Terahertz light detection system

Abstract

To provide a terahertz light detection system capable of detecting weak terahertz waves using quantum property with high sensitivity.SOLUTION: The terahertz optical detection system comprises: a light source; a first beam splitter splitting light from the light source into two pieces of light; a first nonlinear optical crystal receiving first light (Pump A) split by the first beam splitter; a second nonlinear optical crystal receiving second light (Pump B) split by the first beam splitter; a second beam splitter receiving idler light (Idler A) outputted from the first nonlinear optical crystal and idler light (Idler B) outputted from the second nonlinear optical crystal; and a detector detecting outputs (output A and output B) from the second beam splitter. The second nonlinear optical crystal receives signal light (Signal) outputted from the first nonlinear optical crystal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a terahertz light detection system, and more particularly to a terahertz light detection system that can detect weak terahertz waves with high sensitivity by utilizing the quantum nature of light. [Background technology]

[0002] Electromagnetic waves have been broadly divided into low-frequency radio waves and high-frequency light waves, and their technological development has focused on these waves. However, the intermediate frequency band between radio waves and light has been referred to as terahertz (THz) waves since the 1990s, and research into new methods for generating and detecting them has been progressing in recent years. THz waves, which refer to electromagnetic waves in the frequency range of approximately 0.1 to 10 THz, possess both radio wave-like properties, such as the ability to penetrate various materials, including paper, ceramic, and plastic, and optical properties, such as ease of manipulation using optical elements such as lenses and mirrors. Furthermore, when irradiated onto materials, unique electromagnetic absorption spectra emerge due to molecular vibrations and lattice vibrations. Therefore, THz waves are expected to be used in nondestructive testing and spectroscopic measurements in a variety of fields, including science, medicine, and security.

[0003] However, the generation and detection of THz waves is difficult. In particular, THz waves have low energy compared to the energy band gap of semiconductors, making them difficult to detect using devices that utilize electronic transitions, such as photodiodes. Furthermore, existing THz wave detection methods require equipment that operates only at cryogenic temperatures or is very large, making them difficult to handle. In addition, there are issues with detection accuracy, making industrial application difficult at present.

[0004] In recent years, detection methods using nonlinear optical crystals have attracted attention. Nonlinear optical crystals are crystals of materials with high nonlinear optical effects, and the measurement of THz waves by placing them in an interferometer has been studied both theoretically and experimentally (Non-Patent Document 1). Figure 1 in this document shows a conceptual diagram of an optical system in which nonlinear optical crystals NL1 and NL2 are placed in an interferometer. Of the two electromagnetic waves generated by the effect of the nonlinear optical crystals, only THz waves (the route shown in red) pass through the object, and the other light with frequencies in the visible range (the route shown in yellow) is detected. The results showed that information contained in the THz waves passing through the object could be obtained by detecting the visible light that interfered with the beam splitter (BS). The optical system configuration used here is based on the concept of the Zou-Wang-Mandel interferometer, and the experiment suggests that quantum entanglement between the THz waves generated by the nonlinear optical crystals and visible light plays an important role in THz wave detection.

[0005] "Quantum entanglement" refers to the quantum mechanical correlation that exists between multiple particles or states. It is a well-known concept at the core of quantum information and communication technology, and is utilized in protocols such as quantum teleportation and quantum repeaters. In quantum measurements such as those described in Non-Patent Document 1, quantum entangled light consisting of THz waves and visible light is generated due to a nonlinear optical phenomenon called spontaneous parametric down-conversion (SPDC), which occurs due to matter-light interactions within nonlinear optical crystals. SPDC is a process in which one mode of light incident on a nonlinear optical crystal with second-order nonlinear susceptibility is converted into two modes of light as a result of the interaction. The incident light is called the Pump, and the emitted light is called the Idler and Signal, respectively. In other words, in Figure 1 of Non-Patent Document 1, the light irradiated from the light source (the route shown in green) is the Pump, the visible light is the Idler, and the THz wave is the Signal. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Lemos, G., Borish, V., Cole, G. et al. “Quantum imaging with undetected photons”, Nature 512, 409-412(2014). Summary of the Invention [Problem to be solved by the invention]

[0007] Although detection technology for weak signals in the terahertz region is important for imaging and sensing applications, there is still a lack of sufficient technology. From this perspective, the inventors have been studying systems in which electron-lattice systems, with a focus on nonlinear optical crystals, are placed inside an interferometer, and have discovered that it is possible to discuss the time-dependent effects of electron and lattice vibrations excited during the interactions between the pump, idler, and signal in SPDC and the material on the quantum entanglement of the generated light.

[0008] The present invention has been made based on the above background, and an object of the present invention is to provide a terahertz light detection system that can detect weak terahertz waves with high sensitivity by utilizing the quantum nature of light. [Means for solving the problem]

[0009] The terahertz light detection system according to the present invention comprises a light source, a first beam splitter that splits light from the light source into two beams, a first nonlinear optical crystal onto which the first light split by the first beam splitter is incident, a second nonlinear optical crystal onto which the second light split by the first beam splitter is incident, a second beam splitter onto which idler light output from the first nonlinear optical crystal and the idler light output from the second nonlinear optical crystal are incident, and a detector that detects output from the second beam splitter, wherein the second nonlinear optical crystal is arranged to receive signal light output from the first nonlinear optical crystal.

[0010] According to this invention, there is provided a first nonlinear optical crystal and a second nonlinear optical crystal, and the second nonlinear optical crystal is arranged so that the signal light output from the first nonlinear optical crystal is incident thereon. Therefore, by detecting the signal output from the idler light output from the first nonlinear optical crystal and the idler light output from the second nonlinear optical crystal via the second beam splitter, weak terahertz light can be detected with high sensitivity.

[0011] In the terahertz light detection system according to the present invention, the signal light is terahertz light having a frequency lower than that of visible light.

[0012] According to this invention, the signal light is terahertz light, which has a lower frequency than visible light. When this terahertz light is incident from the first nonlinear optical crystal to the second nonlinear optical crystal, the idler light (Idler B) output from the second nonlinear optical crystal and the idler light (Idler A) output from the first nonlinear optical crystal interfere with each other due to the quantum interference effect between the idler lights, and the interfered idler light (output A) output from the second beam splitter can be detected as a signal, and the terahertz light can be calculated based on this output A.

[0013] The terahertz light detection system according to the present invention includes a first transmission means for inputting the idler light output from the first nonlinear optical crystal into the second beam splitter, and a second transmission means for inputting the second light split by the first beam splitter into the second nonlinear optical crystal.

[0014] According to this invention, the first transmission means makes the idler light output from the first nonlinear optical crystal incident on the second beam splitter, and the second transmission means makes the second light split by the first beam splitter incident on the second nonlinear optical crystal. Examples of these transmission means include a reflecting mirror that transmits light through space, and an optical waveguide such as an optical fiber or a waveguide substrate that transmits light outside of space.

[0015] In the terahertz light detection system according to the present invention, the detector is a single photon detector that detects two outputs from the second beam splitter. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a terahertz light detection system that utilizes the quantum nature of light to achieve highly sensitive detection of weak terahertz waves. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a configuration diagram of an embodiment of a terahertz light detection system according to the present invention. [Figure 2] Comparison of idler photon number dynamics after passing through the second beam splitter (BS2): (a) NL2'B and (b) NL2'A. DETAILED DESCRIPTION OF THE INVENTION

[0018] The terahertz light detection system according to the present invention will be described with reference to the drawings. Note that the following embodiments are representative examples, and other modifications and applications that encompass the technical concept of the present invention are also included in the technical scope of the present invention.

[0019] [Terahertz light detection system] As shown in Figure 1, the terahertz light detection system of the present invention comprises a light source, a first beam splitter (BS1) that splits light from the light source into two beams, a first nonlinear optical crystal onto which the first light (Pump A) split by the first beam splitter is incident, a second nonlinear optical crystal onto which the second light (Pump B) split by the first beam splitter (BS1) is incident, a second beam splitter (BS2) onto which the idler light (Idler A) output from the first nonlinear optical crystal and the idler light (Idler B) output from the second nonlinear optical crystal are incident, and a detector that detects the output (output A and output B) from the second beam splitter (BS2), and the second nonlinear optical crystal is arranged to receive the signal light (Signal) output from the first nonlinear optical crystal.

[0020] This invention has a first nonlinear optical crystal and a second nonlinear optical crystal, and the second nonlinear optical crystal is arranged to receive the signal light output from the first nonlinear optical crystal. Therefore, by detecting the signal output from the idler light output from the first nonlinear optical crystal and the idler light output from the second nonlinear optical crystal via the second beam splitter, weak terahertz light can be detected with high sensitivity.

[0021] Each component will be described in detail.

[0022] The terahertz light detection system shown in Figure 1 combines a Mach-Zehnder interferometer with a nonlinear optical crystal. A Mach-Zehnder interferometer is used to measure the phase difference between two paths caused by a sample, measuring the phase difference between two light beams split from a light source. The example in Figure 1 is characterized by combining two nonlinear optical crystals in such a Mach-Zehnder interferometer and arranging the signal light (Signal) output from one nonlinear optical crystal (first nonlinear optical crystal) to be incident on the other nonlinear optical crystal (second nonlinear optical crystal). For example, because the idler light (IdlerA) and the signal light (Signal) are emitted in different directions, the second nonlinear optical crystal is arranged in the direction of emission of the signal light (Signal) from the first nonlinear optical crystal. With this arrangement, weak terahertz light can be detected with high sensitivity by detecting the signals (output A, output B) output by the idler light (Idler A) output from the first nonlinear optical crystal and the idler light (Idler B) output from the second nonlinear optical crystal passing through a beam splitter (second beam splitter: BS2).

[0023] As described above, the signal light (Signal) is light output from the first nonlinear optical crystal, and is terahertz light with a lower frequency than visible light. In the terahertz light detection system according to the present invention, when such terahertz light is incident from the first nonlinear optical crystal to the second nonlinear optical crystal, the idler light (Idler B) output from the second nonlinear optical crystal and the idler light (Idler A) output from the first nonlinear optical crystal interfere with each other due to the quantum interference effect between the idler lights, and the interfered idler light (Output A) output from the second beam splitter can be detected as a signal, and the terahertz light can be calculated based on Output A.

[0024] Although an Nd:YAG laser (wavelength: 1064 nm) is used as the light source, other light sources may be used as long as they can produce results similar to those of the present invention. In the embodiment shown in Figure 1, the input wavelength to the first beam splitter (BS1) is 1064 nm, which becomes Pump A and Pump B.

[0025] The two beam splitters (BS1, BS2) are not particularly limited, and both the first beam splitter (BS1) and the second beam splitter (BS2) may be beam splitters that do not impair the effects of the present invention. Two beams of light (IdlerA, IdlerB) that are phase-matched by two nonlinear optical crystals are incident on the second beam splitter (BS2). The incident light interferes with the second beam splitter (BS2) and is then detected by a single-photon detector. A commercially available high-sensitivity detector can be used as the single-photon detector.

[0026] In the embodiment shown in FIG. 1, first and second transmission means are provided as transmission means. The first transmission means is a means for inputting the idler light output from the first nonlinear optical crystal to the second beam splitter, and the second transmission means is a means for inputting the second light split by the first beam splitter to the second nonlinear optical crystal. The first transmission means inputs the idler light output from the first nonlinear optical crystal to the second beam splitter, and the second transmission means inputs the second light split by the first beam splitter to the second nonlinear optical crystal. Examples of these transmission means include a reflecting mirror for transmission through space, and an optical waveguide such as an optical fiber or a waveguide substrate for transmission outside of space. Each transmission means is arranged at an appropriate position and interval depending on its component.

[0027] Two nonlinear optical crystals are arranged in the optical system. It is desirable to use lithium niobate crystal as the nonlinear optical crystal, but other nonlinear optical crystals may be used as long as they provide the same functions as those of the present invention.

[0028] In the embodiment example of FIG. 1, a first nonlinear optical crystal is disposed between a first beam splitter (BS1) and a first transmission means. This first nonlinear optical crystal transmits the light split by the beam splitter (BS1) and phase-matches the light (Pump A) before transmission. The phase-matched light (Idler A) is directed to a second beam splitter (BS2) by the first transmission means. A second nonlinear optical crystal is disposed between the second transmission means and a second beam splitter (BS2). This second nonlinear optical crystal phase-matches the light (Pump B) split by the beam splitter (BS1) and transmitted by the second transmission means. The phase-matched light (Idler B) is directed to a second beam splitter (BS2).

[0029] The present invention is characterized in that the second nonlinear optical crystal is arranged to receive the signal light (Signal) output from the first nonlinear optical crystal. With this arrangement, weak terahertz light can be detected with high sensitivity by detecting the signal (Outputs A and B) output from the idler light (Idler A) output from the first nonlinear optical crystal and the idler light (Idler B) output from the second nonlinear optical crystal after passing through the second beam splitter. If the Signal is 500 Hz (0.5 THz), the wavelength of Idler A and B is 1065.9 nm. This is the wavelength of Outputs A and B output from the second beam splitter (BS2), and is the light detected by the detector.

[0030] The terahertz light detection system according to the present invention, configured as described above, is characterized in that two nonlinear optical crystals are arranged within a Mach-Zehnder interferometer, and the second nonlinear optical crystal is arranged so that the signal light output from the first nonlinear optical crystal is incident on it. In particular, the system is characterized in that it utilizes nonlinear optical crystals, focusing on quantum entanglement light generated by a nonlinear optical process due to the quantum nature of both matter and light. Specifically, the terahertz light detection system according to the present invention utilizes the nonlinear optical phenomenon using nonlinear optical crystals and the dynamics of quantum entanglement that occurs during this process, whereby the pump induces an excited state in the matter, and quantum entanglement light is generated by the idler and signal through interaction with the matter.

[0031] Intensity changes due to the presence or absence of terahertz light input can be seen in Idlers A and B, but because these intensity changes are weak, direct detection requires a large S / N ratio for the detector. The second beam splitter (BS2) is configured to efficiently detect such intensity changes.

[0032] Figure 2 shows the dynamics of the detected light intensity after Idler A and Idler B pass through the second beam splitter (BS2). (a) shows the N L2'B (b) shows the dynamics of the detected light (output B) after Idler B passes through the second beam splitter (BS2), and (b) shows the dynamics of the detected light (output B) after Idler B passes through the second beam splitter (BS2). L2'A (The dynamics of the light intensity of the detected light (output A) after Idler A passes through the second beam splitter (BS2). Figure 2 shows the case where the signal in Figure 1 is not blocked between the nonlinear optical crystals ("N" in the figure). nonThe graph shows the time change in the output light intensity when terahertz light is present and blocked, corresponding to the presence and absence of terahertz light. A relatively large difference is observed between the two, particularly in the range of Time = 8 to 16. The time unit "Time = 1" here is 300 femtoseconds, but this time unit is not particularly limited. A sufficient difference can be recognized at least 2 to 3 picoseconds after light incidence. While intensity differences are observed in both Outputs A and B depending on the presence or absence of terahertz light, light of approximately the same intensity as the Idler is detected in Output B regardless of the presence or absence of terahertz light. To detect the slight intensity difference due to the presence or absence of terahertz light, a high S / N ratio is required for the detector, as with direct detection of Idler A and Idler B. In contrast, if quantum interference in Output A is ignored, the output is approximately 0, and even considering the effects of quantum interference, the light is extremely weak. Detecting such a weak light intensity difference is easily achieved using a photon counting detection method. In this way, by focusing on the idler that passed through the second beam splitter (BS2) and interfered, the difference in output due to the presence or absence of terahertz light could be clearly seen.

[0033] As described above, the terahertz light detection system according to the present invention was able to demonstrate the effect of interfering quantum entangled light using an interferometer arrangement using Idler. Furthermore, it was confirmed that there was a difference between the outputs A and B from the second beam splitter (BS2) depending on whether or not terahertz light was present. This invention provides insights into the dynamics of quantum entangled light generation in nonlinear optical processes and its optical detection using an interferometer that utilizes quantum entanglement, which were not available with existing technology. In particular, insights into the characteristics of quantum entanglement dynamics are expected to contribute to highly sensitive quantum detection of THz waves.

Claims

1. 1. A terahertz light detection system comprising: a light source; a first beam splitter that splits light from the light source into two beams; a first nonlinear optical crystal onto which the first light split by the first beam splitter is incident; a second nonlinear optical crystal onto which the second light split by the first beam splitter is incident; a second beam splitter onto which idler light output from the first nonlinear optical crystal and the idler light output from the second nonlinear optical crystal are incident; and a detector that detects output from the second beam splitter, wherein the second nonlinear optical crystal is positioned so as to receive signal light output from the first nonlinear optical crystal.

2. The terahertz light detection system according to claim 1 , wherein the signal light is terahertz light having a frequency lower than that of visible light.

3. 2. The terahertz light detection system according to claim 1, further comprising: a first transmission means for inputting the idler light output from the first nonlinear optical crystal into the second beam splitter; and a second transmission means for inputting the second light split by the first beam splitter into the second nonlinear optical crystal.

4. The terahertz light detection system of claim 1 , wherein the detector is a single photon detector that detects two outputs from the second beam splitter.

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