon is an ultraviolet photon and the idler path is swept, the step width Δ must be set to a very small value to avoid undersampling. Therefore, when the idler photon is an ultraviolet photon, the signal path may be swept instead of the idler path. Since the signal photon is a visible photon, the longer its wavelength (center wavelength 927 nm) makes it easier to satisfy the sampling theorem. In this example, when the step width Δ=10 nm, the sampling theorem is naturally satisfied, and when the step width Δ=125 nm, the sampling theorem is also satisfied because 125 nm<<927 nm / 2.

[0241] Figure 34 shows the Fourier spectrum obtained when the signal optical path was swept. The step width was set to 125 nm. It was confirmed that, when the signal optical path was swept, a Fourier spectrum having a peak at an appropriate position could be obtained. On the other hand, when the idler optical path was swept, some of the peaks in the Fourier spectrum were at the maximum wavenumber of the system, 40,000 cm. -1 In this way, the optical path to be swept can be selected according to the system specifications.

[0242] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is defined by the claims, not by the description of the embodiments described above, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0243] 1 laser light source, 201-205 quantum optics, 211-214 lens, 221-223 dichroic mirror, 23 nonlinear optical crystal, 231 first crystal, 232 second crystal, 24 sample holder, 25, 28 moving mirror, 250, 280 driving device, 271 Faraday rotator, 272 λ / 4 wave plate, 26, 29 fixed mirror, 31, 32 photodetector, 4 controller, 41 processor, 42 memory, 43 input / output port, 5 monitor, 401 light source control unit, 402 mirror control unit, 403 count rate calculation unit, 404 Fourier transform unit, 405 first memory unit, 406 second memory unit, 407 transmittance calculation unit, 408 absorption spectrum calculation unit, 409 monitor control unit, 6 dispersive optical element, 7 QPM device, 71, 74 Lenses, 72 Nonlinear optical crystals, 73 Long pass filters, 75 Sharp cut filters, 8 ATR units, 81, 83 Lenses, 82 Prisms, 100, 100A, 200, 300, 300A, 300B, 400, 400A Quantum absorption spectroscopy systems.

Claims

1. 1. A quantum absorption spectroscopy system, comprising: A light source, a quantum optical system, a photodetector, and a computing device, The light source emits pump light; The quantum optical system includes: a nonlinear optical element that generates quantum entangled photon pairs of a signal photon and an idler photon by irradiation with the pump light; a phase conversion unit configured to be able to change the phase of one of the signal photons and the idler photons, The quantum interference is generated between a plurality of physical processes that generate the quantum entangled photon pairs, the photodetector outputs a quantum interference signal corresponding to the number of detections of the signal photons when the phase of one of the photons is changed by the phase conversion unit in a state in which a sample is placed in an optical path of the idler photon, The calculation device calculates the absorption spectroscopic characteristics of the sample by Fourier transform of the quantum interference signal.

2. The computing device includes: calculating a Fourier spectrum by Fourier transform of the quantum interference signal when the sample is disposed in the optical path of the idler photons, and calculating a reference Fourier spectrum by Fourier transform of the quantum interference signal when the sample is not disposed in the optical path of the idler photons; The quantum absorption spectroscopy system of claim 1 , further comprising: a complex transmittance spectrum of the sample calculated based on a ratio between the Fourier spectrum and the reference Fourier spectrum.

3. The quantum absorption spectroscopy system according to claim 2 , wherein the arithmetic unit calculates the absorption spectrum of the sample by squaring an absolute value of a complex transmittance spectrum of the sample.

4. The quantum absorption spectroscopy system according to any one of claims 1 to 3, wherein the calculation device calculates the absorption spectroscopic characteristics of the sample by Fourier transforming the quantum interference signal acquired without wavelength sweeping the quantum entangled photon pair or wavelength decomposing the signal photon in the quantum optical system.

5. 5. The quantum absorption spectroscopy system according to claim 1, wherein the nonlinear optical element is a chirp-type or fan-type quasi-phase matching element.

6. the nonlinear optical element is a quasi-phase matching element including a nonlinear optical crystal, The quantum absorption spectroscopy system according to any one of claims 1 to 4, wherein the quasi-phase matching element is configured such that, when the quantum entangled photon pair is generated multiple times, the wavelengths of a group of idler photons including the idler photon are distributed over a wavelength range determined according to the material and polarization inversion period of the nonlinear optical crystal.

7. the nonlinear optical crystal material includes lithium niobate; 7. The quantum absorption spectroscopy system according to claim 6, wherein the poling period of the nonlinear optical crystal is determined so that the idler photon group includes a plurality of photons having mutually different wavelengths within the wavelength range from 0.4 μm to 5.2 μm.

8. the nonlinear optical crystal material includes gallium phosphide; 7. The quantum absorption spectroscopy system according to claim 6, wherein the poling period of the nonlinear optical crystal is determined so that the idler photon group includes a plurality of photons having mutually different wavelengths within a wavelength range from 0.7 μm to 12 μm.

9. the nonlinear optical crystal material includes gallium arsenide; 7. The quantum absorption spectroscopy system according to claim 6, wherein the poling period of the nonlinear optical crystal is determined so that the group of idler photons includes a plurality of photons having mutually different wavelengths within a wavelength range from 1 μm to 18 μm.

10. the nonlinear optical crystal material includes lithium tantalate; 7. The quantum absorption spectroscopy system according to claim 6, wherein the poling period of the nonlinear optical crystal is determined so that the idler photon group includes a plurality of photons having mutually different wavelengths within the wavelength range from 0.3 μm to 5.5 μm.

11. the nonlinear optical crystal material includes zinc selenide; 7. The quantum absorption spectroscopy system according to claim 6, wherein the poling period of the nonlinear optical crystal is determined so that the idler photon group includes a plurality of photons having mutually different wavelengths within the wavelength range from 0.4 μm to 22 μm.

12. 12. The quantum absorption spectroscopy system according to claim 1, wherein the photodetector is a single-pixel type photodetector.

13. the signal photon is a photon in the visible range, The quantum absorption spectroscopy system of any one of claims 1 to 12, wherein the photodetector is a silicon-based photodetector.

14. The quantum absorption spectroscopy system according to any one of claims 1 to 13, wherein the quantum optical system further includes a total reflection measurement device configured to be capable of measuring the total reflection of the sample.

15. The phase conversion unit is a first movable mirror configured to be movable along a propagation direction of the signal photon; a second movable mirror configured to be movable along the propagation direction of the idler photons; The quantum absorption spectroscopy system further comprises a controller, 15. The quantum absorption spectroscopy system of claim 1, wherein the control device is configured to selectively move one of the first and second moving mirrors.

16. The quantum absorption spectroscopy system according to any one of claims 1 to 14, wherein the phase conversion unit includes a movable mirror configured to be movable along the propagation direction of the signal photons.

17. the nonlinear optical element is configured to generate the idler photons in an ultraviolet range; 5. The quantum absorption spectroscopy system according to claim 1, wherein the arithmetic unit calculates ultraviolet absorption spectroscopic characteristics of the sample.

18. In a quantum optical system configured to cause quantum interference between a plurality of physical processes that generate quantum entangled photon pairs of a signal photon and an idler photon, a step of generating the quantum entangled photon pairs by irradiating a nonlinear optical element with pump light; a step of detecting the signal photon by a photodetector when the phase of one of the signal photon and the idler photon is changed by a phase conversion unit in a state where a sample is placed in an optical path of the idler photon, thereby acquiring a quantum interference signal according to the number of detected photons; and calculating the absorption spectroscopic characteristics of the sample by Fourier transform of the quantum interference signal.

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