Quantum absorption spectroscopy system

Abstract

A quantum absorption spectroscopy system (1) is provided with an exciting light source (11), a quantum interferometer (21), and a spectroscope (31). The exciting light source (11) emits pump light. The quantum interferometer (21) causes quantum interference between a plurality of physical processes in which quantum entanglement pairs of signal photons and idler photons are generated as a result of spontaneous parametric down-conversion of the pump light, and is configured such that a specimen is arranged on a propagation pathway of the idler photons. The spectroscope (31) detects the signal photons from the quantum interferometer (21). The quantum interferometer (21) includes a signal mode fiber portion (SMF) which is optically coupled to at least a portion of a propagation pathway of the signal photons and the propagation pathway of the idler photons.

Description

[Technical Field]

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

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

[0003] In recent years, in quantum technology fields such as quantum metrology, quantum communication, and quantum computing, attempts have been made to realize new functions by utilizing "quantum entangled" photon pairs, in which two photons are quantum-mechanically correlated. Hereinafter, such photon pairs will be referred to as "quantum entangled photon pairs." [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-228091 [Patent Document 2] Special Publication No. 2005-527838 [Non-patent literature]

[0005] [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]

[0006] The present inventors have been conducting research into a "quantum absorption spectroscopy system," which is an absorption spectroscopy system that uses quantum entangled photon pairs. There is a demand for a technology to improve the measurement sensitivity of the quantum absorption spectroscopy system.

[0007] The present disclosure has been made to solve the above problems, and one of the objects of the present disclosure is to improve the measurement sensitivity of a quantum absorption spectroscopy system. [Means for solving the problem]

[0008] (1) A quantum absorption spectroscopy system according to an aspect of the present disclosure includes an excitation light source, a quantum interferometer, and a photodetector. The excitation light source emits pump light. The quantum interferometer is configured to cause quantum interference among a plurality of physical processes in which a quantum entangled photon pair of a signal photon and an idler photon is generated by spontaneous parametric down-conversion of the pump light, and a sample is disposed in a propagation path of the idler photon. The photodetector detects the signal photon from the quantum interferometer. The quantum interferometer includes a single-mode fiber section optically coupled to at least a portion of the propagation path of the signal photon and the propagation path of the idler photon.

[0009] (2) The quantum interferometer further includes a wavelength separation element that separates the quantum entangled photon pair into a signal photon and an idler photon according to wavelength. The single-mode fiber portion includes a first single-mode fiber that is optically coupled to the wavelength separation element and propagates light in the wavelength range of the idler photon. The first single-mode fiber is a tapered fiber.

[0010] (3) The tapered fiber has a non-tapered portion and a tapered waist portion that is thinner than the non-tapered portion, and the diameter of the tapered waist portion is within the wavelength range from the visible region to the far-infrared region.

[0011] (4) The tapered fiber is configured to be replaceable. (5) The quantum interferometer further includes a first mirror that reflects idler photons and a second mirror that reflects signal photons. The first single-mode fiber is optically coupled between the wavelength separation element and the first mirror, and the single-mode fiber section further includes a second single-mode fiber that is optically coupled between the wavelength separation element and the second mirror and propagates light in the wavelength range of the signal photons. At least one of the first and second single-mode fibers is a polarization-maintaining fiber.

[0012] (6) The quantum interferometer further includes a photon pair source that generates quantum entangled photon pairs. The single-mode fiber section further includes a broadband single-mode fiber that is optically coupled between the photon pair source and the wavelength separation element and that propagates light in all wavelength ranges of the pump light, the signal photons, and the idler photons.

[0013] (7) The quantum interferometer further includes a photon pair source that generates quantum entangled photon pairs, and a wavelength separation element that separates the quantum entangled photon pairs into signal photons and idler photons according to wavelength. The single-mode fiber section is optically coupled between the photon pair source and the wavelength separation element, and includes a broadband single-mode fiber that propagates light in all wavelength ranges of the pump light, the signal photons, and the idler photons.

[0014] (8) The broadband single-mode fiber is a photonic crystal fiber. (9) The excitation light source is a pulsed light source. The photodetector is a single-pixel photodetector. The single-mode fiber section includes a wavelength-dispersive single-mode fiber optically coupled to the single-pixel photodetector.

[0015] (10) The quantum interferometer is used in a high-gain region where the signal intensity of the single-pixel photodetector increases nonlinearly as the transmittance of the idler photons through the sample increases. The single-mode fiber section further includes an absorber that absorbs the idler photons.

[0016] (11) The quantum absorption spectroscopy system further includes a processor that executes calculations for analyzing the absorption spectroscopic characteristics of the sample. The quantum interferometer further includes a phase converter configured to be able to change the phase of one of the signal photons and the idler photons. The photodetector outputs a quantum interference signal corresponding to the number of signal photons detected when the phase of one of the photons is changed by the phase converter. The processor calculates the absorption spectroscopic characteristics of the sample by Fourier transforming the quantum interference signal.

[0017] (12) The processor calculates a Fourier spectrum by Fourier transform of the quantum interference signal when the sample is placed in the propagation path of the idler photons, and also calculates a reference Fourier spectrum by Fourier transform of the quantum interference signal when the sample is not placed in the propagation path of the idler photons, and calculates a complex transmittance spectrum of the sample based on the ratio of the Fourier spectrum to the reference Fourier spectrum.

[0018] (13) The processor calculates the absorption spectrum of the sample by squaring the absolute value of the complex transmittance spectrum of the sample.

[0019] (14) The quantum interferometer is configured to generate visible photons as signal photons, and the photodetector is a silicon-based photodetector. [Effects of the Invention]

[0020] According to the present disclosure, the measurement sensitivity of a quantum absorption spectroscopy system can be improved. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a first embodiment. [Figure 2] FIG. 1 is a conceptual diagram for explaining the principle of quantum absorption spectroscopy. [Figure 3] FIG. 1 is a diagram illustrating an example of the configuration of a wideband single-mode fiber. [Figure 4] FIG. 1 is a diagram illustrating the configuration of an infrared single-mode fiber. [Figure 5] FIG. 10 is a diagram showing an example of a simulation result regarding the electric field intensity distribution in the cross section of the tapered waist portion. [Figure 6] 10 is a diagram summarizing the relationship between the diameter of the tapered waist portion and the wavelength range of the idler light. [Figure 7] FIG. 1 is a diagram schematically illustrating an overall configuration of a quantum absorption spectroscopy system according to a first modification of the first embodiment. [Figure 8] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a second modification of the first embodiment. [Figure 9] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a second embodiment. [Figure 10] FIG. 10 shows the relationship between sample transmittance and signal intensity of a single-pixel photodetector in the low-gain region. [Figure 11] FIG. 10 shows the relationship between sample transmittance and signal intensity of a single-pixel photodetector in the high-gain region. [Figure 12] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a first modification of the second embodiment. [Figure 13] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a second modification of the second embodiment. [Figure 14]FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a third embodiment. [Figure 15] FIG. 12 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a first modification of the third embodiment. [Figure 16] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a second modification of the third embodiment. [Figure 17] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a third modification of the third embodiment. [Figure 18] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a fourth modification of the third embodiment. [Figure 19] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a fifth modification of the third embodiment. [Figure 20] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a fourth embodiment. [Figure 21] FIG. 10 is a diagram illustrating an example of the configuration of a QPM device. [Figure 22] FIG. 2 is a diagram illustrating a configuration example of an ATR unit. [Figure 23] FIG. 4 is a functional block diagram for explaining in more detail the arithmetic processing by the controller. [Figure 24] 10 is a flowchart showing a processing procedure of quantum absorption spectroscopy in the fourth embodiment. [Figure 25] FIG. 10 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a modification of the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0022] 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 very 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 very far-infrared range.

[0023] In the present disclosure and its embodiments, the nanometer order includes a range of 1 nm to 1000 nm (= 1 μm), the submicrometer order includes a range of 100 nm to 1 μm, the micrometer order includes a range of 1 μm to 1000 μm (= 1 mm), and the millimeter order includes a range of 1 mm to 100 mm (= 10 cm).

[0024] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and their description will not be repeated. In each embodiment, a configuration for measuring the absorption spectroscopic characteristics of a sample in the near-infrared region using a quantum absorption spectroscopy system according to the present disclosure will be described. However, the wavelength range measurable using the quantum absorption spectroscopy system according to the present disclosure is not limited to the near-infrared region. The quantum absorption spectroscopy system according to the present disclosure can also measure the absorption spectroscopic characteristics of a sample in the ultraviolet, visible, mid-infrared, far-infrared, or ultra-far-infrared region.

[0025] [Embodiment 1] <Overall system configuration> 1 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to Embodiment 1. The quantum absorption spectroscopy system 1 includes an excitation light source 11, a quantum interferometer 21, a spectrometer 31, and a controller 41.

[0026] The pumping light source 11 emits pump light for exciting the nonlinear optical crystal 50 (described later). In the figure, the pump light is indicated by Lp. Hereinafter, the propagation path of the pump light will also be referred to as the "pumping light path." In the first embodiment, the pumping light source 11 emits continuous wave (CW) laser light 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 pumping light source 11. As described later, a pulsed laser may also be used.

[0027] The quantum interferometer 21 is configured to cause quantum interference among multiple physical processes in which entangled photon pairs consisting of a signal photon and an idler photon are generated by irradiation with pump light. As will be explained in detail later in Figure 2, it is important to note that quantum interference differs from ordinary optical interference. In the figure, the propagation path of the signal photon is indicated by Ls, and the propagation path of the idler photon is indicated by Li. In the following, the propagation path of the signal photon may be abbreviated as the "signal optical path," and the propagation path of the idler photon may be abbreviated as the "idler optical path."

[0028] The quantum interferometer 21 is an optical system that applies a configuration similar to, for example, a Michelson interferometer. In this embodiment, the optical elements of the quantum interferometer 21 are optically coupled by optical fibers. The quantum interferometer 21 includes an optical fiber 201, a dichroic splitter 202, an optical fiber 203, a photon pair source 204, an optical fiber 205, a dichroic splitter 206, an optical fiber 207, a polarization controller 208, a mirror 209, an optical fiber 210, a polarization controller 211, a mirror 212, a sample holder 213, and an optical fiber 214.

[0029] The optical fiber 201 optically couples the pumping light source 11 and the dichroic splitter 202. The optical fiber 201 is a single mode fiber (SMF) that propagates light in the wavelength range of the pump light (visible light in this example) in a single mode.

[0030] The dichroic splitter 202 is disposed between the excitation light source 11 and the photon pair source 204. The dichroic splitter 202 reflects light in the wavelength range of the signal light while transmitting light outside the wavelength range (light in the wavelength ranges of the pump light and idler light). The dichroic splitter is also called a wavelength separation coupler. A half mirror may be used instead of the dichroic splitter 202. The same applies to the dichroic splitter 206.

[0031] The optical fiber 203 optically couples the dichroic splitter 202 and the photon pair source 204. The optical fiber 203 is a single-mode fiber that propagates light in the wavelength region of the pump light and light in the wavelength region of the signal light (both of which are visible light in this example) in a single mode.

[0032] The photon pair source 204 includes a nonlinear optical crystal 50, which generates quantum entangled photon pairs of signal light and idler light from pump light. More specifically, the nonlinear optical crystal 50 generates quantum entangled photon pairs by spontaneous parametric down-conversion (SPDC) of the pump light. The nonlinear optical crystal 50 is, for example, a lithium niobate (LiNbO3) crystal. In this case, the signal light is visible light, and the idler light is infrared light (near-infrared light or mid-infrared light).

[0033] However, there is no particular limitation on the type of nonlinear optical crystal 50. Other types of nonlinear optical crystals, such as silver gallium sulfide (AgGaS2) crystals, can also be used. The principle of quantum absorption spectroscopy using the nonlinear optical crystal 50 will be explained with reference to FIG. 2.

[0034] 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, the 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 other compounds.

[0035] The nonlinear optical crystal 50 is an example of a nonlinear optical element for generating quantum entangled photon pairs by SPDC. As a means for generating quantum entangled photon pairs, instead of the nonlinear optical crystal 50, four-wave mixing in a ring resonator, a nonlinear fiber, or an optical waveguide made of silicon (Si) and / or silicon nitride (SiN) (none of which are shown) may be used.

[0036] Optical fiber 205 optically couples photon pair source 204 and dichroic splitter 206. Optical fiber 203 is a broadband single-mode fiber that propagates all of the light in the wavelength range of the pump light, the signal light, and the idler light (visible light, near-infrared light, and mid-infrared light) in a single mode. Therefore, hereinafter, optical fiber 205 is also referred to as "broadband SMF 205." The configuration of broadband SMF 205 will be described with reference to FIG. 3.

[0037] Dichroic splitter 206 is disposed between photon pair source 204 and mirror 209, and between photon pair source 204 and mirror 212. Dichroic splitter 206 transmits light in the wavelength range of the pump light and light in the wavelength range of the signal light, while reflecting light outside the above wavelength ranges (light in the wavelength range of the idler light). Dichroic splitter 206 is an example of a "wavelength separation element" according to the present disclosure.

[0038] The optical fiber 207 optically couples the dichroic splitter 206 and the mirror 209. The optical fiber 207 is a single-mode fiber that propagates light in the wavelength region of the pump light and light in the wavelength region of the signal light (both of which are visible light) in a single mode. Therefore, hereinafter, the optical fiber 207 will also be referred to as a "visible SMF 207." The visible SMF 207 is an example of a "second single-mode fiber" according to the present disclosure.

[0039] The polarization controller 208 is optically coupled to the visible SMF 207. The polarization controller 208 is configured to convert the polarization state of the signal light propagating through the visible SMF 207 into an arbitrary polarization state by applying an external stress to the visible SMF 207. In the example shown in FIG. 1, the polarization controller 208 is paddle-shaped and has three paddles (circular portions). Each paddle functions as a pseudo-wave plate (such as a quarter-wave plate or a half-wave plate). The polarization state of the signal light can be changed by adjusting the angles of the three paddles. However, the polarization controller 208 may be a bulk type or an in-line type.

[0040] Mirror 209 reflects the pump light and the signal light. The pump light and the signal light reflected by mirror 209 propagate through visible SMF 207 and reach dichroic splitter 206. Then, the reflected pump light and the reflected signal light pass through dichroic splitter 206 again and return to photon pair source 204 (nonlinear optical crystal 50). Note that mirror 209 corresponds to the "second mirror" according to the present disclosure.

[0041] The optical fiber 210 optically couples the dichroic splitter 206 and the mirror 212. The optical fiber 210 is a single-mode fiber that propagates light in the wavelength range of the idler light (infrared light) in a single mode. Therefore, hereinafter, the optical fiber 210 is also referred to as an "infrared SMF 210."

[0042] In this embodiment, the infrared SMF 210 is a tapered fiber in which the diameter of a single-mode fiber is partially narrowed by heating and stretching it. The configuration of the infrared SMF 210 will be described with reference to FIG. 4. The infrared SMF 210 is an example of the "first single-mode fiber" according to the present disclosure.

[0043] The polarization controller 211 is optically coupled to the infrared SMF 210. Similar to the polarization controller 208, the polarization controller 211 is configured to convert the polarization state of the idler light propagating in the infrared SMF 210 into an arbitrary polarization state.

[0044] Mirror 212 reflects the idler light. The idler light reflected by mirror 212 propagates through infrared SMF 210 and reaches dichroic splitter 206. The reflected idler light is then reflected again by dichroic splitter 206 and returns to photon pair source 204. Note that mirror 212 corresponds to the "first mirror" according to the present disclosure. The "first mirror" and "second mirror" according to the present disclosure may be reflective Bragg gratings.

[0045] The sample holder 213 is disposed between the dichroic splitter 206 and the mirror 212. The sample holder 213 is configured to hold a sample (indicated by SP in the drawing).

[0046] Optical fiber 214 optically couples dichroic splitter 202 and spectrometer 31. Optical fiber 214 is a single-mode fiber that propagates light (visible light) in the wavelength range of the signal light in a single mode. Of the pump light, signal light, and idler light on the return path that are reflected by mirrors 209 and 212 and returned to photon pair source 204, the signal light is reflected by dichroic splitter 202, propagates through optical fiber 214, and reaches spectrometer 31. On the other hand, the pump light and idler light pass through dichroic splitter 202 and do not reach spectrometer 31.

[0047] The spectrometer 31 splits the signal light that has reached the spectrometer 31. The spectrometer 31 includes a dispersive optical element 301 and a multi-pixel photodetector 302. The dispersive optical element 301 is typically a diffraction grating or a prism, and disperses the signal light in different directions depending on its wavelength.

[0048] The multi-pixel photodetector 302 includes a plurality of pixels (light-receiving elements) arranged in an array. The multi-pixel photodetector 302 is a silicon-based photodetector and has optical properties that enable it to detect visible light. Specifically, the multi-pixel photodetector 302 is a CCD (Charged-Coupled Device) image sensor or a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor. The multi-pixel photodetector 302 detects signal light under the control of the controller 41 and outputs the detection signal to the controller 41. The intensity of the detection signal of the signal light is an intensity according to the number of signal photons detected by the multi-pixel photodetector 302 (more specifically, an intensity that is directly proportional to the number of signal photons detected).

[0049] The controller 41 includes a processor 401 such as a CPU (Central Processing Unit), a memory 402 such as a ROM (Read Only Memory) and a RAM (Random Access Memory), an input / output port 403, and a monitor 404. The controller 41 controls the components of the quantum absorption spectroscopy system 1 (excitation light source 11, spectrometer 31). The controller 41 also executes arithmetic processing to analyze the infrared absorption spectroscopic characteristics (infrared absorption spectrum, refractive index, etc.) of the sample based on the detection signal of the signal light from the multi-pixel photodetector 302. The results of the arithmetic processing by the controller 41 are displayed on the monitor 404, allowing the operator to confirm the infrared absorption spectroscopic characteristics of the sample.

[0050] 1 employs a quantum interferometer similar to a Michelson interferometer. However, the quantum absorption spectroscopy system according to the present disclosure may also include a quantum interferometer having a configuration similar to a Mach-Zehnder interferometer including multiple nonlinear optical elements.

[0051] In the first embodiment, the optical fiber 203, the broadband SMF 205, the visible SMF 207, the infrared SMF 210, and the optical fiber 214 correspond to the "single mode fiber section" (denoted by the reference symbol SMF) according to the present disclosure.

[0052] <Measurement principle> Figure 2 is a conceptual diagram for explaining the principle of quantum absorption spectroscopy. In Figure 1, it was explained that only one nonlinear optical crystal 50 is arranged in the excitation light path, and that the pump light passes through the nonlinear optical crystal 50 twice, on the outbound and return paths. In Figure 2, to make it easier to understand the measurement principle, an example configuration in which two nonlinear optical crystals are arranged in the excitation light path will be explained. The two nonlinear optical crystals will be referred to as a first crystal 51 and a second crystal 52.

[0053] When the first crystal 51 is irradiated with pump light from the excitation light source 11, SPDC in the first crystal 51 splits one photon with a relatively large energy into two photons with smaller energies while satisfying the law of conservation of energy. In the example shown in Fig. 2, a quantum entangled photon pair consisting of one visible photon (signal photon) and one infrared photon (idler photon) is generated from one visible photon (pump photon). Similarly, when the second crystal 52 is irradiated with pump light, a quantum entangled photon pair consisting of one visible photon and one infrared photon is generated. A spectrometer 31 is arranged in the propagation path of the visible photon.

[0054] Quantum interference occurs between an event in which a quantum entangled photon pair is generated by the first crystal 51 (hereinafter referred to as the "first physical process") and an event in which a quantum entangled photon pair is generated by the second crystal 52 (hereinafter referred to as the "second physical process"). More specifically, when the probability amplitude representing the first physical process and the 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 interfere with each other, whereas if the two probability amplitudes are in opposite phases, the first physical process and the second physical process cancel each other out (quantum interference effect). The following describes an example of interference in which the first physical process and the second physical process cancel each other out (destructive interference).

[0055] If the infrared absorber sample is not placed in the idler optical path, the first physical process and the second physical process cannot be distinguished, 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 downstream of the second crystal 52. In other words, the signal photon is not detected by the spectrometer 31.

[0056] On the other hand, if the sample is placed in the idler optical path, the idler photons are absorbed by the sample. This makes it possible to distinguish between the first and second physical processes, resulting in imperfect quantum interference between the first and second physical processes. As a result, the signal photons are detected by the spectrometer 31.

[0057] In this way, in quantum absorption spectroscopy, by detecting one signal photon (visible photon) of a quantum entangled photon pair using the spectrometer 31, it is possible to determine that the other idler photon (infrared photon) has been absorbed by the sample.

[0058] In the conceptual diagram shown in FIG. 2, an example has been described in which the quantum interferometer 21 causes quantum interference between two physical processes (first and second physical processes). However, the quantum interferometer in the present disclosure may cause quantum interference between three or more physical processes. In other words, the quantum interferometer may be configured to cause quantum interference between at least two physical processes. Furthermore, the quantum interferometer may be configured to cause constructive interference between the first physical process and the second physical process.

[0059] <Application of SMF to quantum absorption spectroscopy> Classical infrared absorption spectroscopy systems, such as commercially available FTIR systems, are equipped with a thermal light source (a heating element such as a SiC heater) to generate infrared light. Such thermal light sources have a finite size of a few millimeters to a few centimeters and emit light in all directions. Because a thermal light source emits light in multiple spatial modes with independent phases, it is theoretically impossible to efficiently couple a thermal light source into a single-mode fiber.

[0060] In the quantum absorption spectroscopy system 1 according to this embodiment, the photon pair source 204 generates quantum entangled photon pairs by spontaneous parametric down-conversion (SPDC). SPDC exhibits mode selectivity. That is, SPDC can generate quantum entangled photon pairs in a specific spatial mode (single spatial mode) that satisfies the phase-matching condition. Quantum entangled photon pairs existing in a single spatial mode can be coupled with high efficiency into a single-mode fiber. Thus, quantum absorption spectroscopy is more compatible with a device configuration using a single-mode fiber than classical absorption spectroscopy, which uses a finite-sized thermal light source. However, no examples of infrared quantum absorption spectroscopy measurements using a single-mode fiber have been reported to date.

[0061] This disclosure is based on the discovery by the inventors, who focused on the high compatibility between quantum absorption spectroscopy and single-mode fiber, that constructing the propagation path of quantum entangled photon pairs using single-mode fiber offers various advantages for the practical implementation of quantum absorption spectroscopy systems. First, the measurement sensitivity of quantum absorption spectroscopy is proportional to the clarity of quantum interference. By using a single-mode fiber, quantum interference due to quantum entangled light existing in a single spatial mode is selectively generated, thereby achieving high clarity of quantum interference. Therefore, the measurement sensitivity of the quantum absorption spectroscopy system 1 can be improved. Second, as described below, using a single-mode fiber makes it possible to make the quantum absorption spectroscopy system 1 more robust and more compact.

[0062] Here, we refer to a configuration in which all optical elements are arranged in free space without using single-mode fiber as a "free-space type." In a free-space type system configuration, optical path deviation can occur when the screws that secure the optical elements become loose due to physical vibration. Optical path deviation can also occur due to environmental temperature changes (thermal fluctuations). In addition, in a free-space type system configuration, the quantum absorption spectroscopy system, excluding the controller, becomes a certain size (such as the size of a rectangular parallelepiped with each side measuring several tens of centimeters).

[0063] In contrast, in the quantum interferometer 21 of the quantum absorption spectroscopy system 1, all optical elements are optically coupled by single-mode fiber. Therefore, even if the relative positions of the optical elements are slightly shifted, the occurrence of optical path deviation is suppressed. This increases resistance to physical vibrations and thermal fluctuations, improving the robustness of the quantum absorption spectroscopy system 1. Furthermore, single-mode fiber can be freely bent and wound. This saves space for installing the optical paths between the optical elements, resulting in a significantly smaller quantum absorption spectroscopy system 1 compared to a free-space system configuration. For example, it is possible to compact the quantum absorption spectroscopy system 1 to a size suitable for portability. This allows for measurements at the site where the sample is collected (so-called on-site measurement).

[0064] Furthermore, while a classical infrared absorption spectroscopy system uses a thermal light source in the infrared range and a photodetector in the infrared range, the quantum absorption spectroscopy system 1 uses an excitation light source 11 in the visible range and a multi-pixel photodetector 302 in the visible range. Using a light source in the visible range makes it easier to deal with heat. Furthermore, using a photodetector that is highly sensitive in the visible range eliminates the need for liquid nitrogen cooling to reduce thermal noise. This allows the quantum absorption spectroscopy system 1 to be further miniaturized.

[0065] Generally, the intensity of the irradiated light in a classical infrared absorption spectroscopy system is on the order of milliwatts. Therefore, depending on the sample, changes in the sample's properties (including damage to the sample) may occur due to heating by the irradiated light. In contrast, the idler light propagating through the infrared SMF 210 in the quantum absorption spectroscopy system 1 is very weak, for example on the order of fW. Therefore, heating of the sample during measurement is negligibly small, and changes in the sample's properties can be suppressed.

[0066] <Wideband SMF> FIG. 3 shows an example of the configuration of the wideband SMF 205. The wideband SMF 205 is preferably a photonic crystal fiber (PCF). The wideband SMF 205 is, for example, a refractive index-guided photonic crystal fiber and includes a core 205A and a cladding 205B. A plurality of air holes 205C are regularly arranged in the cladding 205B. By adjusting the size of each of the plurality of air holes 205C and the spacing between the plurality of air holes 205C, single-mode optical transmission over a wide bandwidth is possible. In this embodiment, the wideband SMF 205 is configured to propagate light in all wavelength ranges of the pump light, signal light, and idler light in a single spatial mode (single mode).

[0067] In a refractive index-guided photonic crystal fiber, light is transmitted by repeatedly undergoing total reflection at the boundary between the core and the cladding, just like in a typical optical fiber. The broadband SMF 205 is not limited to a refractive index-guided type, and may be a photonic bandgap photonic crystal fiber that confines and transmits light by Bragg reflection.

[0068] <Tapered fiber> In a classical infrared absorption spectroscopy system, a sample is irradiated with infrared light of a finite size. In this case, the beam shape of the irradiated light may be disturbed (the phase wavefront of the irradiated light may be distorted) due to the condition of the sample surface, such as microscopic irregularities present on the sample surface. This may reduce the signal intensity detected by the photodetector and decrease the measurement sensitivity of the infrared absorption spectroscopy system. Therefore, a tapered fiber is used for the infrared SMF 210 in the quantum absorption spectroscopy system 1.

[0069] 4 is a diagram illustrating the configuration of infrared SMF 210. Infrared SMF 210 has non-tapered portion 210A, tapered portion 210B, tapered waist portion 210C, tapered portion 210D, and non-tapered portion 210E. Tapered portion 210A and tapered portion 210B are disposed at one end of tapered waist portion 210C. Tapered portion 210D and non-tapered portion 210E are disposed at the other end of tapered waist portion 210C. Length L of tapered waist portion 210C is preferably in the range from the order of micrometers to the order of millimeters.

[0070] Tapered waist portion 210C is thinner than non-tapered portions 210A and 210E. If the diameter of tapered portion 210B, which is the connecting region between non-tapered portion 210A and tapered waist portion 210C, changes abruptly, loss in tapered portion 210B will be large. Therefore, by making the diameter change of tapered portion 210B gradual (also expressed as "making it adiabatic"), non-tapered portion 210A and tapered waist portion 210C are smoothly connected. The same applies to the connection between non-tapered portion 210E and tapered waist portion 210C.

[0071] The diameter φ of the tapered waist portion 210C is typically on the order of micrometers (preferably 1 μm or more and less than 20 μm), but depending on the wavelength of the idler light, it can also be on the order of submicrometers (e.g., 300 nm or more and less than 1 μm). That is, the diameter φ is mostly within the wavelength range from the visible to the far-infrared range. When the diameter φ is on the order of nanometers, the infrared SMF 210 can also be called a "nano-optical fiber." Incidentally, the diameter of a typical optical fiber used for communications is approximately 125 μm, and its mode field diameter is approximately 10 μm.

[0072] In the tapered waist portion 210C, the idler light propagates through the entire tapered waist portion 210C as the core and the outside of the tapered waist portion 210C (usually a gas such as air, but it can also be a liquid or vacuum) as the cladding. This forms a seepage region called an evanescent field (near field) near the surface of the tapered waist portion 210C. Figure 4 also shows the simulation results of the electric field intensity distribution of the evanescent field. The tapered waist portion 210C and the sample holder 213 (see Figure 1) are positioned sufficiently close to each other so that the evanescent field and the sample interact with each other.

[0073] 4, a non-tapered portion and a tapered portion exist at both ends of the tapered waist portion 210C. However, the non-tapered portion and the tapered portion may exist only at one end of the tapered waist portion 210C. In this case, a mirror structure (such as a Bragg grating) instead of the mirror 212 may be formed at the other end of the tapered waist portion 210C.

[0074] In the quantum absorption spectroscopy system 1 that employs a tapered fiber, instead of directly irradiating the sample with idler light, the interaction between the evanescent field of the idler light and the sample is measured. This suppresses the decrease in signal intensity caused by the state of the sample surface, thereby further improving the measurement sensitivity of the quantum absorption spectroscopy system 1. In addition, it is possible to measure the infrared absorption spectroscopic characteristics of extremely small samples that are too small to be measured using classical infrared absorption spectroscopy systems.

[0075] In addition, by using a tapered fiber, the idler light can be concentrated in a minute region of approximately the wavelength of visible light or near-infrared light. As will be described in detail in FIG. 5, when compared under the condition that the total number of photons emitted from the light source is the same, the photon number density of the idler light (flux of idler light per unit cross-sectional area) in the tapered waist portion 210C is significantly higher than the photon number density of the irradiated light in a classical infrared absorption spectroscopy system. As a result, the measurement sensitivity of the quantum absorption spectroscopy system 1 can be further improved.

[0076] It is also possible to irradiate a sample with idler light focused to approximately the wavelength using a high-magnification objective lens. However, even in this case, the idler light and the sample can only interact within a range of a few micrometers in the direction of irradiation of the idler light. In contrast, when a tapered fiber is used, the photon number density is enhanced throughout the entire length of the tapered waist portion 210C. The length L of the tapered waist portion 210C can be on the order of millimeters (1 mm or more). Therefore, the evanescent field of the idler light can interact with the sample over a range of 1 mm or more. In other words, the interaction length between the idler light and the sample can be approximately 1,000 times longer than when a high-magnification lens is used.

[0077] 5 is a diagram showing an example of a simulation result regarding the electric field intensity distribution in the cross section of the tapered waist portion 210C. In this simulation, the finite-difference time-domain method (FDTD) by Lumerical's FDTD Solutions was used.

[0078] Figure 5 shows the results of a simulation of how the idler light seeps out of the tapered waist portion 210C for various combinations of the diameter φ of the tapered waist portion 210C and the wavelength of the idler light. The diameter φ of the tapered waist portion 210C was set to 1 μm, 2 μm, 4 μm, or 5 μm. When the diameter φ is 1 μm or 2 μm, the material of the tapered waist portion 210C is infrared glass, specifically chalcogenide glass (As2S3), which is a material of optical fibers commercially available for infrared light propagation. When the diameter φ is 4 μm or 5 μm, the material of the tapered waist portion 210C is Ge. 33 As 12 Se 55 The material was glass (refractive index: 2.4825), and the light intensity of the idler light was approximately 18 pW.

[0079] The wavelength of the idler light for each diameter φ was set to a value at which the idler light propagated in single mode at that diameter φ. Specifically, the wavelength range in which the idler light could propagate in single mode was 3.0 μm to 5.0 μm when the diameter φ was 1 μm, 6.0 μm to 9.1 μm when the diameter φ was 2 μm, 12.3 μm to 16.5 μm when the diameter φ was 4 μm, and 15.3 μm to 19.8 μm when the diameter φ was 5 μm. The results shown in Figure 5 relate to the lower and upper limits of the above wavelength range (i.e., the shortest and longest wavelengths at which the idler light propagates in single mode).

[0080] A typical example will be described where the diameter φ of the tapered waist portion 210C is 1 μm. When the wavelength of the idler light is 3 μm, the electric field is concentrated at the center and near the surface of the tapered waist portion 210C. The magnitude of the electric field at the surface of the tapered waist portion 210C is 110.0 [V / m]. This value is obtained when the light intensity is the same (18 pW) and the beam diameter is 1 mm. 2 This is approximately 700 times larger than when the idler light propagates in free space without using an optical fiber. This corresponds to an increase in the photon number density of the idler light at the surface of the tapered waist portion 210C by approximately 500,000 times. This enables the measurement sensitivity of the infrared absorption spectral characteristics of the sample to be improved by approximately 500,000 times.

[0081] When the wavelength of the idler light was 5 μm, most of the idler light propagated while leaking out of the tapered waist portion 210C. In this case, the magnitude of the electric field at the surface of the tapered waist portion 210C was 21.7 V / m, which was smaller than when the wavelength of the idler light was 3 μm. However, this relationship reversed when the distance from the surface of the tapered waist portion 210C was approximately 1 μm. For example, the magnitude of the electric field at a position 3 μm away from the surface of the tapered waist portion 210C was 13.8 V / m. This value corresponds to an approximately 7,500-fold increase in photon number density in the same comparison as above. This also improves the measurement sensitivity of the infrared absorption spectroscopic characteristics of the sample by approximately 7,500 times.

[0082] When the diameter φ of the tapered waist portion 210C is 2 μm, 4 μm, or 5 μm, the electric field is concentrated near the center and surface of the tapered waist portion 210C, just as when the diameter φ is 1 μm, when the wavelength of the idler light is short. On the other hand, when the wavelength of the idler light is long, the electric field significantly leaks out of the tapered waist portion 210C, and most of the idler light propagates outside the tapered waist portion 210C. Although a detailed description will not be repeated, the number of times the photon number density of the idler photons is increased compared to propagation in free space (and as a result, the number of times the measurement sensitivity of the infrared absorption spectral characteristics of the sample can be improved) can be similarly estimated from the magnitude of the electric field shown in FIG. 5.

[0083] Thus, in the wavelength range where single-mode propagation is possible, the infrared absorption spectroscopic characteristics of a sample can be measured with higher sensitivity when the idler beam has a short wavelength than when the idler beam has a long wavelength. However, when the idler beam has a short wavelength, the electric field is concentrated near the surface of the tapered waist portion 210C, so the sample must be brought closer to the tapered waist portion 210C. This increases the risk of the sample coming into contact with the tapered waist portion 210C and damaging or contaminating the tapered waist portion 210C. In contrast, when the idler beam has a long wavelength, it is possible to avoid damage or contamination of the tapered waist portion 210C due to contact with the sample, while achieving significantly higher sensitivity than propagation in free space.

[0084] Fig. 6 is a diagram summarizing the relationship between the diameter φ of the tapered waist portion 210C and the wavelength range of the idler light. As shown in Fig. 6, by setting the diameter φ of the tapered waist portion 210C in the range from 0.5 μm to 5 μm, the idler light can be propagated in a single mode over the entire wavelength range from approximately 1 μm to approximately 20 μm. In other words, an infrared SMF having an appropriate diameter φ depending on the wavelength of the idler light can be 210 By selecting the idler light, the infrared SMF 210 It is possible to make the light propagate in a single mode.

[0085] For example, if the wavelength of the idler light is 5 μm, it can propagate in single mode whether the diameter φ is 1 μm or 1.5 μm. In this way, in the range where wavelengths that can propagate in single mode overlap, infrared SMF with a large diameter φ 210 This makes it difficult for the tapered waist portion 210C to be damaged, so it is recommended to select 210 The durability of the product can be improved.

[0086] In addition, when the wavelength of the idler light is 2 μm or less, silica glass is used instead of infrared glass for the infrared SMF. 210 The refractive index of silica-based glass is lower than that of infrared glass. Therefore, compared to infrared glass, silica-based glass allows single-mode propagation even if the diameter φ of the tapered waist portion 210C is made larger. For example, if the wavelength of the idler light is 2 μm, infrared glass requires the diameter φ to be 0.5 μm, whereas silica-based glass allows the diameter φ to be 1 μm. This allows for the infrared SMF 210 Furthermore, since silica glass is cheaper than infrared glass, the cost of materials can be reduced.

[0087] Light propagating through a single-mode fiber such as the infrared SMF 210 is not limited to single-mode light. Light of two or more (specifically, two to four) spatial modes may propagate through these single-mode fibers. In other words, the "single-mode fiber section" according to the present disclosure may include a few-mode fiber (FEF).

[0088] When the diameter φ of the tapered waist portion 210C is 1 μm and the wavelength of the idler light is approximately 2.5 μm, the infrared SMF 210 functions as a few-mode fiber. The wavelength of the idler light for which the infrared SMF 210 functions as a few-mode fiber is approximately 5 μm when the diameter φ is 2 μm, approximately 11 μm when the diameter φ is 4 μm, and approximately 13 μm when the diameter φ is 5 μm.

[0089] By using the infrared SMF210 as both a single-mode fiber and a few-mode fiber, the measurable wavelength range can be broadened, although the clarity of quantum interference may be reduced compared to when the infrared SMF210 is used only as a single-mode fiber. By shortening the wavelength of the idler light even further compared to when the infrared SMF210 is used as a few-mode fiber, the bandwidth of the infrared SMF210 can be further broadened. In this case, although the clarity of quantum interference may be further reduced, the photon number density increases, which is expected to improve measurement sensitivity compared to when the idler light propagates in free space.

[0090] Furthermore, for example, when the wavelength of the idler light is 5 μm, the diameter φ is required to be 1 μm or 1.5 μm in order to propagate the idler light in a single mode, whereas if the idler light is propagated in a few modes, the diameter φ can be set to 2 μm. In this way, by utilizing the function as a few-mode fiber, the diameter φ of the tapered waist portion 210C can be made thicker, and thus the infrared SMF 210 The durability of the product can be improved.

[0091] [First Modification of First Embodiment] 7 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a first modification of the first embodiment. Quantum absorption spectroscopy system 1A differs from quantum absorption spectroscopy system 1 according to the first embodiment (see FIG. 1) in that quantum absorption spectroscopy system 1A includes quantum interferometer 21A instead of quantum interferometer 21. Quantum interferometer 21A differs from quantum interferometer 21 in that it includes optical fibers 215 and 216 instead of visible SMF 207 and infrared SMF 210, and in that it does not include polarization controllers 208 and 211.

[0092] While the visible SMF 207 and the infrared SMF 210 are typical single-mode fibers (SMFs), the optical fibers 215 and 216 are polarization-maintaining fibers (PMFs). The optical fiber 215 maintains the polarization of light (visible light) in the wavelength region of the signal photons. On the other hand, the optical fiber 216 maintains the polarization of light (infrared light) in the wavelength region of the idler photons. Therefore, in the first modification, the polarization controllers 208 and 211 for correcting the polarization disturbance of photons propagating through the single-mode fibers can be omitted. Note that only one of the visible SMF 207 and the infrared SMF 210 may be replaced with a polarization-maintaining fiber.

[0093] [Modification 2 of Embodiment 1] 8 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a second modification of the first embodiment. Quantum absorption spectroscopy system 1B differs from quantum absorption spectroscopy system 1 according to the first embodiment (see FIG. 1) in that quantum absorption spectroscopy system 1B includes quantum interferometer 21B instead of quantum interferometer 21. Quantum interferometer 21B differs from quantum interferometer 21 in that it does not include optical fibers 201 and 203 and broadband SMF 205. As shown in FIG. 8, a configuration may be employed in which some optical elements included in quantum interferometers 21 and 21A are optically coupled by a single-mode fiber, while the remaining optical elements are not coupled to the single-mode fiber (a configuration in which light propagates in free space).

[0094] In the quantum absorption spectroscopy systems 1A and 1B as well, the propagation path of the quantum entangled photon pairs is configured using a single-mode fiber. Therefore, the first and second modifications of the first embodiment also improve the measurement sensitivity of the quantum absorption spectroscopy systems 1A and 1B, and also improve the robustness of the quantum absorption spectroscopy systems 1A and 1B. Furthermore, in the first and second modifications of the first embodiment as well, the idler light is focused using a tapered fiber. Therefore, the measurement sensitivity of the quantum absorption spectroscopy systems 1A and 1B can be further improved.

[0095] The single mode fibers in the quantum interferometers 21, 21A, and 21B are optically coupled to the single mode fibers. target It is desirable to modularize the idler optical path together with the optical elements coupled to it. In particular, it is desirable to modularize the idler optical path, which employs the infrared SMF 210, a tapered fiber. This makes it possible to select and measure an appropriate infrared SMF 210 for the application from a variety of infrared SMFs 210 with different diameters φ of the tapered waist portion 210C. Furthermore, if the tapered waist portion 210C is damaged during sample installation, for example, the infrared SMF 210 can be easily replaced with a new one.

[0096] Instead of modularizing the entire infrared SMF 210, the tapered fiber region (the region including the tapered portions 210B and 210D and the tapered waist portion 210C) may be modularized partially. Also, instead of the tapered fiber region, a ridge-type waveguide or hollow-beam type optical waveguide having a width on the order of submicrometers to micrometers may be used.

[0097] [Embodiment 2] In the first embodiment, a configuration has been described in which the signal light is split by the spectrometer 31. In the second embodiment, a configuration in which the signal light is split into wavelengths without using the spectrometer 31 will be described.

[0098] <Overall system configuration> 9 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to embodiment 2. Quantum absorption spectroscopy system 2 differs from quantum absorption spectroscopy system 1 according to embodiment 1 (see FIG. 1 ) in that it includes an excitation light source 12 instead of excitation light source 11, a quantum interferometer 22 instead of quantum interferometer 21, and a single-pixel photodetector 32 instead of spectrometer 31. Quantum interferometer 22 differs from quantum interferometer 21 in that it includes optical fiber 217 instead of optical fiber 214.

[0099] While the pumping light source 11 in the first embodiment emits continuous wave laser light, the pumping light source 12 emits pulsed laser light, which makes it possible to generate quantum entangled photon pairs with high efficiency (see the high gain region in FIG. 11).

[0100] The optical fiber 217 optically couples the dichroic splitter 202 and the single-pixel photodetector 32. The optical fiber 217 is a single-mode fiber that propagates light (visible light) in the wavelength range of the signal light in a single mode. In the second embodiment, the optical fiber 217 is a wavelength dispersion fiber in which the propagation time of the signal light varies depending on the wavelength. When the signal light propagates through the optical fiber 217, the short-wavelength component of the signal light arrives at the single-pixel photodetector 32 relatively slowly, while the long-wavelength component of the signal light arrives at the single-pixel photodetector 32 relatively quickly.

[0101] The single-pixel photodetector 32 is a silicon-based photodetector including a single pixel and has optical properties capable of detecting visible light. Specifically, the single-pixel photodetector 32 is a photodiode such as a PIN photodiode or an APD (Avalanche Photodiode). Alternatively, a phototube or a photomultiplier tube may be used instead of a photodiode. Also, a superconducting single photon detector (SSPD) may be used to obtain a high signal-to-noise ratio. The single-pixel photodetector 32 detects signal light under the control of the controller 41 and outputs the detection signal to the controller 41.

[0102] The controller 41 measures the wavelength of the signal photon based on the detection time of the signal photon. More specifically, in the second embodiment, the excitation light source 12, which is a pulsed light source, is employed. Therefore, multiple quantum entangled photon pairs are generated within a very short time interval in response to the irradiation of the pump light, which is a pulsed wave, and as a result, multiple signal photons arrive at the optical fiber 217 approximately simultaneously. As these signal photons propagate through the optical fiber 217, which is a wavelength dispersion fiber, a delay corresponding to the wavelength of the signal photon occurs. Therefore, the time difference between the irradiation time of the pump light and the arrival time of the signal photon at the single-pixel photodetector 32 contains information about the wavelength of the signal photon. Therefore, by recording this time difference, the controller 41 can measure the wavelength of the signal photon. Other configurations of the quantum absorption spectroscopy system 2 are similar to the corresponding configurations of the quantum absorption spectroscopy system 1 according to the first embodiment, and therefore, description thereof will not be repeated.

[0103] As described above, in the second embodiment, as in the first embodiment, the optical elements of the quantum interferometer 22 are optically coupled by a single-mode fiber. This improves the measurement sensitivity of the quantum absorption spectroscopy system 2 and also improves the robustness of the quantum absorption spectroscopy system 2. Also in the second embodiment, the idler light is focused using the infrared SMF 210, which is a tapered fiber. This further improves the measurement sensitivity of the quantum absorption spectroscopy system 2.

[0104] Furthermore, in the second embodiment, the signal light is dispersed using the optical fiber 217, which is a wavelength dispersion fiber, so that the spectrometer 31 can be replaced with the single-pixel photodetector 32. Therefore, according to the second embodiment, it is possible to further reduce the size of the quantum absorption spectroscopy system 2 and reduce the cost of components.

[0105] <Gain Dependence> Further study results regarding the measurement sensitivity of the quantum absorption spectroscopy system 2 will be described. The generation rate of quantum entangled photon pairs is determined by a combination of the intensity of the pump light and the conversion efficiency of the element for generating quantum entangled photon pairs (e.g., the nonlinear optical crystal 50). Compared to the base rate defined by the generation bandwidth of quantum entangled photon pairs, the region where quantum entangled photon pairs are generated at a sufficiently high rate is called the "high-gain region," and the region where quantum entangled photon pairs are generated at a low rate is called the "low-gain region."

[0106] As a specific example, when photons generated in the infrared region among quantum entangled photon pairs have a generation bandwidth of 300 nm centered on a wavelength of 4500 nm, the reference rate is a rate at which one quantum entangled photon pair is generated approximately every 100 fs. In the low-gain region, where the generation rate of quantum entangled photon pairs is lower than this reference rate, if the nonlinear optical crystal 50 is excited by a continuous wave, the intensity of the pump light can be 1 μW or less. On the other hand, in the high-gain region, for example, multiple pairs (several pairs to approximately 100 pairs) of quantum entangled photon pairs are generated every 100 fs. When the nonlinear optical crystal 50 is excited by a continuous wave, the intensity of the pump light needs to be approximately 1 mW. However, when the nonlinear optical crystal 50 is excited by a pulsed wave, the pump light localized in the time domain has a momentary high optical intensity, so the generation rate of quantum entangled photon pairs may reach the high-gain region at a lower average intensity. As will be explained below, the intensity of the detected signal from the single-pixel photodetector 32 tends to differ between the high-gain region and the low-gain region.

[0107] FIG. 10 is a diagram showing the relationship between the sample transmittance in the low-gain region and the signal intensity of the single-pixel photodetector 32. FIG. 11 is a diagram showing the relationship between the sample transmittance in the high-gain region and the signal intensity of the single-pixel photodetector 32. In FIGS. 10 and 11, the horizontal axis represents the sample transmittance (the transmittance of idler photons through the sample). The vertical axis represents the signal intensity (the intensity of the detected signal from the single-pixel photodetector 32). This signal intensity can also be considered an index showing the clarity of quantum interference.

[0108] In the low-gain region, the signal intensity increases linearly as the sample transmittance increases. In contrast, in the high-gain region, the signal intensity increases nonlinearly as the sample transmittance increases. In the high-gain region (i.e., the region where the sample only slightly absorbs idler photons), the signal intensity barely increases as the sample transmittance increases. In the range where the sample transmittance is close to 100%, the slope (rate of change) of the signal intensity is close to zero. This means that, although the measurement sensitivity of the quantum absorption spectroscopy system 2 is expected to improve in the high-gain region where quantum entangled photon pairs are generated with high efficiency, the slight absorption of idler photons by the sample cannot be detected with high accuracy.

[0109] Therefore, when using the high-gain region, it is desirable to install an absorber 213A of the idler light, for example, in the sample holder 213, as shown in FIG. 9. The absorber 213A is preferably a material with a known absorption spectrum in the infrared region or a material with a flat absorption spectrum in the infrared region, such as an infrared absorption filter. The sample transmittance is intentionally reduced by having the absorber 213A absorb the idler light. In the example shown in FIG. 11, the sample transmittance can be reduced to approximately 40% to 60%. This increases the amount of change in signal intensity associated with changes in sample transmittance compared to when the absorber 213A is not installed. As a result, the measurement sensitivity of the quantum absorption spectroscopy system 2 can be improved.

[0110] [Modification 1 of Embodiment 2] 12 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a first modification of the second embodiment. Quantum absorption spectroscopy system 2A differs from quantum absorption spectroscopy system 2 according to the second embodiment (see FIG. 9) in that quantum absorption spectroscopy system 2A includes quantum interferometer 22A instead of quantum interferometer 22. Quantum interferometer 22A differs from quantum interferometer 22 in that it includes optical fibers 215 and 216 instead of visible SMF 207 and infrared SMF 210, and in that it does not include polarization controllers 208 and 211. Optical fibers 215 and 216 are polarization-maintaining fibers. The polarization-maintaining fibers have been described in detail in the first modification of the first embodiment (see FIG. 7), and therefore description thereof will not be repeated.

[0111] [Modification 2 of Embodiment 2] 13 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to Modification 2 of Embodiment 2. Quantum absorption spectroscopy system 2B differs from quantum absorption spectroscopy system 2 according to Embodiment 2 (see FIG. 9) in that quantum interferometer 22B is provided instead of quantum interferometer 22. Quantum interferometer 22B differs from quantum interferometer 22 in that it does not include optical fibers 201 and 203 and broadband SMF 205.

[0112] In the quantum absorption spectroscopy systems 2A and 2B as well, the propagation path of the quantum entangled photon pairs is configured using a single-mode fiber. Therefore, according to the first and second modifications of the second embodiment as well, it is possible to improve the measurement sensitivity of the quantum absorption spectroscopy systems 2A and 2B and also improve the robustness of the quantum absorption spectroscopy systems 2A and 2B. Furthermore, in the first and second modifications of the second embodiment as well, the idler light is focused using a tapered fiber. Therefore, it is possible to further improve the measurement sensitivity of the quantum absorption spectroscopy systems 2A and 2B.

[0113] [Embodiment 3] In the third embodiment, a configuration for measuring the infrared absorption spectral characteristics of a sample without using a tapered fiber will be described.

[0114] 14 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to embodiment 3. Quantum absorption spectroscopy system 3 differs from quantum absorption spectroscopy system 1 according to embodiment 1 (see FIG. 1) in that quantum absorption spectroscopy system 3 includes quantum interferometer 23 instead of quantum interferometer 21. Quantum interferometer 23 differs from quantum interferometer 21 in that quantum interferometer 23 includes infrared SMF 218, which does not include tapered waist portion 210C (see FIG. 4), instead of infrared SMF 210.

[0115] In the third embodiment, the sample is placed near the end of infrared SMF 218. Idler light emitted from the end of infrared SMF 218 into free space passes through the sample and is reflected by mirror 212. The idler light reflected by mirror 212 passes through the sample again and returns to infrared SMF 218 from the end of infrared SMF 218. Other configurations of quantum absorption spectroscopy system 3 are similar to the corresponding configurations of quantum absorption spectroscopy system 1 according to the first embodiment, and therefore description thereof will not be repeated.

[0116] As described above, in the third embodiment, similarly to the first and second embodiments, the optical elements of the quantum interferometer 23 are optically coupled by a single-mode fiber. This improves the measurement sensitivity of the quantum absorption spectroscopy system 3, and also improves the robustness of the quantum absorption spectroscopy system 3.

[0117] The improvement in measurement sensitivity achieved by using a single-mode fiber is due to the fact that the spatial mode of the light used in the measurement is uniquely selected. In quantum absorption spectroscopy systems that do not perform mode selection using a single-mode fiber (such as the measurement system used in Non-Patent Document 1), all spatial modes of the quantum entangled photon pairs generated from the nonlinear optical crystal contribute to the quantum interference signal, resulting in superposition of quantum interference signals with different phases. As a result, the clarity of the quantum interference decreases, and the measurement sensitivity decreases. In contrast, according to the present embodiment, which performs mode selection using a single-mode fiber, higher clarity of the quantum interference can be expected, and therefore improved measurement sensitivity can be expected.

[0118] [Modification 1 of the Third Embodiment] 15 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a first modification of the third embodiment. Quantum absorption spectroscopy system 3A differs from quantum absorption spectroscopy system 3 according to the third embodiment (see FIG. 14) in that quantum absorption spectroscopy system 3A includes quantum interferometer 23A instead of quantum interferometer 23. Quantum interferometer 23A differs from quantum interferometer 23 in that it includes optical fibers 215 and 219 instead of visible SMF 207 and infrared SMF 218, and in that it does not include polarization controllers 208 and 211. Optical fibers 215 and 219 are polarization-maintaining fibers. The polarization-maintaining fibers have been described in detail in the first modification of the first embodiment (see FIG. 7), and therefore description thereof will not be repeated.

[0119] [Modification 2 of Embodiment 3] 16 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to Modification 2 of Embodiment 3. Quantum absorption spectroscopy system 3B differs from quantum absorption spectroscopy system 3 according to Embodiment 3 (see FIG. 14) in that quantum interferometer 23B is provided instead of quantum interferometer 23. Quantum interferometer 23B differs from quantum interferometer 23 in that it does not include infrared SMF 218 and polarization controller 211.

[0120] The quantum absorption spectroscopy system 3B also includes an optical fiber 203, a broadband SMF 205, and a visible SMF 207. As a result, quantum entangled photon pairs existing in a single spatial mode are coupled to the single-mode fiber. Furthermore, the pumping optical path, the propagation path of the quantum entangled photon pairs before wavelength separation, and the signal optical path are stabilized. Therefore, the second modification of the third embodiment also improves the measurement sensitivity of the quantum absorption spectroscopy system 3B and the robustness of the quantum absorption spectroscopy system 3B.

[0121] [Modification 3 of Embodiment 3] 17 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a third modification of the third embodiment. The quantum absorption spectroscopy system 3C differs from the quantum absorption spectroscopy system 3 according to the third embodiment (see FIG. 14) in that it includes a quantum interferometer 23C instead of the quantum interferometer 23. The quantum interferometer 23C differs from the quantum interferometer 23 in that it does not include the visible SMF 207, the infrared SMF 218, and the polarization controllers 208 and 211.

[0122] The quantum absorption spectroscopy system 3C also includes an optical fiber 203 and a broadband SMF 205. This allows quantum entangled photon pairs existing in a single spatial mode to be coupled to the single-mode fiber. Furthermore, the excitation light path and the propagation path of the broadband quantum entangled photon pairs before separation are stabilized. Therefore, according to the third modification of the third embodiment, it is possible to improve the measurement sensitivity of the quantum absorption spectroscopy system 3C and also improve the robustness of the quantum absorption spectroscopy system 3C.

[0123] [Fourth Modification of the Third Embodiment] 18 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a fourth modification of the third embodiment. The quantum absorption spectroscopy system 3D differs from the quantum absorption spectroscopy system 3 according to the third embodiment (see FIG. 14) in that it includes a quantum interferometer 23D instead of the quantum interferometer 23. The quantum interferometer 23D differs from the quantum interferometer 23 in that it does not include optical fibers 201 and 203 and a broadband SMF 205.

[0124] The quantum absorption spectroscopy system 3D also includes a visible SMF 207 and an infrared SMF 218. This allows quantum entangled photon pairs existing in a single spatial mode to be coupled into a single-mode fiber, enabling highly clear quantum interference to be observed. Furthermore, the signal and idler optical paths are stabilized. Therefore, the fourth modification of the third embodiment also improves the measurement sensitivity of the quantum absorption spectroscopy system 3D and the robustness of the quantum absorption spectroscopy system 3D.

[0125] In the quantum absorption spectroscopy system 3D, the signal and idler optical paths are made of single-mode fibers to improve measurement sensitivity and robustness, while the adjustment elements for the excitation optical path (such as the dichroic splitter 202) are placed in free space, which allows for adjustment of the pump light, which directly affects the generation rate and spatial mode of quantum entangled photon pairs.

[0126] [Fifth Modification of the Third Embodiment] 19 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a fifth modification of the third embodiment. The quantum absorption spectroscopy system 3E differs from the quantum absorption spectroscopy system 3 according to the third embodiment (see FIG. 14) in that it includes a quantum interferometer 23E instead of the quantum interferometer 23. The quantum interferometer 23E differs from the quantum interferometer 23 in that it does not include the broadband SMF 205, the visible SMF 207, or the infrared SMF 218.

[0127] The quantum absorption spectroscopy system 3E also includes optical fibers 201 and 203. This stabilizes the excitation light path. This also stabilizes the propagation paths (signal light path and idler light path) of quantum entangled photon pairs generated by irradiation with pump light. Furthermore, quantum entangled photon pairs existing in a single spatial mode are coupled to a single-mode fiber (optical fiber 203). This allows only quantum interference in a specific spatial mode to be selectively observed, thereby improving the clarity of quantum interference. Therefore, according to the fifth modification of the third embodiment, it is possible to improve the measurement sensitivity of the quantum absorption spectroscopy system 3E and also improve the robustness of the quantum absorption spectroscopy system 3E.

[0128] Furthermore, in quantum absorption spectroscopy system 3E, excitation light source 11 and photon pair source 204 are coupled by optical fibers 201 and 203, so that it is possible to modularize a part of the system for generating quantum entangled photon pairs. For example, optical fiber 201, dichroic splitter 202, and optical fiber 203 may be modularized, and in addition, photon pair source 204 may also be modularized.

[0129] [Embodiment 4] In the fourth embodiment, a configuration for measuring the infrared spectral characteristics of a sample over a wider band is described, based on the system configuration of the first embodiment (see FIG. 1). In the fourth embodiment, the absorption spectral characteristics of the sample are calculated by Fourier transform. Therefore, when the infrared region is the measurement target, the quantum absorption spectroscopy (QAS) in the fourth embodiment can be specifically called quantum Fourier transform infrared spectroscopy (Q-FTIR). However, the quantum absorption spectroscopy system according to the fourth embodiment can also measure the absorption spectral characteristics of a sample in the ultraviolet or visible region.

[0130] <System configuration> 20 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to the fourth embodiment. Quantum absorption spectroscopy system 4 differs from quantum absorption spectroscopy system 1 according to the first embodiment (see FIG. 1 ) in that quantum interferometer 24 is provided instead of quantum interferometer 21, single-pixel photodetector 32 is provided instead of spectrometer 31, and controller 42 is provided instead of controller 41. Quantum interferometer 24 differs from quantum interferometer 21 in that photon pair source 220 is provided instead of photon pair source 204 and movable mirror 221 is provided instead of mirror 212. Photon pair source 220 includes a quasi-phase-matched (QPM) device 60 instead of bulk nonlinear optical crystal 50.

[0131] 21 is a diagram showing an example of the configuration of a QPM device 60. The QPM device 60 includes a lens 61, a nonlinear optical crystal 62, a long-pass filter 63, a lens 64, and a sharp-cut filter 65.

[0132] The material of the nonlinear optical crystal 62 is, for example, magnesium-doped stoichiometric lithium tantalate (also referred to as Mg:SLT). Other materials that may be used for the nonlinear optical crystal 62 include gallium phosphide (GaP), gallium arsenide (GaAs), lithium tantalate (LiTaO), and zinc selenide (ZnSe). The material of the nonlinear optical crystal 62 may also be an organic material. The material of the nonlinear optical crystal 62 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 substituted with other atoms or atomic groups.

[0133] The nonlinear optical crystal 62 has a periodically poled structure. The arrows in the figure indicate the spontaneous polarization direction. The nonlinear optical crystal 62 has a rectangular parallelepiped shape. When pump light is incident on one end (first end) 621 of the opposing end faces of the nonlinear optical crystal 62, signal photons and idler photons are emitted from the other end (second end) 622. The nonlinear optical crystal 62 has, for example, five poled structures (also called sections or segments) between the first end 621 and the second end 622. The nonlinear optical crystal 62 is typically divided into several tens of sections. As shown in the figure, the width of the sections (poled period) gradually increases from the first end 621 to the second end 622.

[0134] By selecting an appropriate material for the nonlinear optical crystal 62 and appropriately designing the number of sections and the polarization period, it is possible to generate idler light with a flat intensity distribution across a wide bandwidth. The QPM device 60 shown in FIG. 21 is a chirp-type device in which the polarization period varies along the optical path. However, the QPM device 60 may also be a fan-type (fan-out structure) device in which the polarization period varies in a fan-like fashion. The QPM device 60 may also be a ridge waveguide type (including a chirp-type) device, which is expected to further improve the generation efficiency of quantum entangled photon pairs.

[0135] The long-pass filter 63 and the sharp-cut filter 65 each cut off pump light having wavelengths shorter than a specific wavelength. The long-pass filter 63 and the sharp-cut filter 65 may be omitted.

[0136] In the fourth embodiment, it is not essential to employ photon pair source 220 including QPM device 60. Quantum absorption spectroscopy system 4 may include photon pair source 204 including nonlinear optical crystal 50, similar to the first to third embodiments.

[0137] Returning to FIG. 20, movable mirror 221 is configured to be movable along the propagation direction of the idler light. Specifically, movable mirror 221 is provided with a drive device. The drive device is an electric actuator controlled by controller 42, and is, for example, a motor drive device (servo motor, stepping motor, etc.) that mechanically displaces in accordance with a control command from controller 42. The drive device may also be a piezoelectric element that displaces in accordance with an applied voltage from controller 42. The idler light path can be swept by periodically changing the position of movable mirror 221 using the drive device (causing movable mirror 221 to move back and forth).

[0138] Movable mirror 221 is an example of a configuration for changing the phase (more specifically, the effective optical path length) of one of the signal photon and the idler photon relative to the phase of the other photon. Movable mirror 221 changes the optical path length of the idler optical path, but mirror 209 of the signal optical path may be replaced with a movable mirror to change the optical path length of the signal optical path. Also, a phase modulator (not shown) such as an electro-optic modulator (EOM) may be used instead of or in addition to movable mirror 221. For example, the optical path length may be roughly changed using movable mirror 221, and then fine changes in the optical path length may be achieved using a phase modulator. Movable mirror 221 and the phase modulator correspond to the "phase conversion unit" according to the present disclosure.

[0139] Instead of the transmission method in which idler light is transmitted through a sample, reflected light can also be used. For example, an attenuated total reflection (ATR) measurement method may be applied to the quantum absorption spectroscopy system 4, and an ATR unit may be employed.

[0140] 22 is a diagram showing an example of the configuration of an ATR unit. The ATR unit 70 includes a lens 71, a prism 72, and a lens 73. The lens 71, the prism 72, and the lens 73 are arranged in this order along the propagation direction of the idler photons.

[0141] Prism 72 has a high refractive index and is configured to come into contact with the sample surface. Idler photons that enter prism 72 from lens 71 are totally reflected at the interface between prism 72 and the sample. At this time, idler photons (evanescent waves) that leak into the sample side are absorbed by the sample surface, so the infrared absorption spectroscopic characteristics of the sample surface can be measured by detecting the totally reflected light.

[0142] When microscopic irregularities exist on the sample surface, in a typical transmission method, idler photons can be scattered and / or reflected by the sample surface, resulting in multiple modes. This can result in a decrease in signal intensity and an increase in noise, reducing the accuracy of measuring the infrared absorption spectroscopic characteristics of the sample. In contrast, with ATR, the prism 72 is configured to contact the sample surface, making the above-mentioned issues unlikely to occur, even if microscopic irregularities exist on the sample surface. This allows for highly accurate measurement of the infrared absorption spectroscopic characteristics of the sample.

[0143] In the ATR, it is necessary to adjust the optical axis of the idler beam so that it enters one end of the prism and exits from the other end of the prism toward a subsequent mirror. When the idler beam path is swept by the reciprocating movement of the movable mirror 221 disposed in the idler beam path, the optical axis of the idler beam may periodically change as the movable mirror 221 moves back and forth. It is not easy to construct a quantum interferometer 24 so that the optical axis of the idler beam is always maintained at an appropriate position even during the sweeping of the idler beam path. Therefore, although not shown, when the sample holder 213 is replaced with the ATR unit 70, it is desirable to replace the mirror 209 disposed in the signal beam path with a movable mirror and sweep the signal beam path. This reduces the difficulty of constructing the quantum interferometer 24.

[0144] <Calculation processing by controller> When the spectrometer 31 is used as in the first embodiment, signal light of a different frequency is detected for each of the multiple pixels included in the multi-pixel photodetector 302. In other words, signal light separated into monochromatic components is detected in each pixel of the multi-pixel photodetector 302. On the other hand, the quantum absorption spectroscopy system 4 according to the fourth embodiment includes a single-pixel photodetector 32 instead of the spectrometer 31. The signal photons are guided to the single-pixel photodetector 32 without being spatially separated, and signal light including all frequency components is detected by a single pixel. Even when the signal light includes all frequency components in this way, the controller 42 can calculate the infrared absorption spectroscopic characteristics of the sample (the Fourier spectrum, complex transmittance spectrum, and infrared absorption spectrum of the sample) based on the detection signal of the single-pixel photodetector 32 by performing the following arithmetic processing.

[0145] 23 is a functional block diagram for explaining in more detail the calculation processing by the controller 42. The controller 42 includes a light source control unit 421, a mirror control unit 422, a count rate calculation unit 423, a Fourier transform unit 424, a first storage unit 425, a second storage unit 426, a transmittance calculation unit 427, an absorption spectrum calculation unit 428, and a monitor control unit 429.

[0146] The light source control unit 421 controls the optical output (laser power) of the excitation light source 11. The laser power during measurement by the quantum absorption spectroscopy system 4 is basically kept constant.

[0147] Mirror control unit 422 controls the sweep (reciprocating motion) of movable mirror 221. When the position of movable mirror 221 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 moving movable mirror 221 back and forth, the detection signal of the signal light from single-pixel photodetector 32 is acquired as a time waveform of the idler propagation time t0.

[0148] The count rate calculation unit 423 calculates the signal photon count rate P based on the detection signal of the signal light from the single pixel photodetector 32. s is calculated as a function of the idler propagation time t0. The signal photon count rate P 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 424.

[0149] The Fourier transform unit 424 calculates the signal photon count rate P s In the quantum absorption spectroscopy system 4, the signal photon count rate P s The signal photon count rate P (t0) is acquired when the sample is placed in the sample holder 213. s The Fourier spectrum obtained by Fourier transform of (t0) is called "A s On the other hand, the signal photon count rate P obtained when no sample is placed on the sample holder 213 is s The Fourier spectrum obtained by Fourier transform of (t0) is called "A s 0The Fourier transform unit 424 converts the Fourier spectrum A s (ω) is output to the first storage unit 425 and the monitor control unit 429, and the Fourier spectrum A s 0 (ω) is output to the second storage unit 426. s 0 (ω) corresponds to the "reference Fourier spectrum" according to the present disclosure.

[0150] The first storage unit 425 stores the Fourier spectrum A when the sample is placed on the sample holder 213. s The second storage unit 426 stores the Fourier spectrum A (ω) in a non-volatile manner when no sample is placed on the sample holder 213. s 0 (ω) is stored in a non-volatile manner. The stored Fourier spectrum (A s (ω) or A s 0 (ω) is read out by the transmittance calculation unit 427 as appropriate.

[0151] The transmittance calculation unit 427 calculates the Fourier spectrum A s (ω) and Fourier spectrum A s 0 The transmittance calculation unit 427 calculates the complex transmittance spectrum τ(ω) of the sample based on (ω). The transmittance calculation unit 427 outputs the calculation result of the complex transmittance spectrum τ(ω) to the absorption spectrum calculation unit 428 and the monitor control unit 429.

[0152] The absorption spectrum calculation unit 428 calculates the infrared absorption spectrum of the sample based on the complex transmittance spectrum τ(ω) of the sample. The absorption spectrum calculation unit 428 outputs the calculation result of the infrared absorption spectrum to the monitor control unit 429.

[0153] The monitor control unit 429 receives the calculation result (Fourier spectrum A of the sample) from the controller 42. s (ω), the complex transmittance spectrum τ(ω), and the infrared absorption spectrum are displayed on the monitor 404.

[0154] Next, the functions of some blocks will be explained in detail. Below, parameters marked "signal" are parameters related to signal photons. Parameters marked "idler" are parameters related to idler photons.

[0155] First, we will explain the calculation process by the count rate calculation unit 423. The state vector |Ψ> obtained by superposing two events (first and second physical processes) in which quantum entangled photon pairs are generated by the QPM device 60 is described as in the following formula (1).

[0156]

number

[0157] 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 ) and 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, and 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, we can distinguish between before and 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 in response to the optical path difference between the first and second physical processes is expressed as φ. p It is expressed as:

[0158] When the optical loss due to the sample placed in the idler path is evaluated according to the beam splitter model, the idler annihilation operator a i1,out is expressed as the following equation (2).

[0159]

number

[0160] In equation (2), the complex transmittance (complex transmission amplitude) of the sample is expressed as τ, and the complex reflectance (complex reflection amplitude) of the sample is expressed as r. In the beam splitter model that represents the j-th (j = 1, 2) transmission through the sample, the vacuum field incident from a port different from the port where the idler photon is input is expressed as a vj It is expressed as:

[0161] The mode of the signal light generated in the first physical process is adjusted to spatially coincide with the mode of the signal light generated in the second physical process. Also, the mode of the idler light generated in the first physical process is adjusted to spatially coincide with the mode of the idler light generated in the second physical process. These modes can be expressed by the same creation / annihilation operator, except for the phase change due to propagation, as shown in the following equations (3) and (4).

[0162]

number

[0163] In equations (3) and (4), t1 represents the propagation time until the signal light generated in the first physical process reaches the QPM device 60 again. Similarly, t0 represents the propagation time until the idler light generated in the first physical process reaches the QPM device 60 again when the sample is not placed in the idler light path.

[0164] Next, the electric field E of the signal light in the single-pixel photodetector 32 s (+) (t) is expressed as the following equation (5).

[0165]

number

[0166] Signal photon count rate P s is expressed as the following equation (6) using the state vector |Ψ> shown in equation (1).

[0167]

number

[0168] By substituting equations (1) to (4) into equation (5), we obtain the following equation (7), which represents the time waveform of quantum interference (quantum beat) between the first and second physical processes that generate quantum entangled photon pairs.

[0169]

number

[0170] where the signal frequency ω s the center frequency ω s0 We redefine it using the detuning Ω from ω. s =ω s0 -Ω. 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).

[0171]

number

[0172] A configuration in which the dispersive optical element 301 is arranged in front of the multi-pixel photodetector 302, as in the first embodiment, is also possible (see FIG. 1). By providing the dispersive optical element 301, the signal photons that reach each of the multiple pixels included in the multi-pixel photodetector 302 are limited in advance to signal photons in a specific wavelength range, and the light intensity of the signal photons is measured for each wavelength. However, devices that include a spectrometer generally tend to be large and expensive. Furthermore, wavelength scanning by a spectrometer takes time, which can be an obstacle in terms of shortening the measurement time.

[0173] In contrast to this, in the fourth embodiment, the signal photons in the entire wavelength range generated by the QPM device 60 are detected by the single-pixel photodetector 32 without passing through the dispersive optical element 301. This is because the frequency (signal frequency) ω of the signal photons detected by the single-pixel photodetector 32 is s is included in the frequency band, and the signal photon count rate P s This is based on the idea that all possibilities should be considered when calculating . This idea is also reflected in the fact that the frequency component (detuning Ω) is integrated in the above equation (8).

[0174] In the fourth 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 equation (9) is obtained from equation (8).

[0175]

number

[0176] 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 srepresents 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 holds. 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 the reciprocating motion of the movable mirror 221 (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).

[0177]

number

[0178] Next, the calculation process by the Fourier transform unit 424 will be described. The Fourier transform unit 424 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 is obtained as shown in the following formula (11): 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.

[0179]

number

[0180] Next, the calculation process by the transmittance calculation unit 427 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 0The 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 of (ω) is calculated, the following equation (12) is derived.

[0181]

number

[0182] 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.

[0183] Finally, we will explain the calculation process performed by the absorption spectrum calculation unit 428. The absorption spectrum calculation unit 428 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).

[0184] Thus, in the fourth embodiment, the single signal photon count rate P obtained using the single pixel photodetector 32 is s (t0) is Fourier transformed to obtain Fourier spectrum A s By simply calculating (ω), broadband infrared absorption spectroscopy can be achieved. According to the fourth embodiment, in principle, photodetection for each wavelength is not required, and the complex transmittance spectrum of a sample over a broadband can be measured with a single photodetection. This effect is particularly noticeable when a broadband QPM device 60 is used.

[0185] <Measurement flow> 24 is a flowchart showing the processing procedure of quantum absorption spectroscopy in the fourth embodiment. This flowchart is called from the main routine and executed when, for example, an input device (not shown), such as an operation button, accepts an operation by the measurer. Each step is basically realized by software processing by the controller 42, but may also be realized by hardware processing by an electronic circuit disposed within the controller 42. Hereinafter, step will be abbreviated as "S".

[0186] In S1, a sample is placed on the sample holder 213 arranged in the idler optical path. The sample is usually placed by the operator. However, it is also possible to automate the process by providing a feed device (not shown) that transports the sample.

[0187] In S2, the controller 42 controls the excitation light source 11 to start outputting pump light.

[0188] In S3, controller 42 controls the drive device provided on movable mirror 221 to start or continue high-speed reciprocating motion. In general, the signal-to-noise ratio of the infrared absorption spectrum can be improved by integrating the light detection results. Therefore, in S3, movable mirror 221 reciprocates several tens of times per second over a range of sub-millimeter (e.g., several tens of μm) to several centimeters (e.g., up to 5 cm). However, if integration is not required or if high speed is required, such as when acquiring a time-varying infrared absorption spectrum, then only one reciprocating motion may be required.

[0189] In S4, the controller 42 calculates the signal photon count rate P based on the detection signal of the single pixel photodetector 32. s Calculate (t0).

[0190] In S5, controller 42 determines whether a condition has been met to end the reciprocating motion of movable mirror 221. For example, controller 42 may determine whether a signal photon count rate P sWhen (t0) is calculated, it can be determined that the termination condition is met. If the termination condition is not met (NO in S5), the controller 42 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. If the termination condition is met (YES in S5), the controller 42 advances the process to S6.

[0191] In S6, the controller 42 controls the excitation light source 11 to stop outputting the pump light, and also controls the drive device of the movable mirror 221 to stop the reciprocating motion of the movable mirror 221.

[0192] In S7, the controller 42 calculates the signal photon count rate P with the sample placed in the idler optical path. s (t0) is Fourier transformed to obtain the Fourier spectrum A s Calculate (ω).

[0193] Although not shown, before the execution of the series of processes S1 to S7, a similar process (so-called background measurement) is performed in a state where the sample is not placed in the idler optical path to obtain a Fourier spectrum A s 0 In S8, the controller 42 calculates the Fourier spectrum A (ω) calculated by the series of processes. s (ω) and the previously acquired Fourier spectrum A s 0 (ω), the complex transmittance spectrum τ(ω) of the sample is calculated. If background measurement has not been performed, the Fourier spectrum A can be obtained by performing the same processes as in S1 to S7 without placing a sample. s 0 (ω) can be obtained.

[0194] In S8, the controller 42 calculates 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 of τ(ω) to τ(ω).

[0195] In S9, the controller 42 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.

[0196] In S10, the controller 42 outputs the measurement results of the infrared absorption spectroscopic characteristics of the sample obtained by the processes in S1 to S9 (Fourier spectrum A of the sample). s (ω), the complex transmittance spectrum τ(ω) of the sample, and the infrared absorption spectrum of the sample) are displayed on the monitor 404.

[0197] As described above, in the fourth embodiment, all signal light emitted from the quantum interferometer 24 is detected by the single-pixel photodetector 32 without being spectrally resolved (wavelength-resolved or frequency-resolved) by the dispersive optical element 301 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 from all bands, broadband infrared absorption spectroscopy is possible. Furthermore, the phase information of the sample can be obtained from the difference in the Fourier spectrum depending on whether or not the sample is placed in the idler path.

[0198] [Modification of the fourth embodiment] In the fourth embodiment, a configuration has been described in which quantum Fourier transform infrared spectroscopy (Q-FTIR) is applied to the quantum absorption spectroscopy system 1 (see FIG. 1) according to the first embodiment. However, Q-FTIR can also be applied to the quantum absorption spectroscopy systems according to the other embodiments described above.

[0199] 25 is a diagram schematically illustrating the overall configuration of a quantum absorption spectroscopy system according to a modification of the fourth embodiment. The quantum absorption spectroscopy system 4A differs from the quantum absorption spectroscopy system 2 according to the fourth embodiment (see FIG. 20) in that it includes a quantum interferometer 24A instead of the quantum interferometer 24. The quantum interferometer 24A differs from the quantum interferometer 24 in that it includes an infrared SMF 218 that does not include a tapered waist portion 210C, instead of the infrared SMF 210 that is provided with a tapered waist portion 210C (see FIG. 4), as in the third embodiment.

[0200] In this way, Q-FTIR can also be applied to the quantum absorption spectroscopy system according to embodiment 3. Although detailed description will not be repeated, Q-FTIR may also be applied to the quantum absorption spectroscopy systems according to modifications 1 and 2 of embodiment 1 or modifications 1 to 5 of embodiment 3. Furthermore, photodetectors that can be used in quantum absorption spectroscopy systems to which Q-FTIR is applied are not limited to single-pixel photodetectors. Q-FTIR can also be applied to system configurations that include multi-pixel photodetectors.

[0201] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is defined by the claims, not by the description of the above embodiments, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0202] 1, 1A, 1B, 2, 2A, 2B, 3, 3A to 3E, 4, 4A quantum absorption spectroscopy system, 11, 12 excitation light source, 21, 21A, 21B, 22, 22A, 22B, 23, 23A to 23E, 24, 24A quantum interferometer, 201 optical fiber, 202 dichroic splitter, 203 optical fiber, 204 photon pair source, 205 optical fiber, 205A core, 205B cladding, 205C air hole, 206 dichroic splitter, 207 optical fiber (visible SMF), 208 polarization controller, 209 mirror, 210 optical fiber (infrared SMF), 210A, 210E non-tapered section, 210B, 210D tapered section, 210C tapered waist section, 211 Polarization controller, 212 Mirror, 213 Sample holder, 213A Absorber, 214-219 Optical fiber, 220 Photon pair source, 221 Moving mirror, 31 Spectrometer, 301 Dispersive optical element, 302 Multi-pixel photodetector, 32 Single-pixel photodetector, 41, 42 Controller, 401 Processor, 402 Memory, 403 Input / output port, 404 Monitor, 421 Light source control unit, 422 Mirror control unit, 423 Count rate calculation unit, 424 Fourier transform unit, 425 First memory unit, 426 Second memory unit, 427 Transmittance calculation unit, 428 Absorption spectrum calculation unit, 429 Monitor control unit, 50 Nonlinear optical crystal, 51 First crystal, 52 Second crystal, 60 QPM device, 61 Lens, 62 Nonlinear optical crystal, 621 First end, 622 Second end, 63 Long-pass filter, 64 lens, 65 sharp-cut filter, 70 ATR unit, 71 lens, 72 prism, 73 lens.

Claims

1. an excitation light source that emits pump light; a quantum interferometer configured to cause quantum interference among a plurality of physical processes in which quantum entangled photon pairs of a signal photon and an idler photon are generated by spontaneous parametric down-conversion of the pump light, and to place a sample in a propagation path of the idler photon; a photodetector for detecting the signal photons from the quantum interferometer; A quantum absorption spectroscopy system, wherein the quantum interferometer includes a single-mode fiber section optically coupled to at least a portion of the propagation path of the signal photons and the propagation path of the idler photons.

2. the quantum interferometer further includes a wavelength separation element that separates the quantum entangled photon pair into the signal photon and the idler photon according to wavelength; the single-mode fiber portion includes a first single-mode fiber optically coupled to the wavelength separation element and propagating light in the wavelength range of the idler photons; The quantum absorption spectroscopy system of claim 1 , wherein the first single mode fiber is a tapered fiber.

3. The tapered fiber is a non-tapered portion; a tapered waist portion that is narrower than the non-tapered portion, The quantum absorption spectroscopy system according to claim 2 , wherein the diameter of the tapered waist portion is included in a wavelength range from the visible range to the far infrared range.

4. The quantum absorption spectroscopy system according to claim 2 or 3, wherein the tapered fiber is configured to be replaceable.

5. The quantum interferometer comprises: a first mirror that reflects the idler photons; a second mirror that reflects the signal photons; the first single-mode fiber is optically coupled between the wavelength separation element and the first mirror; the single-mode fiber portion further includes a second single-mode fiber optically coupled between the wavelength separation element and the second mirror, the second single-mode fiber propagating light in the wavelength range of the signal photons; 5. The quantum absorption spectroscopy system according to claim 2, wherein at least one of the first and second single-mode fibers is a polarization-maintaining fiber.

6. the quantum interferometer further includes a photon pair source that generates the quantum entangled photon pairs; 6. The quantum absorption spectroscopy system according to claim 2, wherein the single-mode fiber section further includes a broadband single-mode fiber optically coupled between the photon pair source and the wavelength separation element, and propagating light in all wavelength regions of the pump light, the signal photons, and the idler photons.

7. The quantum interferometer comprises: a photon pair source that generates the quantum entangled photon pairs; a wavelength separation element that separates the quantum entangled photon pair into the signal photon and the idler photon according to wavelength; 2. The quantum absorption spectroscopy system of claim 1, wherein the single-mode fiber section is optically coupled between the photon pair source and the wavelength separation element and includes a broadband single-mode fiber that propagates light in all wavelength ranges of the pump light, the signal photons, and the idler photons.

8. 8. The quantum absorption spectroscopy system according to claim 6, wherein the broadband single-mode fiber is a photonic crystal fiber.

9. the excitation light source is a pulsed light source, the photodetector is a single pixel photodetector; 9. The quantum absorption spectroscopy system of claim 1, wherein the single-mode fiber section includes a wavelength-dispersive single-mode fiber optically coupled to the single-pixel photodetector.

10. the quantum interferometer is used in a high gain region where the signal intensity of the single-pixel photodetector increases nonlinearly as the transmittance of the idler photons through the sample increases; The quantum absorption spectroscopy system of claim 9 , wherein the single mode fiber section further includes an absorber that absorbs the idler photons.

11. a processor for executing a calculation process for analyzing the absorption spectroscopic characteristics of the sample; the quantum interferometer further includes a phase converter configured to be able to change the phase of one of the signal photons and the idler photons; 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; 9. The quantum absorption spectroscopy system according to claim 1, wherein the processor calculates the absorption spectroscopic characteristics of the sample by Fourier transforming the quantum interference signal.

12. The processor: In addition to calculating a Fourier spectrum by Fourier transform of the quantum interference signal in a state where the sample is placed in the propagation path of the idler photons, calculating a reference Fourier spectrum by Fourier transform of the quantum interference signal in a state where the sample is not placed in the propagation path of the idler photons; The quantum absorption spectroscopy system of claim 11 , wherein a complex transmittance spectrum of the sample is calculated based on a ratio between the Fourier spectrum and the reference Fourier spectrum.

13. The quantum absorption spectroscopy system of claim 12 , wherein the processor calculates the absorption spectrum of the sample by squaring the absolute value of the complex transmittance spectrum of the sample.

14. the quantum interferometer is configured to generate visible photons as the signal photons; The quantum absorption spectroscopy system of any one of claims 1 to 13, wherein the photodetector is a silicon-based photodetector.

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