Quantum entangled photon pair generating device and quantum entangled photon pair generating method

Abstract

To provide a quantum entangled photon pair generator and a quantum entangled photon pair generation method capable of obtaining a quantum entangled photon pair of higher quality at a lower cost.SOLUTION: The optically entangled photon pair generator includes a pumping light generator that generates pulsed pumping light having a first peak frequency and a first frequency bandwidth, a nonlinear optical unit that includes a nonlinear optical medium, receives input of the pumping light, and generates a signal photon and an idler photon, a first frequency filter that receives input of the signal photon and extracts the signal photon having a frequency bandwidth less than one time the first frequency bandwidth, and a second frequency filter that receives input of the idler photon and extracts the idler photon having a frequency bandwidth less than one time the first frequency bandwidth.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a quantum entangled photon pair generating device and a quantum entangled photon pair generating method. [Background technology]

[0002] A method disclosed in Patent Document 1 is known as a method for generating polarization-entangled photon pairs, which is an example of quantum entangled photon pairs. In this method, first, both ends of an optical fiber serving as a nonlinear optical medium are connected to a polarization beam splitter (PBS) to form an optical fiber loop. Next, pump light with a peak frequency fp is input to the PBS. The PBS separates the pump light into a vertically polarized component and a horizontally polarized component and outputs them to each end of the optical fiber. In the optical fiber loop, the separated pump light propagates in opposite directions, generating signal photons and idler photons through degenerate four-wave mixing. The signal photons and idler photons propagate through the optical fiber loop together with the pump light. Here, the signal photons and idler photons have the same polarization state as the pump light that generates them. Furthermore, if the peak frequencies of the signal photons and idler photons are fs and fi, respectively, then 2fp = fs + fi holds.

[0003] Next, the PBS polarization-combines the pump light, signal photons, and idler photons that have propagated in opposite directions through the optical fiber loop and outputs the combined light. A frequency filter is then used to extract the signal and idler photons from the light output from the PBS. This allows the signal and idler photons that make up a polarization-entangled photon pair to be obtained. The polarization-entangled photon pair obtained in this way is thought to be useful, for example, for realizing stable quantum communication in the communication wavelength band. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3999752 Summary of the Invention [Problem to be solved by the invention]

[0005] Quantum entangled photon pairs are required to be of high quality, and in recent years there has been a demand to obtain even higher quality quantum entangled photon pairs.

[0006] The present invention has been made in view of the above, and aims to provide a quantum entangled photon pair generating device and a quantum entangled photon pair generating method that can obtain higher quality quantum entangled photon pairs. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a quantum entangled photon pair generating device comprising: a pump light generating device that generates pulsed pump light having a first peak frequency and a first frequency bandwidth; a nonlinear optical unit that includes a nonlinear optical medium, receives input of the pump light, and generates signal photons and idler photons; a first frequency filter that receives input of the signal photons and extracts signal photons with a frequency bandwidth that is less than one time the first frequency bandwidth; and a second frequency filter that receives input of the idler photons and extracts idler photons with a frequency bandwidth that is less than one time the first frequency bandwidth.

[0008] The first frequency bandwidth may be less than 40 GHz.

[0009] The pump light generating device may include an original pump light generating unit that generates original pump light having a third frequency bandwidth that includes the first peak frequency and is wider than the first frequency bandwidth, and a third frequency filter that receives input of the original pump light and extracts an optical component that includes the first peak frequency and is in the first frequency bandwidth as the pump light.

[0010] The transmittance spectra of the first frequency filter and the second frequency filter may have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width.

[0011] The transmittance spectrum of the third frequency filter may have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width.

[0012] One aspect of the present invention is a quantum entangled photon pair generating device comprising: a first pump light generating device that generates first pump light having a first peak frequency and a first frequency bandwidth; a second pump light generating device that generates second pump light having a second peak frequency different from the first peak frequency and a second frequency bandwidth; a nonlinear optical unit that receives input of the first pump light and the second pump light and generates signal photons and idler photons; a first frequency filter that receives input of the signal photons and extracts signal photons with a frequency bandwidth that is less than one time the first frequency bandwidth and the second frequency bandwidth; and a second frequency filter that receives input of the idler photons and extracts idler photons with a frequency bandwidth that is less than one time the first frequency bandwidth and the second frequency bandwidth.

[0013] The first frequency bandwidth and the second frequency bandwidth may be less than 40 GHz.

[0014] The first pump light generating device may include a first pump light generating unit that generates first pump light having a third frequency bandwidth that includes the first peak frequency and is wider than the first frequency bandwidth, and a third frequency filter that receives input of the first pump light and extracts an optical component that includes the first peak frequency and is in the first frequency bandwidth as the first pump light.The second pump light generating device may include a second pump light generating unit that generates second pump light having a fourth frequency bandwidth that includes the second peak frequency and is wider than the second frequency bandwidth, and a fourth frequency filter that receives input of the second pump light and extracts an optical component that includes the peak frequency of the second pump light and is in the second frequency bandwidth as the second pump light.

[0015] The transmittance spectra of the first frequency filter and the second frequency filter may have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width.

[0016] The transmittance spectra of the third frequency filter and the fourth frequency filter may have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width.

[0017] For all quantum entangled photon pairs that satisfy the following conditions: the frequency bandwidths through which the first frequency filter and the second frequency filter extract signal photons or idler photons are the same and are 0.42 to less than 1 times the first frequency bandwidth; and the absolute value of the difference between the first peak frequency and the peak frequency of the signal photon is 0.05 to 0.7 THz, the CAR may be 62 or more when the generation efficiency (cps) is 1000 at liquid nitrogen temperature.

[0018] The maximum CAR may be 1000 or more at liquid nitrogen temperature for all quantum entangled photon pairs that satisfy the following conditions: the frequency bandwidths through which the first frequency filter and the second frequency filter extract signal photons or idler photons are the same and are 0.27 times or more and less than 1 time the first frequency bandwidth; and the absolute value of the difference between the first peak frequency and the peak frequency of the signal photon is 0.05 THz or more and 0.2 THz or less.

[0019] One aspect of the present invention is a method for generating quantum entangled photon pairs, which comprises inputting pump light into a nonlinear optical medium to generate signal light and idler light, extracting signal photons having a frequency bandwidth that is less than one time the frequency bandwidth of the pump light using a first frequency filter, and extracting idler photons having a frequency bandwidth that is less than one time the frequency bandwidth of the pump light using a second frequency filter. [Effects of the Invention]

[0020] According to the present invention, it is possible to obtain higher quality quantum entangled photon pairs. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram of a quantum entangled photon pair generating device and a detection system according to the first embodiment. [Figure 2] FIG. 2 is a diagram illustrating an example of the power spectrum of the pump light. [Figure 3] FIG. 3 is a diagram illustrating an example of the power spectrum of the light output from the nonlinear optical unit. [Figure 4] FIG. 4 is a diagram showing an example of the power spectrum of the signal photon and the idler photon. [Figure 5] FIG. 5 is a diagram showing an example of a transmittance spectrum when the frequency filter is a transmission filter. [Figure 6] FIG. 6 is a schematic diagram of a quantum entangled photon pair generating device according to the second embodiment. [Figure 7] FIG. 7 is a diagram showing the relationship between generation efficiency and CAR in Experimental Example 1. [Figure 8] FIG. 8 is a diagram showing the relationship between generation efficiency and CAR in Experimental Example 2. [Figure 9] FIG. 9 is a diagram showing the relationship between generation efficiency and CAR in Experimental Example 3. [Figure 10] FIG. 10 is a schematic diagram of a quantum entangled photon pair generating device according to the third embodiment. [Figure 11] FIG. 11 is a diagram showing an example of the power spectra of the pump light, the signal photon, and the idler photon in the third embodiment. [Figure 12] FIG. 12 is a diagram showing another example of the power spectra of the pump light, the signal photons, and the idler photons in the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments will be described with reference to the drawings. However, the present invention is not limited to these embodiments. In addition, in the description of the drawings, identical or corresponding elements are appropriately designated by the same reference numerals, and duplicate explanations are appropriately omitted. It should be noted that the drawings are schematic, and the dimensional relationships and ratios of each element may differ from the actual ones. Even between the drawings, there may be parts in which the dimensional relationships and ratios differ from each other.

[0023] The present inventors have conducted extensive research to improve the quality of quantum entangled photon pairs and have found that the simultaneous output of Raman scattered photons caused by pump light in a nonlinear optical medium as noise photons when the quantum entangled photon pairs are output is a factor in the degradation of quality in terms of the Coincidences-to-Accidentals Ratio (CAR). The present inventors then came up with the idea that the quality of quantum entangled photon pairs can be improved by removing noise photons by extracting photons of a predetermined frequency bandwidth from each of the signal photon and idler photon that make up the quantum entangled photon pair, and have completed the present invention.

[0024] (Embodiment 1) 1 is a schematic configuration diagram of a quantum entangled photon pair generating device and detection system according to embodiment 1. A quantum entangled photon pair generating device 10 includes a pump light generating device 11, a nonlinear optical unit 12, and frequency filters 13 and 14. A detection system 100 includes photon number detectors 101 and 102, and a processing device 103.

[0025] <Configuration of quantum entangled photon pair generator 10> First, the configuration of the quantum entangled photon pair generating device 10 will be described. The pump light generating device 11 generates and outputs pulsed pump light PL1. FIG. 2 is a diagram showing an example of the power spectrum of the pump light PL1. The pump light PL1 has a peak frequency fp and a frequency bandwidth Δfp. The peak frequency fp is an optical frequency used in optical fiber communications, for example, and is, for example, 192.8 THz (1554.94 nm in wavelength). The frequency bandwidth Δfp is, for example, less than 40 GHz (for example, more than approximately 10 ps as the pulse time width). Here, the peak frequency fp is an example of a first peak frequency, and the frequency bandwidth Δfp is an example of a first frequency bandwidth. In this specification, unless otherwise specified, frequency bandwidth refers to a 3 dB bandwidth.

[0026] The pump light generating device 11 includes a light source such as, but not limited to, a fiber laser or a semiconductor laser. The method of generating pulsed light in the pump light generating device 11 is also not particularly limited, and may be, for example, a mode-locking method, a method of directly modulating a CW (Continuous Wave) laser with a pulsed driving current, or a method of inputting CW laser light into a modulator and extracting it at a predetermined duty ratio.

[0027] Returning to Fig. 1, the nonlinear optical unit 12 includes a nonlinear optical medium 12a, receives input of pump light PL1, and generates signal photons SL1 and idler photons IL1. Specifically, in the nonlinear optical unit 12, when pump light PL1 is input to the nonlinear optical medium 12a, signal photons SL1 and idler photons IL1 are generated by degenerate four-wave mixing. Degenerate four-wave mixing is a type of nonlinear optical effect.

[0028] 3 is a diagram showing an example of the power spectrum of light output from the nonlinear optical unit. Pump light PL11 is pump light PL1 after signal photon SL1 and idler photon IL1 are generated. Furthermore, the signal photon SL1 and idler photon IL1 have spectra that are approximately symmetric in frequency with respect to the pump light PL11 (pump light PL1) and have a somewhat broad bandwidth. Here, the signal photon SL1 and idler photon IL1, which are symmetric in frequency with respect to the pump light PL11 (pump light PL1), have a strong correlation and form a quantum entangled photon pair. For example, the signal photon and idler photon shown with the same pattern in FIG. 3 are symmetric in frequency with respect to the pump light PL11 (pump light PL1), and form a quantum entangled photon pair.

[0029] The configuration of the nonlinear optical unit 12 is not particularly limited. For example, when generating polarization-entangled photon pairs, a configuration in which an optical fiber loop is formed as described in Patent Document 1 can be adopted, using an optical fiber such as a dispersion-shifted optical fiber (DSF) with relatively high optical nonlinearity as the nonlinear optical medium 12a. In this case, it is preferable that the zero-dispersion wavelength of the DSF is the same as or close to the wavelength corresponding to the peak frequency fp. Furthermore, for example, when generating photon-number-related quantum entangled photon pairs, the nonlinear optical unit 12 can be configured such that, when pump light PL1 is input from one end of the DSF, pump light PL11, signal photons SL1, and idler photons IL1 are output from the other end.

[0030] 1, the frequency filter 13 is an example of a first frequency filter, and receives input of a signal photon SL1 and extracts a signal photon SL2 having a frequency bandwidth that is less than one time the frequency bandwidth Δfp. The frequency filter 13 is, for example, a transmission filter, and may be, for example, a filter typically used in optical communications, such as a dielectric multilayer filter or an AWG (Arrayed Waveguide Gratings), but is not particularly limited thereto. The frequency filter 13 may also be a reflection filter.

[0031] Furthermore, frequency filter 14 is an example of a second frequency filter, and receives input of idler photons IL2 and extracts idler photons IL2 having a frequency bandwidth that is less than 1 times the frequency bandwidth Δfp. Like frequency filter 13, frequency filter 14 is also, for example, a transmission filter, but is not particularly limited thereto.

[0032] 4 is a diagram showing an example of the power spectrum of the signal photon SL2 and the idler photon IL2. The frequency bandwidth Δfs of the signal photon SL2 is approximately the same as the frequency bandwidth (e.g., the transmission frequency bandwidth) of the frequency filter 13. Furthermore, the frequency bandwidth Δfi of the idler photon IL2 is approximately the same as the frequency bandwidth (e.g., the transmission frequency bandwidth) of the frequency filter 14. In this embodiment, Δfs=Δfi, but they do not necessarily have to be equal.

[0033] Fig. 5 is a diagram showing an example of a transmittance spectrum when the frequency filters 13 and 14 are transmission filters. In Fig. 5, the frequency filters 13 and 14 have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width.

[0034] <Configuration of detection system 100> Next, the configuration of the detection system 100 will be described. Photon number detector 101 receives signal photons SL2 and counts the number of photons. Photon number detector 102 receives idler photons IL2 and counts the number of photons. Photon number detectors 101 and 102 each output a signal including count information to processing device 103. Note that photon number detectors 101 and 102 are configured to include, for example, a photomultiplier tube or a photodiode. Processing device 103 is configured to include, for example, a computer, and processes information input from photon number detectors 101 and 102, and stores and displays the photon count results, for example.

[0035] In the quantum entangled photon pair generating device 10 configured as described above, the frequency filters 13 and 14 that extract the signal photon SL2 and the idler photon IL2 extract photons with a frequency bandwidth that is less than 1 times the frequency bandwidth Δfp of the pump light PL1, thereby eliminating noise photons. As a result, higher quality quantum entangled photon pairs can be obtained.

[0036] Furthermore, since inexpensive filters that are normally used in optical communications can be used as the frequency filters 13 and 14, higher quality quantum entangled photon pairs can be obtained at lower cost.

[0037] Furthermore, in known quantum entangled photon pair generating devices, an expensive device with a frequency bandwidth of 40 GHz or more (for example, a pulse time width of less than approximately 10 ps) is often used as a pump light generating device to obtain high-quality quantum entangled photon pairs. However, in this embodiment, high-quality quantum entangled photon pairs can be obtained using a relatively inexpensive pump light generating device 11 with a frequency bandwidth Δfp of, for example, less than 40 GHz.

[0038] Furthermore, in this embodiment, the frequency filters 13 and 14 have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width, which effectively removes noise photons such as Raman scattered photons. This is because the generation efficiency of signal light and idler light from pump light is proportional to the square of the pump light power, while the generation efficiency of Raman scattered light from pump light is proportional to the pump light power. As a result, if the frequency filters 13 and 14 have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width, photons in a frequency band in which the power of the signal light and idler light is higher than the power of the Raman scattered light can be effectively extracted.

[0039] (Embodiment 2) Fig. 6 is a schematic configuration diagram of a quantum entangled photon pair generating device according to embodiment 2. The quantum entangled photon pair generating device 10A has a configuration in which the pump light generating device 11 of the quantum entangled photon pair generating device 10 according to embodiment 1 shown in Fig. 1 is replaced with a pump light generating device 11A.

[0040] The pump light generating device 11A includes an original pump light generating unit 11A1 and a frequency filter 11A2. The original pump light generating unit 11A1 generates original pump light PL1A having a third frequency bandwidth that includes a peak frequency fp and is wider than the frequency bandwidth Δfp. The original pump light generating unit 11A1 includes a light source such as a fiber laser or a semiconductor laser. The frequency filter 11A2 is an example of a third frequency filter, and receives input of the original pump light PL1A and extracts the optical component that includes the peak frequency fp and has the frequency bandwidth Δfp as the pump light PL1. The frequency filter 11A2 is, for example, a transmission filter, similar to the frequency filters 13 and 14, but is not particularly limited thereto. Furthermore, for example, the transmittance spectrum of the frequency filter 11A2 has a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width.

[0041] The quantum entangled photon pair generating device 10A configured as described above can achieve the following effect in addition to the effect achieved by the quantum entangled photon pair generating device 10. That is, the pump light generating device 11A can output pump light PL1 of a desired frequency bandwidth by setting the frequency bandwidth of the frequency filter 11A2 from original pump light PL1A of a relatively wide frequency band such as the third frequency bandwidth.

[0042] (Experimental Example 1) A quantum entangled photon pair generator having the configuration shown in FIG. 6 was fabricated as experimental apparatus 1 for conducting experiments to verify the effects of the present invention. The peak frequency (i.e., fp) of the original pump light output from the original pump light generation unit in experimental apparatus 1 was set to 192.8 THz, and the third frequency bandwidth was set to 14.1 GHz. The pulse width was 50 ps. Furthermore, a transmission filter with a frequency bandwidth (i.e., Δfp) of 5.7 GHz and a transmission filter with a frequency bandwidth of 13.5 GHz were prepared as the third frequency filters. Furthermore, the third frequency filter used had a shape in which the 0.5 dB width of the transmittance spectrum was 0.4 times the 3 dB width. The peak power of the pump light input from the pump light generation unit to the nonlinear optical unit in experimental apparatus 1 was a minimum of 0.1 mW and a maximum of 18.5 mW.

[0043] The nonlinear optical section employed a configuration in which an optical fiber loop was formed, as in Patent Document 1, to enable the generation of polarization-entangled photon pairs. The DSF used had a length of 300 m, a nonlinear coefficient of 2.5 / m·W, and a zero-dispersion wavelength of 1561.24 nm. The polarization direction of the pump light was tilted by 45 degrees relative to the horizontal polarization when input to the nonlinear section, and horizontally polarized light was extracted. The nonlinear optical section was maintained at liquid nitrogen temperature.

[0044] Two types of transmission filters with different frequency bandwidths were prepared as the first and second frequency filters. One had a frequency bandwidth (i.e., Δfs = Δfi) of 5.7 GHz, and the other had a frequency bandwidth of 13.5 GHz. The transmission peak frequency (i.e., fs) of the first frequency filter was 193.5 THz (1549.315 nm), and the transmission peak frequency (i.e., fi) of the second frequency filter was 192.1 THz (1560.606 nm). In other words, the absolute value of the difference between the first peak frequency and the peak frequency of the signal photon was 0.7 THz. The first and second frequency filters used had a shape in which the 0.5 dB width of the transmittance spectrum was 0.4 times the 3 dB width.

[0045] In the above experimental setup 1, the combination of the first, second, and third frequency filters was changed to configure an experimental setup with Δfs / Δfp of 0.42 and 1.0. Then, using a known detection system configured as shown in Figure 1, the relationship between the generation efficiency (coincidence counts [cps]) of quantum entangled signal and idler photons and the CAR was investigated.

[0046] FIG. 7 shows the relationship between the number of coincidences and the CAR in Experimental Example 1. Table 1 shows the CAR when the number of coincidences is 1000 cps. In FIG. 7, four data points indicated by dashed lines are data when Δfp is 13.5 GHz and Δfs (=Δfi) is 13.5 GHz (i.e., Δfs / Δfp = 1, Experimental Example 1-3). Three data points indicated by dashed lines are data when Δfp is 5.7 GHz and Δfs (=Δfi) is 5.7 GHz (i.e., Δfs / Δfp = 1, Experimental Example 1-2). Five data points indicated by solid lines are data when Δfp is 13.5 GHz and Δfs (=Δfi) is 5.7 GHz (i.e., Δfs / Δfp = 0.42, Experimental Example 1-1). As shown in FIG. 7, it was confirmed that the quality was high when Δfs / Δfp was 0.42, which is less than 1. In particular, an extremely high value of 66 was obtained as the CAR at 1000 cps.

[0047] [Table 1]

[0048] Furthermore, through careful investigations, including further experiments, the inventors have confirmed that for all quantum entangled photon pairs that satisfy the conditions that Δfs = Δfi, Δfs is 0.42 times or more and less than 1 time Δfp, and the absolute value of (fp-fs) is 0.05 THz or more and 0.7 THz or less, a CAR of 62 or more can be obtained at a generation efficiency (cps) of 1000 at liquid nitrogen temperature.

[0049] Hereinafter, it will be explained using mathematical expressions that when Δfs is less than 1 time Δfp, that is, when Δfs / Δfp<1, CAR monotonically decreases.

[0050] Let p be the power of the pump light, α be the average generation rate of quantum entangled photon pairs per unit time when Δfs = Δfp multiplied by Δfp, and β be the average generation rate of Raman scattering multiplied by Δfp. Let d be the number of dark noise counts in the photon number detector for signal photons and the photon number detector for idler photons, and η be their efficiencies.

[0051] The coincidence count CC{Δfs / Δfp}, which is the number of correlated photons counted simultaneously per unit time by both photon detectors, is expressed by the following formula: CC{Δfs / Δfp}=(Δfs / Δfp)αp 2 η 2 The accidental count AC{Δfs / Δfp}, which is the number of two uncorrelated photons counted simultaneously, is expressed by the following formula: AC{Δfs / Δfp}=(((Δfs / Δfp)αp 2 +(Δfs / Δfp)βp+d / η)η) 2 The CAR {Δfs / Δfp}, an index of the quality of quantum entangled light, is expressed by the following formula: CAR{Δfs / Δfp}=CC{Δfs / Δfp} / AC{Δfs / Δfp} However, if counting of dark noises is ignored, CAR{Δfs / Δfp} is expressed by the following formula. CAR{Δfs / Δfp}=αpη / {(Δfs / Δfp)(αp+β) 2 pη+2(αp+β)d}

[0052] In the above formula, taking the first derivative of Δfs / Δfp, CAR'{Δfs / Δfp}=-α(αp+β) 2 p 2 η 2 / {(Δfs / Δfp)(αp+β) 2 pη+2(αp+β)d} 2 <0 Therefore, CAR decreases monotonically with Δfs / Δfp.

[0053] (Experimental Example 2) Experimental apparatus 2 was constructed in the same way as experimental apparatus 1. However, the nonlinear optical section was configured so that when pump light was input from one end of the DSF, pump light, signal light, and idler light were output from the other end, enabling the generation of quantum entangled photon pairs in terms of the photon number. The DSF used had a length of 300 m, a nonlinear coefficient of 2.5 / m·W, and a zero-dispersion wavelength of 1561.24 nm. Furthermore, horizontally polarized pump light was input to the nonlinear section, and horizontally polarized light was extracted. The nonlinear optical section was maintained at liquid nitrogen temperature.

[0054] Two types of transmission filters with different frequency bandwidths were prepared as the first and second frequency filters. One had a frequency bandwidth (i.e., Δfs = Δfi) of 5.7 GHz, and the other had a frequency bandwidth of 13.5 GHz. The transmission peak frequency (i.e., fs) of the first frequency filter was 193.0 THz (1553.329 nm), and the transmission peak frequency (i.e., fi) of the second frequency filter was 192.6 THz (1556.555 nm). In other words, the absolute value of the difference between the first peak frequency and the peak frequency of the signal photon was 0.2 THz. The first and second frequency filters used had a shape in which the 0.5 dB width of the transmittance spectrum was 0.4 times the 3 dB width.

[0055] In the above experimental setup 2, we changed the combination of the first, second, and third frequency filters to configure experimental setups with Δfs / Δfp of 0.42 and 1.0. Then, using a known detection system configured as shown in Figure 1, we investigated the relationship between the generation efficiency of quantum entangled signal and idler light and the CAR.

[0056] FIG. 8 shows the relationship between the number of coincidences and the CAR in Experimental Example 2. Table 2 shows the CAR when the number of coincidences is 40 cps. In FIG. 8, the data indicated by the dashed-dotted line is the data when Δfp is 13.5 GHz and Δfs (=Δfi) is 13.5 GHz (i.e., Δfs / Δfp = 1, Experimental Example 2-3). The data indicated by the dashed line is the data when Δfp is 5.7 GHz and Δfs (=Δfi) is 5.7 GHz (i.e., Δfs / Δfp = 1, Experimental Example 2-2). The data indicated by the solid line is the data when Δfp is 13.5 GHz and Δfs (=Δfi) is 5.7 GHz (i.e., Δfs / Δfp = 0.42, Experimental Example 2-1). As shown in FIG. 8, it was confirmed that the quality was high when Δfs / Δfp was 0.42, which is less than 1. In particular, the highest CAR value at 40 cps was an extremely high value of 1208, which is above 1000.

[0057] The optical losses due to the second frequency filter were 17.0 dB, 9.5 dB, and 9.7 dB in Experimental Examples 2-1, 2-2, and 2-3, respectively. That is, in Experimental Example 2-1, the highest CAR value was obtained despite the large optical loss.

[0058] [Table 2]

[0059] (Experimental Example 3) Experimental apparatus 3 was fabricated in the same manner as experimental apparatus 2. However, transmission filters with two different frequency bandwidths were prepared as the first and second frequency filters. One of the frequency bandwidths (i.e., Δfs = Δfi) was 5.7 GHz, and the other was 3.6 GHz. The transmission peak frequency (i.e., fs) of the first frequency filter was 193.0 THz (1553.329 nm), and the transmission peak frequency (i.e., fi) of the second frequency filter was 192.6 THz (1556.555 nm). In other words, the absolute value of the difference between the first peak frequency and the peak frequency of the signal photon was 0.2 THz. In addition, the first and second frequency filters used had a shape in which the 0.5 dB width of the transmittance spectrum was 0.4 times the 3 dB width.

[0060] In the above experimental setup 3, experimental setups with Δfs / Δfp of 0.42 and 0.27 were constructed by changing the combination of the first, second, and third frequency filters. Then, using a known detection system configured as shown in Figure 1, the relationship between the generation efficiency of the quantum entangled signal light and idler light and the CAR was investigated.

[0061] FIG. 9 shows the relationship between the number of coincidences and the CAR in Experimental Example 3. Table 3 shows the maximum CAR values. The data shown by the dashed line in FIG. 9 is the data when Δfp is 13.5 GHz and Δfs (=Δfi) is 5.7 GHz (i.e., Δfs / Δfp = 0.42, Experimental Example 3-2). The data shown by the solid line is the data when Δfp is 13.5 GHz and Δfs (=Δfi) is 3.6 GHz (i.e., Δfs / Δfp = 0.27, Experimental Example 3-1). As shown in FIG. 9, in both cases, an extremely high maximum CAR value of over 1000 was obtained. In particular, it was confirmed that the quality was higher (CAR = 1879) when Δfs / Δfp was 0.27 than when it was 0.42.

[0062] The optical losses due to the second frequency filter were 18.5 dB and 13.2 dB in Experimental Examples 3-1 and 3-2, respectively. That is, in Experimental Example 3-1, a high CAR value was obtained despite the large optical loss.

[0063] [Table 3]

[0064] Furthermore, through careful investigations, including further experiments, the inventors have confirmed that for all quantum entangled photon pairs that satisfy the conditions that Δfs = Δfi, Δfs is 0.27 times or more and less than 1 time Δfp, and the absolute value of (fp-fs) is 0.05 THz or more and 0.2 THz or less, the maximum CAR value is 1000 or more at liquid nitrogen temperature.

[0065] However, as Δfs / Δfp approaches 0, the rate of decline in generation efficiency becomes larger relative to the rate of increase in CAR. For example, as can be seen from Figure 9, the maximum CAR value when Δfs / Δfp is 0.27 is about 10% higher than the maximum CAR value when Δfs / Δfp is 0.42, but the generation efficiency at that time is about 40% lower. Because the generation efficiency drops even further relative to the rate of increase in CAR when Δfs / Δfp is less than 0.27, it is preferable that Δfs / Δfp be 0.27 or higher.

[0066] (Embodiment 3) 10 is a schematic diagram of a quantum entangled photon pair generating device according to embodiment 3. The quantum entangled photon pair generating device 10B has a configuration in which the pump light generating device 11 of the quantum entangled photon pair generating device 10 according to embodiment 1 shown in FIG. 1 is replaced with pump light generating devices 11Ba and 11Bb.

[0067] The pump light generating device 11Ba generates and outputs pulsed pump light PL1Ba. The pump light PL1Ba has a peak frequency fpBa and a frequency bandwidth ΔfpBa as a bandwidth. The pump light generating device 11Bb generates and outputs pulsed pump light PL1Bb. The pump light PL1Bb has a peak frequency fpBb and a frequency bandwidth ΔfpBb as a bandwidth. Here, the peak frequency fpBa is different from the peak frequency fpBb. Furthermore, the frequency bandwidth ΔfpBa and the frequency bandwidth ΔfpBb are the same, but may be different. The frequency bandwidth ΔfpBa and the frequency bandwidth ΔfpBb are, for example, less than 40 GHz.

[0068] Here, the pump light generating device 11Ba is an example of a first pump light generating device, and the pump light PL1Ba is an example of the first pump light. The peak frequency fpBa is an example of a first peak frequency, and the frequency bandwidth ΔfpBa is an example of a first frequency bandwidth. The pump light generating device 11Bb is an example of a second pump light generating device, and the pump light PL1Bb is an example of the second pump light. The peak frequency fpBb is an example of a second peak frequency, and the frequency bandwidth ΔfpBa is an example of a second frequency bandwidth.

[0069] The nonlinear optical unit 12 receives the pump light PL1Ba and the pump light PL1Bb as input and generates a signal photon SL1B and an idler photon IL1B. Specifically, when the pump light PL1Ba and the pump light PL1Bb are input to the nonlinear optical medium 12a in the nonlinear optical unit 12, the signal photon SL1B and the idler photon IL1B are generated by nondegenerate four-wave mixing. Nondegenerate four-wave mixing is a type of nonlinear optical effect.

[0070] The frequency filter 13 receives the input of the signal photon SL1B and extracts the signal photon SL2B with a frequency bandwidth ΔfsB that is less than one times the frequency bandwidths ΔfpBa and ΔfpBb. Also, the frequency filter 14 receives the input of the idler photon IL1B and extracts the idler photon IL2B with a frequency bandwidth ΔfiB that is less than one times the frequency bandwidths ΔfpBa and ΔfpBb.

[0071] FIG. 11 is a diagram showing an example of the power spectra of the pump lights PL1Ba and PL1Bb, the signal photon SL2B, and the idler photon IL2B in Embodiment 3. Assuming that the peak frequency of the signal photon SL2B is fsB and the peak frequency of the idler photon IL2B is fiB, fpBa + fpBb = fsB + fiB holds. Also, the example shown in FIG. 10 is a case where the difference between fpBa and fp2Bb is relatively small, and fiB < fpBb < fpBa < fsB holds.

[0072] However, the arrangement of the power spectra of the pump lights PL1Ba and PL1Bb, the signal photon SL2B, and the idler photon IL2B in Embodiment 3 is not limited to that shown in FIG. 11. FIG. 12 is a diagram showing another example of the power spectra of the pump lights PL1Ba and PL1Bb, the signal photon SL2B, and the idler photon IL2B in Embodiment 3. In the example shown in FIG. 11, fpBa + fpBb = fsB + fiB holds as in the example shown in FIG. 11, but different from the example shown in FIG. 11, fpBb < fiB < fsB < fpBa holds.

[0073] Even in the quantum entanglement photon pair generation device 10B configured as described above, the same effects as those of the quantum entanglement photon pair generation device 10 can be obtained.

[0074] In the quantum entangled photon pair generating device 10B, the pump light generating devices 11Ba and 11Bb may be replaced with a pump light generating device such as the pump light generating device 11A of the quantum entangled photon pair generating device 10A according to the second embodiment. In this case, the first pump light generating device replacing the pump light generating device 11Ba includes, for example, a first original pump light generating unit and a third frequency filter. The first original pump light generating unit generates first original pump light having a third frequency bandwidth that includes frequency fpBa (an example of a first peak frequency) and is wider than frequency bandwidth ΔfpBa (an example of a first frequency bandwidth). The third frequency filter receives input of the first original pump light and extracts a light component that includes frequency fpBa and has frequency bandwidth ΔfpBa as first pump light PL1Ba. In addition, the second pump light generating device replacing the pump light generating device 11Bb includes, for example, a second original pump light generating unit and a fourth frequency filter. The second pump light generating unit generates first pump light having a fourth frequency bandwidth that includes frequency fpBb (an example of a second peak frequency) and is wider than frequency bandwidth ΔfpBb (an example of a second frequency bandwidth). The fourth frequency filter receives input of the second pump light and extracts an optical component that includes frequency fpBb and has frequency bandwidth ΔfpBb as second pump light PL1Bb. This provides the same effects as in the second embodiment.

[0075] Furthermore, in quantum entangled photon pair generating device 10B, for all entangled photon pairs that satisfy the conditions that ΔfsB and ΔfiB are the same and are 0.42 to less than 1 times ΔfpBa and ΔfpBb, and the absolute value of the difference between fpBa and fsB is 0.05 to 0.7 THz, the CAR can be made 62 or more when the generation efficiency (cps) is 1000 at liquid nitrogen temperature. Furthermore, for all entangled photon pairs that satisfy the conditions that ΔfsB and ΔfiB are the same and are 0.27 to less than 1 times ΔfpBa and ΔfpBb, and the absolute value of the difference between fpBa and fsB is 0.05 to 0.2 THz, the maximum CAR can be made 1000 or more at liquid nitrogen temperature.

[0076] In addition, in any of the above embodiments, a method for generating quantum entangled photon pairs can be performed by inputting pump light into a nonlinear optical medium to generate signal light and idler light, extracting signal photons with a frequency bandwidth that is less than one time the frequency bandwidth of the pump light using a first frequency filter, and extracting idler photons with a frequency bandwidth that is less than one time the frequency bandwidth of the pump light using a second frequency filter.

[0077] Furthermore, the present invention is not limited to the above-described embodiments. The present invention also includes configurations in which the above-described components are appropriately combined. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible. [Explanation of symbols]

[0078] 10, 10A, 10B: Photon pair generator 11, 11A, 11Ba, 11Bb: Pump light generating device 11A1: Original pump light generation unit 12: Nonlinear optical section 12a: Nonlinear optical medium 11A2, 13, 14: Frequency filters 100: Detection system 101, 102: Photon number detector 103: Processing equipment IL1, IL1B, IL2, IL2B: Idler photon PL1, PL11, PL1Ba, PL1Bb: pump light PL1A: Original pump light SL1, SL1B, SL2, SL2B: signal photons

Claims

1. a pump light generating device that generates pulsed pump light having a first peak frequency and a first frequency bandwidth; a nonlinear optical section including a nonlinear optical medium, receiving the pump light and generating signal photons and idler photons; a first frequency filter that receives the signal photons and extracts signal photons having a frequency bandwidth that is less than one time the first frequency bandwidth; a second frequency filter that receives the idler photons and filters out idler photons having a frequency bandwidth that is less than one time the first frequency bandwidth; Equipped with Quantum entangled photon pair generator.

2. The first frequency bandwidth is less than 40 GHz. The quantum entangled photon pair generating device according to claim 1 .

3. the pump light generating device includes an original pump light generating unit that generates original pump light having a third frequency bandwidth that includes the first peak frequency and is wider than the first frequency bandwidth; a third frequency filter that receives the original pump light and extracts, as the pump light, an optical component that includes the first peak frequency and is in the first frequency bandwidth; Equipped with The quantum entangled photon pair generating device according to claim 1 .

4. The transmittance spectra of the first frequency filter and the second frequency filter have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width. The quantum entangled photon pair generating device according to claim 1 .

5. The transmittance spectrum of the third frequency filter has a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width. The quantum entangled photon pair generating device according to claim 3 .

6. a first pump light generating device that generates first pump light having a first peak frequency and a first frequency bandwidth; a second pump light generating device that generates second pump light having a second peak frequency different from the first peak frequency and having a second frequency bandwidth; a nonlinear optical unit that receives the first pump light and the second pump light and generates signal photons and idler photons; a first frequency filter that receives the signal photons and extracts signal photons having a frequency bandwidth that is less than one time the first frequency bandwidth and the second frequency bandwidth; a second frequency filter that receives the idler photons and filters out idler photons having a frequency bandwidth that is less than one time the first frequency bandwidth and the second frequency bandwidth; Equipped with Quantum entangled photon pair generator.

7. The first frequency bandwidth and the second frequency bandwidth are less than 40 GHz. The quantum entangled photon pair generating device according to claim 6.

8. the first pump light generating device includes: a first pump light generating unit that generates first pump light having a third frequency bandwidth that includes the first peak frequency and is wider than the first frequency bandwidth; and a third frequency filter that receives input of the first pump light and extracts an optical component that includes the first peak frequency and is in the first frequency bandwidth as the first pump light; The second pump light generating device includes: a second pump light generating unit that generates second pump light having a fourth frequency bandwidth that includes the second peak frequency and is wider than the second frequency bandwidth; and a fourth frequency filter that receives input of the second pump light and extracts an optical component that includes the peak frequency of the second pump light and has the second frequency bandwidth as the second pump light. The quantum entangled photon pair generating device according to claim 6.

9. The transmittance spectra of the first frequency filter and the second frequency filter have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width. The quantum entangled photon pair generating device according to claim 6.

10. The transmittance spectra of the third frequency filter and the fourth frequency filter have a shape in which the 0.5 dB width is less than 0.6 times the 3 dB width. The quantum entangled photon pair generating device according to claim 8.

11. the first frequency filter and the second frequency filter have the same frequency bandwidth for extracting signal photons or idler photons, and the frequency bandwidth is 0.42 times or more and less than 1 time the first frequency bandwidth; For all quantum entangled photon pairs that satisfy the condition that the absolute value of the difference between the first peak frequency and the peak frequency of the signal photon is 0.05 THz or more and 0.7 THz or less, the CAR is 62 or more when the generation efficiency (cps) is 1000 at liquid nitrogen temperature. The quantum entangled photon pair generating device according to any one of claims 1 to 9.

12. the frequency bandwidths through which the first frequency filter and the second frequency filter extract signal photons or idler photons are the same and are 0.27 times or more and less than 1 time the first frequency bandwidth, For all quantum entangled photon pairs that satisfy the condition that the absolute value of the difference between the first peak frequency and the peak frequency of the signal photon is 0.05 THz or more and 0.2 THz or less, the maximum CAR value is 1000 or more at liquid nitrogen temperature. The quantum entangled photon pair generating device according to any one of claims 1 to 9.

13. Pump light is input to the nonlinear optical medium to generate signal light and idler light; a first frequency filter extracting signal photons having a frequency bandwidth that is less than one time the frequency bandwidth of the pump light; a second frequency filter extracting idler photons having a frequency bandwidth that is less than one time the frequency bandwidth of the pump light; A method for generating quantum entangled photon pairs.

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