Quantum wavelength converter, single photon source with herald

Abstract

To achieve a quantum wavelength converter easily connected with an optical fiber.SOLUTION: A quantum wavelength converter (2) includes: an optical fiber (8) including a plurality of resonators (34, 36, 38 and 40) having a common resonant optical path in at least a part; and a laser beam source (6) connected with the optical fiber and for inputting pump light having at least one or more frequencies into the optical fiber. The plurality of resonators satisfy a four light wave mixing process in the common resonant optical path or the energy conservation law of parametric conversion and phase matching conditions.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to quantum wavelength converters and heralded single photon sources comprising such quantum wavelength converters. [Background technology]

[0002] Quantum wavelength conversion techniques that convert the wavelength of photons are generally known to be realized by nonlinear optical processes, including parametric down-conversion and four-wave mixing. These nonlinear optical processes are also used to generate photon pairs, and are important as the operating principle of heralded single-photon sources.

[0003] Non-Patent Documents 1 and 2 disclose a method for realizing the above-mentioned quantum wavelength conversion using a nonlinear optical crystal bulk such as a KTP crystal. Non-Patent Documents 3 and 4 disclose a method for realizing the above-mentioned quantum wavelength conversion using a nonlinear optical crystal waveguide such as a PPLN waveguide. Furthermore, Non-Patent Documents 5 and 6 disclose a method for realizing the above-mentioned quantum wavelength conversion using a ring-type microcavity on a chip. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] J. Huang and P. Kumar, “Observation of quantum frequency conversion” Phys. Rev. Lett. 68, 2153 (1992). [Non-patent document 2] ZY Ou and L. Madel, “Violation of Bell's Inequality and Classical Probability in a Two-Photon Correlation Experiment” Phys. Rev. Lett. 61, 50 (1988). [Non-patent document 3] B. Albrecht et al., “A waveguide frequency converter connecting rubidium-based quantum memories to the telecom C-band” Nature Commun. 5, 3376 (2014). [Non-patent document 4] M. Bock et al., “Highly efficient heralded single-photon source for telecom wavelengths based on a PPLN waveguide” Opt. Express 24, 23992 (2016). [Non-patent document 5] A. Singh et al., “Quantum frequency conversion of a quantum dot single-photon source on a nanophotonic chip” Optica 6, 563 (2019). [Non-patent document 6] R. Wakabayashi et al., “Time-bin entangled photon pair generation from Si micro-ring resonator” Opt. Express 23, 1103 (2015). Summary of the Invention [Problem to be solved by the invention]

[0005] The nonlinear optical crystal bulk, nonlinear optical crystal waveguide, and ring-type microresonator used in the quantum wavelength conversion techniques described in Non-Patent Documents 1 to 6 are all difficult to couple to optical fibers. Therefore, when a quantum wavelength converter using the techniques described in Non-Patent Documents 1 to 6 is introduced into a device that uses optical fiber as a quantum channel for propagating photons, a mechanism is required to couple the optical fiber to the quantum wavelength converter. Therefore, the quantum wavelength converter increases the complexity of the device that includes the quantum wavelength converter. [Means for solving the problem]

[0006] In order to solve the above-described problems, a quantum wavelength converter according to one aspect of the present disclosure includes an optical fiber including therein a plurality of resonators having at least a portion of a common resonant optical path; and a laser light source connected to the optical fiber and configured to input pump light having at least one frequency into the optical fiber, wherein the plurality of resonators include a first resonator having a first resonant frequency ω1 as a resonant frequency, a second resonator having a second resonant frequency ω2 as a resonant frequency, a third resonator having a third resonant frequency ω3 as a resonant frequency, and a fourth resonator having a fourth resonant frequency ω4 different from any of the first resonant frequency ω1, the second resonant frequency ω2, and the third resonant frequency ω3; i The i-th resonant frequency ω i is the effective refractive index within the optical fiber for light of (a), c is the speed of light, γ is the nonlinear parameter of the optical fiber, and P is the intensity of the pump light.

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[0007] A quantum wavelength converter according to another aspect of the present disclosure includes an optical fiber including a plurality of resonators therein, each having at least a portion of a common resonant optical path; and a laser light source connected to the optical fiber and configured to input pump light into the optical fiber, wherein the plurality of resonators include a first resonator having a first resonant frequency ω1, a third resonator having a third resonant frequency ω3, and a fourth resonator having a fourth resonant frequency ω4 different from both the first resonant frequency ω1 and the third resonant frequency ω3; i The i-th resonant frequency ω i is the effective refractive index within the optical fiber for light of (a), c is the speed of light, γ is the nonlinear parameter of the optical fiber, and P is the intensity of the pump light.

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[0008] A quantum wavelength converter according to another aspect of the present disclosure includes an optical fiber including a plurality of resonators therein, each having at least a portion of a common resonant optical path; and a laser light source connected to the optical fiber and configured to input pump light into the optical fiber, wherein the plurality of resonators include a first resonator having a first resonant frequency ω1 as a resonant frequency, a second resonator having a second resonant frequency ω2 as a resonant frequency, and a fourth resonator having a fourth resonant frequency ω4 as a resonant frequency that is different from both the first resonant frequency ω1 and the second resonant frequency ω2; i The i-th resonant frequency ω i is the effective refractive index within the optical fiber for light of (a), c is the speed of light, γ is the nonlinear parameter of the optical fiber, and P is the intensity of the pump light.

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[0009] According to one aspect of the present disclosure, a quantum wavelength converter is achieved that can be easily coupled to an optical fiber, and when introduced into a quantum device using an optical fiber, can reduce the complexity of the quantum device. [Brief explanation of the drawings]

[0010] [Figure 1] 1A and 1B are a schematic plan view and a schematic enlarged view of a quantum wavelength converter according to a first embodiment of the present disclosure, respectively; [Figure 2] FIG. 1 is a schematic enlarged view of a single-photon source according to a first embodiment of the present disclosure. [Figure 3]1A and 1B are schematic diagrams showing input and output of photons in a resonator structure according to a first embodiment of the present disclosure, and an energy diagram showing energy transition in the resonator structure; [Figure 4] 10A and 10B are a schematic plan view and a schematic enlarged view of a quantum wavelength converter according to a second embodiment of the present disclosure, respectively; [Figure 5] 10A and 10B are schematic diagrams showing input and output of photons in a resonator structure according to a second embodiment of the present disclosure, and an energy diagram showing energy transition in the resonator structure. [Figure 6] 10A and 10B are a schematic plan view and a schematic enlarged view of a quantum wavelength converter according to a third embodiment of the present disclosure, respectively; [Figure 7] 10A and 10B are schematic diagrams showing input and output of photons in a resonator structure according to a third embodiment of the present disclosure, and an energy diagram showing energy transition in the resonator structure. [Figure 8] 10A and 10B are a schematic plan view and a schematic enlarged view of a quantum wavelength converter according to a fourth embodiment of the present disclosure, respectively; [Figure 9] 10A and 10B are schematic diagrams showing input and output of photons in a resonator structure according to a fourth embodiment of the present disclosure, and an energy diagram showing energy transition in the resonator structure. [Figure 10] 10A and 10B are a schematic plan view and a schematic enlarged view of a quantum wavelength converter according to a fifth embodiment of the present disclosure, respectively; [Figure 11] 10A and 10B are schematic diagrams showing input and output of photons in a resonator structure according to a fifth embodiment of the present disclosure, and an energy diagram showing energy transition in the resonator structure. [Figure 12] FIG. 10 is a schematic plan view of a heralded single-photon source according to a sixth embodiment of the present disclosure. [Figure 13] 10A and 10B are schematic diagrams showing input and output of photons in a resonator structure according to a sixth embodiment of the present disclosure, and an energy diagram showing energy transition in the resonator structure. [Figure 14] FIG. 10 is a schematic plan view of a heralded single-photon source according to a seventh embodiment of the present disclosure. [Figure 15]10A and 10B are schematic diagrams showing input and output of photons in a resonator structure according to a seventh embodiment of the present disclosure, and an energy diagram showing energy transition in the resonator structure. [Figure 16] FIG. 10 is a schematic plan view of a heralded single-photon source according to an eighth embodiment of the present disclosure. [Figure 17] 13A and 13B are schematic diagrams showing input and output of photons in a resonator structure according to an eighth embodiment of the present disclosure, and an energy diagram showing energy transition in the resonator structure. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Embodiment 1] <Quantum wavelength converter> The quantum wavelength converter according to the present embodiment is a device for converting the wavelength of a single photon, for example, by converting the wavelength of an input single photon having a specific wavelength and outputting a single photon having a wavelength different from the input wavelength. In particular, the quantum wavelength converter according to the present embodiment is a device for inputting a single photon from a single-photon source and pump light from a laser light source into a resonator structure including multiple resonators having a common resonant optical path, and outputting a wavelength-converted single photon from the resonator structure.

[0012] A quantum wavelength converter according to this embodiment and a resonator structure included in the quantum wavelength converter will be described with reference to Fig. 1. Fig. 1 is a schematic plan view of a quantum wavelength converter 2 according to this embodiment, and an enlarged schematic view of a resonator structure 4 included in the quantum wavelength converter 2 in the schematic plan view.

[0013] As shown in Fig. 1, the quantum wavelength converter 2 includes a resonator structure 4, a laser light source 6, and a single-mode optical fiber 8. The quantum wavelength converter 2 further includes a single-photon source 10, a dichroic mirror 12, and an optical terminator 14. Fig. 1 also shows a quantum device X to which wavelength-converted single photons generated by the quantum wavelength converter 2 using a method described below are input. The quantum wavelength converter 2 of this embodiment will be described in detail later, but will be described by taking as an example a quantum wavelength converter that converts a single photon input from the single-photon source 10 to the resonator structure 4 into a single photon of a wavelength required by the quantum device X.

[0014] <Resonator structure> 1, the resonator structure 4 includes a nano-optical fiber portion 16 and both end portions 8A connected to the nano-optical fiber portion 16 via tapered portions 18. Here, for example, both end portions 8A are parts of the optical fiber 8 included in the quantum wavelength converter 2. The nano-optical fiber portion 16 is formed in a heated portion of the optical fiber 8 by, for example, heating the portion using one of various heating methods including a ceramic heater or an oxygen-water flame, and then pulling the heated portion from both ends. In other words, the resonator structure 4 includes the nano-optical fiber portion 16 and both end portions 8A as parts of the optical fiber 8.

[0015] The resonator structure 4 further includes a common FBG (fiber Bragg grating) 20, a first FBG 22, a second FBG 24, a third FBG 26, and a fourth FBG 28 inside both end portions 8A or the nano-optical fiber portion 16. In particular, in this embodiment, of the core portion 30 through which photons propagate and the cladding portion 32 around the core portion 30 included in both end portions 8A, the above-mentioned FBGs are formed in the core portion 30.

[0016] The common FBG 20, the first FBG 22, the second FBG 24, the third FBG 26, and the fourth FBG 28 reflect a portion of photons having a specific frequency. In this specification, the first FBG 22 reflects a portion of photons having the first resonant frequency ω1, the second FBG 24 reflects a portion of photons having the second resonant frequency ω2, the third FBG 26 reflects a portion of photons having the third resonant frequency ω3, and the fourth FBG 28 reflects a portion of photons having the fourth resonant frequency ω4. In addition, the common FBG 20 reflects a portion of photons having any of the first resonant frequency ω1, the second resonant frequency ω2, the third resonant frequency ω3, and the fourth resonant frequency ω4.

[0017] In this embodiment, the first resonant frequency ω1, the second resonant frequency ω2, the third resonant frequency ω3, and the fourth resonant frequency ω4 are all different from one another. In other words, in this embodiment, at least the fourth resonant frequency ω4 is different from all of the first resonant frequency ω1, the second resonant frequency ω2, and the third resonant frequency ω3.

[0018] In this embodiment, a common FBG 20 is formed on one of the two end portions 8A formed on both ends of the nanooptical fiber portion 16, and a first FBG 22, a second FBG 24, a third FBG 26, and a fourth FBG 28 are formed on the other. Therefore, a common optical path including at least a propagation path of photons of the nanooptical fiber portion 16 is formed between the common FBG 20 and each of the first FBG 22, the second FBG 24, the third FBG 26, and the fourth FBG 28.

[0019] Therefore, in the resonator structure 4, a first resonator 34, a second resonator 36, a third resonator 38, and a fourth resonator 40 are formed by the common FBG 20 and the first FBG 22, the second FBG 24, the third FBG 26, and the fourth FBG 28, respectively. In this embodiment, the first resonator 34, the second resonator 36, the third resonator 38, and the fourth resonator 40 have a first resonant frequency ω1, a second resonant frequency ω2, a third resonant frequency ω3, and a fourth resonant frequency ω4 as their resonant frequencies, respectively. In other words, in each resonator included in the resonator structure 4, photons having the corresponding resonant frequency resonate.

[0020] In particular, the first resonator 34, the second resonator 36, the third resonator 38, and the fourth resonator 40 have, as a common resonant optical path, the photon propagation path of the nano-optical fiber portion 16, which is at least a part of the optical fiber 8. In other words, the resonator structure 4 includes a plurality of resonators, at least some of which have a common resonant optical path.

[0021] Furthermore, each of the plurality of resonators has a pair of fiber Bragg gratings whose reflection bands include the respective resonant frequencies. In particular, the common FBG 20 is formed as the same fiber Bragg grating between the plurality of resonators. Note that at least one of the fiber Bragg gratings of the plurality of resonators included in the resonator structure 4 may be the common FBG 20.

[0022] In this embodiment, as described above, the first FBG 22, the second FBG 24, the third FBG 26, and the fourth FBG 28 are individually formed at one of the two end portions 8A. Therefore, in this embodiment, the resonator lengths L1, L2, L3, and L4 of the first resonator 34, the second resonator 36, the third resonator 38, and the fourth resonator 40, respectively, can be designed independently.

[0023] In this embodiment, the first FBG 22, the second FBG 24, the third FBG 26, and the fourth FBG 28 are formed individually, but this is not limited to this. For example, in this embodiment, a continuous fiber Bragg grating may be formed at one of the end portions 8A, and chirps may be formed at each of multiple positions of the fiber Bragg grating. As a result, in this embodiment, each resonator may be formed by reflecting photons of a specific wavelength at the position where each chirp of the fiber Bragg grating is formed.

[0024] <Laser light source, single photon source> Returning to the plan view of the quantum wavelength converter 2 in FIG. 1 , the laser light source 6 is a light source for inputting pump light having at least one frequency into each resonator of the resonator structure 4 via an optical fiber 8. For example, the laser light source 6 may generate pump light including multiple frequency bands, or may generate multiple pump light including a specific frequency. In particular, in this embodiment, the laser light source 6 generates a first pump light PL1 having a second resonant frequency ω2 and a second pump light PL2 having a third resonant frequency ω3, and inputs the first pump light PL1 and the second pump light PL2 into each resonator of the resonator structure 4.

[0025] Next, the single-photon source 10 will be described in detail with reference to Fig. 2. Fig. 2 is an enlarged schematic diagram of the single-photon source 10 included in the quantum wavelength converter 2, which is part of the schematic plan view of the quantum wavelength converter 2 shown in Fig. 1. As shown in Fig. 2, the single-photon source 10 includes, among the components included in the resonator structure 4, a nano-optical fiber portion 16 and both end portions 8A connected to the nano-optical fiber portion 16 via tapered portions 18.

[0026] However, the single-photon source 10 includes a common FBG 20 in the core 30 at one end 8A of each fiber Bragg grating included in the resonator structure 4, and includes only a first FBG 22 in the core 30 at the other end 8A. Therefore, the single-photon source 10 includes only the first resonator 34 of each resonator included in the resonator structure 4.

[0027] Furthermore, the single-photon source 10 includes a quantum system 42 formed on the nano-optical fiber portion 16. The quantum system 42 includes at least a ground level and an excited level that is a level higher than the ground level, and includes, for example, atoms, ions, diamonds having nitrogen defects, quantum dots, etc. In this embodiment, the level difference between the ground level and the excited level of the quantum system 42 corresponds to the energy of a photon having a first resonant frequency ω1.

[0028] The single-photon source 10 may generate a single photon having a first resonant frequency ω1, for example, by using a state transition between the ground state and the excited state of the quantum system 42. In this case, the single-photon source 10 may include, for example, a laser light source (not shown) capable of irradiating the quantum system 42 with control light that causes a state transition between the ground state and the excited state of the quantum system 42.

[0029] The single-photon source 10 may generate a single photon having the first resonant frequency ω1 in the first resonator 34, for example, based on the Purcell effect, in which spontaneous emission of a single photon accompanying a state transition between a ground state and an excited state is emphasized by coupling between the quantum system 42 and the first resonator 34. In other words, the single-photon source 10 may generate a single photon by utilizing the phenomenon in which the state of the quantum system 42 is excited by the control light from the laser light source described above, and a single photon is generated when the state returns to the ground state.

[0030] Alternatively, the single-photon source 10 may generate single photons in the first resonator 34 by irradiating the quantum system 42 with control light whose amplitude gradually increases from 0. In this case, the waveform of the single photon can be controlled by controlling the change in the amplitude of the control light over time.

[0031] 1 , the single-photon source 10 is connected to the resonator structure 4 via an optical fiber 8. Therefore, the single-photon source 10 inputs the generated single photons having the first resonant frequency ω1 as an input single-photon IF to each resonator of the resonator structure 4. The quantum wavelength converter 2 may include a combiner 44 for mixing the first pump light PL1 and the second pump light PL2 from the laser light source 6 and the input single-photon IF from the single-photon source 10 and inputting the mixed light into each resonator of the resonator structure 4.

[0032] <Four-wave mixing process> As described above, in this embodiment, the first pump light PL1 and the second pump light PL2 from the laser light source 6 and the input single-photon IF from the single-photon source 10 are input to each resonator of the resonator structure 4. In this embodiment, the resonator structure 4 generates output photons OF having a fourth resonant frequency ω4 through a four-wave mixing process of the input first pump light PL1, second pump light PL2, and input single-photon IF. In this embodiment, the output photons OF are single photons.

[0033] The generation of output photons OF by the resonator structure 4 will be described in more detail with reference to Fig. 3. Fig. 3 shows a schematic diagram 4A for explaining the input and output of photons in the resonator structure 4 and an energy diagram D1 showing state transitions occurring in the resonator structure 4.

[0034] 3, a single input photon IF having a first resonant frequency ω1, a first pump light PL1 having a second resonant frequency ω2, and a second pump light PL2 having a third resonant frequency ω3 are input to the resonator structure 4. As a result, an output photon OF having a fourth resonant frequency ω4 is output from the resonator structure 4 as shown in schematic diagram 4A by a method described below.

[0035] In this embodiment, the following equations (1) and (2) hold for the first resonance frequency ω1, the second resonance frequency ω2, the third resonance frequency ω3, and the fourth resonance frequency ω4.

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[0036] Here, in the nano-optical fiber portion 16 included in the common optical path of each resonator of the resonator structure 4, a ground level g and an excitation level e are virtually set as shown in the energy diagram D1 of Fig. 3. The energy difference between the ground level g and the excitation level e is set to be approximately the same as the sum of the energy of a photon having the first resonant frequency ω1 and the energy of a photon having the second resonant frequency ω2. Note that, since ω1 + ω2 = ω3 + ω4 holds from the above formula (1), the energy difference between the ground level g and the excitation level e is also approximately the same as the sum of the energy of a photon having the third resonant frequency ω3 and the energy of a photon having the fourth resonant frequency ω4.

[0037] When the above two equations are satisfied, the single input photon IF, the first pump light PL1, and the second pump light PL2 cause the state transitions shown in energy diagram D1 in the nano optical fiber section 16. Specifically, the single input photon IF and the first pump light PL1 cause excitation from the ground level g to the excited level e. In addition, in conjunction with this excitation, the second pump light PL2 causes a transition from the excited level e to an intermediate level between the two levels. As a result, an output photon OF having a fourth resonant frequency ω4 is generated, with energy corresponding to the difference between the energy of the second pump light PL2 and the energy difference between the ground level g and the excited level e.

[0038] As a result, a four-wave mixing process occurs in the nano-optical fiber portion 16. In other words, each resonator of the resonator structure 4 according to this embodiment satisfies the law of energy conservation and the phase matching condition of the four-wave mixing process in the nano-optical fiber portion 16, which is a common resonant optical path. Here, of the above two equations, equation (1) represents the law of energy conservation of the four-wave mixing process, and equation (2) represents the phase matching condition of the four-wave mixing process.

[0039] Here, each resonator of the resonator structure 4, which includes the nano-optical fiber portion 16 as a common resonant optical path, couples with light having the first resonant frequency ω1, the second resonant frequency ω2, the third resonant frequency ω3, and the fourth resonant frequency ω4. Therefore, each resonator of the resonator structure 4 promotes the above-mentioned four-wave mixing process in the nano-optical fiber portion 16, and promotes the generation of output photons OF in the resonator structure 4.

[0040] <Free Spectral Range Design> Here, the resonance condition of each resonator of the resonator structure 4 is (n i ω i ·Li / c)+φ i =m i In the resonance condition equation, c is the speed of light, n i is the group refractive index in the i-th resonator, Li is the resonator length of the i-th resonator, φ i is the i-th resonance frequency ω i the phase change of a photon in a fiber Bragg grating that reflects the photon, m i is a natural number. In general, n i and φ i Therefore, when the resonator length Li is the same and the dispersion of the fiber Bragg grating included in the resonator structure 4 is ignored, the i-th resonant frequency ω that satisfies the resonance condition of each resonator of the resonator structure 4 is i In this case, the above formula (2) does not generally hold.

[0041] Therefore, in this embodiment, the resonator length Li of each resonator in the resonator structure 4 is designed individually, or each resonator is designed taking into consideration the dispersion added by the fiber Bragg grating of each resonator. As a result, in this embodiment, each resonator in the resonator structure 4 is designed so that the above formula (2) holds. For example, as described above, by individually forming the first FBG 22, the second FBG 24, the third FBG 26, and the fourth FBG 28 for the common FBG 20, it becomes possible to individually design the resonator length Li of each resonator.

[0042] As a result, in each resonator, the resonance condition (ni ω i ·Li / c)+φ i =m i The resonator lengths Li that satisfy π can be individually designed. Note that, as long as the above formula (2) is satisfied, the resonator lengths Li of the resonators included in the resonator structure 4 are not particularly limited. Therefore, the magnitude relationship between the resonator lengths Li of the resonators included in the resonator structure 4 is not limited to the magnitude relationship shown in FIG. 1 etc.

[0043] Therefore, in the resonator structure 4 according to this embodiment, it is easy to design each resonator so as to satisfy the above formula (2). Furthermore, in the resonator structure 4, one of the pair of fiber Bragg gratings of each resonator is formed individually, so that the resonator length Li of each resonator can be independently stabilized against disturbances such as temperature changes or vibrations.

[0044] <Dichroic mirror, optical terminator> As a result, the resonator structure 4 generates output photons OF having a fourth resonant frequency ω4. Referring back to FIG. 1, the output photons OF output from the resonator structure 4 are incident on the dichroic mirror 12 via the optical fiber 8. However, the resonator structure 4 also emits a single input photon IF from the single-photon source 10, and the first pump light PL1 and second pump light PL2 from the laser light source 6, which are incident on the dichroic mirror 12.

[0045] Dichroic mirror 12 is a mirror that transmits or reflects photons of a specific frequency. In this embodiment, dichroic mirror 12 is installed so that transmitted photons enter quantum device X and reflected photons enter optical terminator 14. Note that quantum wavelength converter 2 according to this embodiment may include multiple dichroic mirrors 12 that transmit or reflect photons of different frequencies.

[0046] In this embodiment, the dichroic mirror 12 reflects photons having the first resonant frequency ω1, the second resonant frequency ω2, and the third resonant frequency ω3, and transmits photons having the fourth resonant frequency ω4. Therefore, the dichroic mirror 12 reflects the single input photon IF having the first resonant frequency ω1, the first pump light PL1 having the second resonant frequency ω2, and the second pump light PL2 having the third resonant frequency ω3, and makes them incident on the optical terminator 14. The dichroic mirror 12 also transmits the output photon OF having the fourth resonant frequency ω4, making it incident on the quantum device X. In other words, the dichroic mirror 12 limits the photons emitted from the resonator structure 4 that enter the quantum device X to only the output photon OF. The quantum wavelength converter 2 according to this embodiment may include, instead of the dichroic mirror 12, a diffraction grating that diffracts the light emitted from the resonator structure 4 in different directions for each wavelength. The quantum wavelength converter 2 according to this embodiment may also include, instead of the dichroic mirror 12, a WDM filter that demultiplexes the light emitted from the resonator structure 4.

[0047] Optical terminator 14 is an optical element that converts the energy of incident photons into energy such as heat, thereby eliminating the photons without reflecting or dispersing them, etc. Optical terminator 14 enables quantum wavelength converter 2 to reduce the emission of the single input photon IF, first pump light PL1, and second pump light PL2 emitted from resonator structure 4 to the outside.

[0048] However, the quantum wavelength converter 2 may further include another resonator structure 4 instead of the optical terminator 14, and the photons reflected by the dichroic mirror 12 may be incident on the resonator structure 4. In this way, the quantum wavelength converter 2 may generate output photons OF in each of the multiple resonator structures 4.

[0049] <Summary of First Embodiment> The quantum wavelength converter 2 of this embodiment can convert a single input photon IF having a first resonant frequency ω1 generated by the single-photon source 10 into an output photon OF having a fourth resonant frequency ω4, which is a frequency different from the first resonant frequency ω1. The quantum wavelength converter 2 also inputs the output photon OF to the quantum device X. As a result, even if there is a difference between the frequency of the photon generated by the single-photon source 10 and the frequency of the photon required by the quantum device X, the quantum wavelength converter 2 can convert the frequency of the photon generated by the single-photon source 10 and input it to the quantum device X.

[0050] In this embodiment, for example, as shown in the energy diagram D1 of FIG. 3, the fourth resonant frequency ω4 is set to a frequency lower than any of the other resonant frequencies. However, this is not limiting, and the fourth resonant frequency ω4 may be higher than any of the other resonant frequencies. In this embodiment, for example, the fourth resonant frequency ω4 may be higher than the third resonant frequency ω3. In this embodiment, the fourth resonant frequency ω4 may be changed by changing the design of the frequencies of the first pump light PL1 and the second pump light PL2, the resonator length of each resonator included in the resonator structure 4, etc., so as to satisfy the above formulas (1) and (2).

[0051] The quantum wavelength converter 2 of this embodiment includes, as part of the optical fiber 8, a common resonant optical path for each resonator that receives pump light or the like and generates wavelength-converted photons. Therefore, even if a quantum device that emits photons before wavelength conversion or a quantum device that requires wavelength-converted photons includes an optical fiber, the quantum wavelength converter 2 of this embodiment can easily be coupled to the optical fiber. Therefore, even when the quantum wavelength converter 2 is incorporated into a quantum device that includes an optical fiber, the quantum wavelength converter 2 reduces the complexity of the structure of the quantum device.

[0052] The quantum wavelength converter 2 converts the wavelength of a single input photon IF generated by the single-photon source 10, but is not limited to this. For example, the quantum wavelength converter 2 may convert the wavelength of a photon having a first resonant frequency ω1 output from a quantum operation unit in a quantum computer or a quantum repeater in a quantum communication device. Alternatively, the quantum wavelength converter 2 may further convert the wavelength of an output photon OF generated by another quantum wavelength converter 2.

[0053] Generally, in a quantum operation unit or quantum repeater that includes an atom as a quantum system, when the atom interacts with a photon, the transition between the atomic ground state and its immediately higher excited state is often utilized to ensure sufficient coherence time in the atom. In this case, the wavelength of the photon is generally required to be less than 1 μm in order to interact with the atom.

[0054] On the other hand, to transmit photons used in a quantum operation unit to another quantum operation unit to perform further operations using the photons, or to transmit photons between multiple quantum repeaters, a single-mode optical fiber is generally used. In general, to sufficiently reduce photon loss in the optical fiber, the wavelength of the photons propagating through the optical fiber is often included in the communication wavelength band, which is between 1.3 μm and 1.6 μm.

[0055] The quantum wavelength converter 2 according to this embodiment may convert photons having a wavelength of less than 1 μm output from the quantum operation unit or quantum repeater described above into photons having a wavelength of 1.3 μm or more and 1.6 μm or less, and propagate them through an optical fiber. Another quantum wavelength converter 2 may then convert the photons propagating through the optical fiber back into photons having a wavelength of less than 1 μm, and then input the photons to another quantum operation unit or quantum repeater. In this case, the quantum wavelength converter 2 enables low-loss propagation of photons through multiple quantum operation units or quantum repeaters.

[0056] In this embodiment, the common resonant optical path in each resonator of the resonator structure 4 includes a nano-optical fiber portion 16 having a diameter smaller than both end portions 8A. Therefore, due to the small mode cross-sectional area, the above-mentioned four-wave mixing process occurs more significantly in the nano-optical fiber portion 16 than in other portions of the optical fiber 8. Therefore, the quantum wavelength converter 2 according to this embodiment can improve the conversion efficiency from a single input photon IF to an output photon OF.

[0057] In this embodiment, each resonator in the resonator structure 4 has a pair of fiber Bragg gratings. Therefore, the quantum wavelength converter 2 can have a simpler configuration for each resonator in the resonator structure 4 than when the quantum wavelength converter 2 has a pair of optical elements such as mirrors, or a ring resonator, etc. Furthermore, at least some of the fiber Bragg gratings among the multiple resonators are the same common FBG 20. Therefore, the quantum wavelength converter 2 can have a simpler configuration for each resonator in the resonator structure 4 than when each resonator has its own independent pair of fiber Bragg gratings. By forming one of the pair of fiber Bragg gratings in each resonator as a common grating and the other as an individual grating, the quantum wavelength converter 2 can more easily design the resonator length of each resonator, as described above. The quantum wavelength converter 2 may input pump light having a first resonant frequency ω1 emitted from a laser light source 6 into the resonator structure 4 instead of the single input photon IF. In this case, by sufficiently increasing the intensity of each pump light emitted from the laser light source 6, the resonator structure 4 generates coherent photons having a fourth resonant frequency ω4. In other words, the quantum wavelength converter 2 may also function as an optical parametric oscillator. The quantum wavelength converter according to each embodiment described in this specification, including but not limited to this embodiment, may also function as an optical parametric oscillator that generates coherent photons having a fourth resonant frequency ω4 from pump light from a laser light source.

[0058] [Embodiment 2] <Wavelength conversion using parametric down-conversion> 4 is a schematic plan view of a quantum wavelength converter 46 according to this embodiment, and an enlarged schematic view of a resonator structure 48 included in the quantum wavelength converter 46. The quantum wavelength converter 46 according to this embodiment differs in configuration from the quantum wavelength converter 2 according to the previous embodiment only in that it includes a resonator structure 48 instead of the resonator structure 4. In the following embodiments, unless otherwise specified, components that have the same configuration or perform the same function between different embodiments will be designated by the same component numbers, and detailed descriptions thereof will be omitted.

[0059] The resonator structure 48 according to this embodiment differs in configuration from the resonator structure 4 according to the previous embodiment in that it does not include the second FBG 24 and the second resonator 36. Therefore, the resonator structure 48 includes a first resonator 34, a third resonator 38, and a fourth resonator 40, each having a first resonant frequency ω1, a third resonant frequency ω3, and a fourth resonant frequency ω4 as its resonant frequencies. The resonator structure 48 also includes at least a nano-optical fiber portion 16 as a common resonant optical path for the first resonator 34, the third resonator 38, and the fourth resonator 40.

[0060] Furthermore, in this embodiment, the laser light source 6 inputs the second pump light PL2 having the third resonant frequency ω3 to the resonator structure 48, and does not emit the first pump light PL1. Therefore, the resonator structure 48 according to this embodiment receives the second pump light PL2 from the laser light source 6 and the single input photon IF generated by the single-photon source 10. In this embodiment, the resonator structure 48 generates output photons OF having the fourth resonant frequency ω4 by parametric down-conversion using the input second pump light PL2 and the input single-photon IF.

[0061] The generation of output photons OF by the resonator structure 48 will be described in more detail with reference to Fig. 5. Fig. 5 shows a schematic diagram 48A for explaining the input and output of photons in the resonator structure 48 and an energy diagram D2 showing state transitions occurring in the resonator structure 48.

[0062] 5, a single input photon IF having a first resonant frequency ω1 and a second pump light PL2 having a third resonant frequency ω3 are input to the resonator structure 48. As a result, an output photon OF having a fourth resonant frequency ω4 is output from the resonator structure 48 by a method described below, as shown in schematic diagram 48A. In this embodiment, the output photon OF is also a single photon.

[0063] Here, in this embodiment, for the first resonant frequency ω1, the third resonant frequency ω3, and the fourth resonant frequency ω4, the above-mentioned equations (1) and (2) are replaced with the following equations (3) and (4).

number

number

[0064] When the above two equations are satisfied, the single input photon IF and the second pump light PL2 cause the state transition shown in energy diagram D2 in the nano optical fiber portion 16. Specifically, the single input photon IF causes excitation from the ground level g to the excited level e. In addition, in conjunction with this excitation, the second pump light PL2 causes a transition from the excited level e to an intermediate level between the two levels. As a result, an output photon OF having a fourth resonant frequency ω4 is generated, with energy corresponding to the difference between the energy of the second pump light PL2 and the energy difference between the ground level g and the excited level e.

[0065] As a result, parametric down-conversion occurs in the nano-optical fiber portion 16. In other words, each resonator of the resonator structure 48 according to this embodiment satisfies the law of energy conservation and the phase matching condition for parametric down-conversion in the nano-optical fiber portion 16, which is a common resonant optical path. Here, of the above two equations, equation (3) represents the law of energy conservation for parametric down-conversion, and equation (4) represents the phase matching condition for parametric down-conversion.

[0066] Here, the four-wave mixing process, which is a third-order nonlinear optical effect, occurs significantly even in materials with inversion symmetry, whereas the parametric down-conversion, which is a second-order nonlinear optical effect, occurs significantly in materials without inversion symmetry. Generally, for example, when a single-mode optical fiber is made of silica glass, the optical fiber has inversion symmetry. However, in a nano-optical fiber, the effect of the surface, which does not have inversion symmetry as a structure, becomes significant, and a second-order nonlinear optical effect is observed. Therefore, in each resonator including the nano-optical fiber portion 16 in a common resonant optical path, as in the resonator structure 48 according to this embodiment, not only the four-wave mixing process but also the parametric down-conversion occurs significantly.

[0067] As a result, the resonator structure 48 generates an output photon OF having a fourth resonant frequency ω4. Referring back to Fig. 4, the output photon OF output from the resonator structure 48 passes through the dichroic mirror 12 and enters the quantum device X. Furthermore, the single input photon IF from the resonator structure 48 and the second pump light PL2 are reflected by the dichroic mirror 12, enter the optical terminator 14, and are lost.

[0068] Like the quantum wavelength converter 2, the quantum wavelength converter 46 of this embodiment can convert a single input photon IF having a first resonant frequency ω1 generated by the single-photon source 10 into an output photon OF having a fourth resonant frequency ω4 and input it to the quantum device X. Furthermore, the resonator structure 48 of the quantum wavelength converter 46 does not include the second resonator 36, compared to the resonator structure 4 of the quantum wavelength converter 2, and therefore includes one fewer resonator. Therefore, the quantum wavelength converter 46 can more simply configure the resonator structure 48 than the quantum wavelength converter 2, and the resonator structure 48 can be designed more easily. Because the quantum wavelength converter 46 converts the wavelength of photons using parametric down-conversion, it is more useful than the quantum wavelength converter 2 when it is desired to convert a single photon input to the resonator structure 48 into a photon with a longer wavelength and lower frequency.

[0069] [Embodiment 3] <Wavelength conversion using parametric upconversion> 6 is a schematic plan view of a quantum wavelength converter 50 according to this embodiment, and a schematic enlarged view of a resonator structure 52 included in the quantum wavelength converter 50. The quantum wavelength converter 50 according to this embodiment differs in configuration from the quantum wavelength converter 2 according to the first embodiment only in that it includes a resonator structure 52 instead of the resonator structure 4.

[0070] The resonator structure 52 according to this embodiment differs in configuration from the resonator structure 4 in that it does not include the third FBG 26 and the third resonator 38. Therefore, the resonator structure 52 includes a first resonator 34, a second resonator 36, and a fourth resonator 40, each having a first resonant frequency ω1, a second resonant frequency ω2, and a fourth resonant frequency ω4 as its resonant frequencies. The resonator structure 52 also includes at least a nano-optical fiber portion 16 as a common resonant optical path for the first resonator 34, the second resonator 36, and the fourth resonator 40.

[0071] Furthermore, in this embodiment, the laser light source 6 inputs the first pump light PL1 having the second resonant frequency ω2 to the resonator structure 52, and does not emit the second pump light PL2. Therefore, the resonator structure 52 according to this embodiment receives the first pump light PL1 from the laser light source 6 and a single input photon IF generated by the single-photon source 10. In this embodiment, the resonator structure 52 generates an output photon OF having a fourth resonant frequency ω4 by parametric up-conversion using the input first pump light PL1 and the input single-photon IF.

[0072] The generation of output photons OF by the resonator structure 52 will be described in more detail with reference to Fig. 7. Fig. 7 shows a schematic diagram 52A for explaining the input and output of photons in the resonator structure 52 and an energy diagram D3 showing state transitions occurring in the resonator structure 52.

[0073] 7, a single input photon IF having a first resonant frequency ω1 and a first pump light PL1 having a second resonant frequency ω2 are input to the resonator structure 52. As a result, an output photon OF having a fourth resonant frequency ω4 is output from the resonator structure 52 by a method described below, as shown in schematic diagram 52A. In this embodiment, the output photon OF is also a single photon.

[0074] Here, in this embodiment, for the first resonant frequency ω1, the second resonant frequency ω2, and the fourth resonant frequency ω4, the above-mentioned equations (1) and (2) are replaced with the following equations (5) and (6).

number

number

[0075] When the above two equations are satisfied, the single input photon IF and the first pump light PL1 cause a state transition shown in energy diagram D3 in the nano optical fiber portion 16. Specifically, the single input photon IF and the first pump light PL1 cause excitation from the ground level g to the excited level e. As a result, an output photon OF having an energy corresponding to the energy difference between the ground level g and the excited level e and a fourth resonant frequency ω4 is generated.

[0076] As a result, parametric up-conversion occurs in the nano-fiber portion 16. In other words, each resonator of the resonator structure 52 according to this embodiment satisfies the law of energy conservation and the phase matching condition for parametric up-conversion in the nano-fiber portion 16, which is a common resonant optical path. Here, of the above two equations, equation (5) represents the law of energy conservation for parametric up-conversion, and equation (6) represents the phase matching condition for parametric up-conversion.

[0077] Here, the parametric up-conversion is a second-order nonlinear optical effect like the parametric down-conversion, and therefore occurs significantly in materials that do not have inversion symmetry. However, in the nano-optical fiber portion 16, where the effect of the surface that does not have inversion symmetry as a structure becomes significant, the parametric up-conversion also occurs significantly.

[0078] As a result, the resonator structure 52 generates an output photon OF having a fourth resonant frequency ω4. Referring back to Fig. 6, the output photon OF output from the resonator structure 52 passes through the dichroic mirror 12 and enters the quantum device X. Furthermore, the single input photon IF from the resonator structure 52 and the first pump light PL1 are reflected by the dichroic mirror 12, enter the optical terminator 14, and are lost.

[0079] Like the quantum wavelength converter 2, the quantum wavelength converter 50 of this embodiment can convert a single input photon IF having a first resonant frequency ω1 generated by the single-photon source 10 into an output photon OF having a fourth resonant frequency ω4 and input it to the quantum device X. Furthermore, compared to the resonator structure 4 of the quantum wavelength converter 2, the resonator structure 52 of the quantum wavelength converter 50 does not include the third resonator 38, and therefore includes one fewer resonator. Therefore, compared to the quantum wavelength converter 2, the quantum wavelength converter 50 can more simply configure the resonator structure 52 and make the design of the resonator structure 52 easier. Because the quantum wavelength converter 50 converts the wavelength of photons using parametric up-conversion, it is more useful than the quantum wavelength converter 2 and the quantum wavelength converter 46 when it is desired to convert a single photon input to the resonator structure 52 into a photon with a shorter wavelength and a higher frequency.

[0080] [Embodiment 4] <Wavelength conversion of pump light> 8 is a schematic plan view of a quantum wavelength converter 54 according to this embodiment, and a schematic diagram showing an enlarged view of a resonator structure 56 included in the quantum wavelength converter 54. The quantum wavelength converter 54 according to this embodiment differs from the quantum wavelength converter 2 according to the first embodiment only in that it includes a resonator structure 56 instead of the resonator structure 4, and does not include a single-photon source 10. Therefore, the quantum wavelength converter 54 does not need to include a multiplexer 44.

[0081] The resonator structure 56 according to this embodiment has the same configuration as the resonator structure 48, except for the relationship in the resonator lengths of the resonators. In particular, in this embodiment, L3 is longer than L1. For example, the resonator structure 56 corresponds to the resonator structure 4 in which the second FBG 24 is identical to the first FBG 22, and the first resonator 34 is identical to the second resonator 36. In this case, in this embodiment, the second resonant frequency ω2 can be considered to be identical to the first resonant frequency ω1.

[0082] Therefore, the resonator structure 56 includes a first resonator 34, a third resonator 38, and a fourth resonator 40, each having a first resonant frequency ω1, a third resonant frequency ω3, and a fourth resonant frequency ω4 as its resonant frequencies. The resonator structure 56 also includes at least a nano-optical fiber portion 16 as a common resonant optical path for the first resonator 34, the third resonator 38, and the fourth resonator 40.

[0083] In this embodiment, the laser light source 6 inputs the first pump light PL1 having the second resonant frequency ω2 and the second pump light PL2 having the third resonant frequency ω3 to the resonator structure 56. In this embodiment, the second resonant frequency ω2 can be considered to be the same as the first resonant frequency ω1, and therefore the first pump light PL1 can be considered to have the first resonant frequency ω1. In this embodiment, the resonator structure 56 generates output photons OF having the fourth resonant frequency ω4 through a four-wave mixing process of the input first pump light PL1 and second pump light PL2.

[0084] The generation of output photons OF by the resonator structure 56 will be described in more detail with reference to Fig. 9. Fig. 9 shows a schematic diagram 56A for explaining the input and output of photons in the resonator structure 56 and an energy diagram D4 showing state transitions occurring in the resonator structure 56.

[0085] 9, a first pump light PL1 having a first resonant frequency ω1 and a second pump light PL2 having a third resonant frequency ω3 are input to the resonator structure 56. As a result, as shown in schematic diagram 56A, an output photon OF having a fourth resonant frequency ω4 is output from the resonator structure 56 by a method described below. In this embodiment, the output photon OF may be a single photon, or may include multiple photons emitted from the resonator structure 56 at approximately the same time.

[0086] In this embodiment, for the first resonant frequency ω1, the third resonant frequency ω3, and the fourth resonant frequency ω4, the above-described formulas (1) and (2) are satisfied with ω1=ω2.

[0087] In this embodiment, the energy difference between the ground level g and the excited level e in the nano optical fiber portion 16 is set to be approximately equal to twice the energy of a photon having the first resonant frequency ω1, as shown in the energy diagram D4 in Fig. 9. Note that, since 2ω1 = ω3 + ω4 holds from the above formula (5), the energy difference between the ground level g and the excited level e is also approximately equal to the sum of the energy of a photon having the third resonant frequency ω3 and the energy of a photon having the fourth resonant frequency ω4.

[0088] When the above two formulas are satisfied, the first pump light PL1 and the second pump light PL2 cause the state transition shown in energy diagram D4 in the nano-fiber portion 16. Specifically, the first pump light PL1 causes excitation from the ground level g to an energy level equal to half the energy difference between the ground level g and the excited level e, and further excitation by the first pump light PL1 occurs continuously. Therefore, the first pump light PL1 causes excitation from the ground level g to the excited level e. This excitation from the ground level g to the excited level e occurs more efficiently in proportion to the intensity of the first pump light PL1.

[0089] In addition, in conjunction with this excitation, the second pump light PL2 causes a transition from the excited level e to an intermediate level between the two levels. As a result, an output photon OF having a fourth resonant frequency ω4 is generated, the output photon OF having energy corresponding to the difference between the energy of the second pump light PL2 and the energy difference between the ground level g and the excited level e.

[0090] As a result, a four-wave mixing process occurs in the nano-optical fiber portion 16. In other words, each resonator of the resonator structure 56 according to this embodiment satisfies the law of energy conservation and the phase matching condition of the four-wave mixing process in the nano-optical fiber portion 16, which is a common resonant optical path. The four-wave mixing process in this embodiment is a degenerate four-wave mixing process corresponding to the four-wave mixing process in embodiment 1 when ω1 = ω2. However, the degenerate four-wave mixing process in this embodiment is a third-order nonlinear optical effect, just like the non-degenerate four-wave mixing process in embodiment 1, and therefore occurs significantly in the optical fiber 8.

[0091] As a result, the resonator structure 56 generates output photons OF having the fourth resonant frequency ω4. Referring back to Fig. 8, the output photons OF output from the resonator structure 56 pass through the dichroic mirror 12 and enter the quantum device X. In addition, the first pump light PL1 and the second pump light PL2 from the resonator structure 56 are reflected by the dichroic mirror 12, enter the optical terminator 14, and are then lost.

[0092] The quantum wavelength converter 54 according to this embodiment can generate output photons OF having a fourth resonant frequency ω4 by converting the wavelength of the pump light from the laser light source 6, and input the output photons OF to the quantum device X.

[0093] Furthermore, compared to resonator structure 4 of quantum wavelength converter 2, resonator structure 56 of quantum wavelength converter 54 does not include second resonator 36, and therefore includes one fewer resonator. Therefore, compared to quantum wavelength converter 2, quantum wavelength converter 54 can configure resonator structure 56 more simply, and the design of resonator structure 56 is also easier.

[0094] [Embodiment 5] <Second harmonic generator> 10 is a schematic plan view of a quantum wavelength converter 58 according to this embodiment, and an enlarged schematic view of a resonator structure 60 included in the quantum wavelength converter 58. The quantum wavelength converter 58 according to this embodiment differs from the quantum wavelength converter 54 according to the previous embodiment only in that the quantum wavelength converter 58 includes a resonator structure 60 instead of the resonator structure 56.

[0095] The resonator structure 60 according to this embodiment differs from the resonator structure 4 in that it does not include the second FBG 24, the third FBG 26, the second resonator 36, and the third resonator 38. For example, the resonator structure 60 corresponds to the resonator structure 52 in which the second FBG 24 is the same as the first FBG 22, and the first resonator 34 is the same as the second resonator 36. In this case, in this embodiment, the second resonant frequency ω2 can be considered to be the same as the first resonant frequency ω1.

[0096] Therefore, the resonator structure 60 includes a first resonator 34 and a fourth resonator 40, which have a first resonant frequency ω1 and a fourth resonant frequency ω4 as their resonant frequencies, respectively. The resonator structure 60 also includes at least a nano-optical fiber portion 16 as a common resonant optical path for the first resonator 34 and the fourth resonator 40.

[0097] In this embodiment, the laser light source 6 inputs the first pump light PL1 having the second resonant frequency ω2 to the resonator structure 56. In this embodiment, the second resonant frequency ω2 can be considered to be the same as the first resonant frequency ω1, and therefore the first pump light PL1 can be considered to have the first resonant frequency ω1. In this embodiment, the resonator structure 60 generates output photons OF having a fourth resonant frequency ω4 by parametric up-conversion using the input first pump light PL1.

[0098] The generation of output photons OF by the resonator structure 60 will be described in more detail with reference to Fig. 11. Fig. 11 shows a schematic diagram 60A for explaining the input and output of photons in the resonator structure 60 and an energy diagram D5 showing state transitions occurring in the resonator structure 60.

[0099] 11 , a first pump light PL1 having a first resonant frequency ω1 is input to the resonator structure 60. As a result, as shown in the schematic diagram 60A, an output photon OF having a fourth resonant frequency ω4 is output from the resonator structure 60 by a method described later. In this embodiment, the output photon OF may be a single photon, or may include multiple photons emitted from the resonator structure 56 at approximately the same time.

[0100] In this embodiment, for the first resonance frequency ω1 and the fourth resonance frequency ω4, the above-described formulas (5) and (6) are satisfied with ω1=ω2.

[0101] In this embodiment, the energy difference between the ground level g and the excited level e in the nano optical fiber portion 16 is set to be approximately equal to twice the energy of a photon having the first resonant frequency ω1, as shown in the energy diagram D5 in Fig. 11. Note that, since 2ω1 = ω4 holds from the above formula (5), the energy difference between the ground level g and the excited level e is also approximately equal to the energy of a photon having the fourth resonant frequency ω4.

[0102] When the above two equations are satisfied, the first pump light PL1 causes a state transition shown in energy diagram D5 in the nano optical fiber portion 16. Specifically, the first pump light PL1 causes excitation from the ground level g to an energy level that is half the energy difference between the ground level g and the excited level e. Here, if the intensity of the first pump light PL1 is sufficiently strong, further excitation by the first pump light PL1 occurs continuously. Therefore, the first pump light PL1 causes excitation from the ground level g to the excited level e. As a result, an output photon OF having a fourth resonant frequency ω4 and having energy equivalent to the energy difference between the ground level g and the excited level e is generated.

[0103] As a result, parametric up-conversion occurs in the nano-optical fiber portion 16. In other words, each resonator of the resonator structure 60 according to this embodiment satisfies the energy conservation law and the phase matching condition for parametric up-conversion in the nano-optical fiber portion 16, which is a common resonant optical path.

[0104] As a result, the resonator structure 60 generates output photons OF having the fourth resonant frequency ω4. Referring back to Fig. 10, the output photons OF output from the resonator structure 60 are transmitted through the dichroic mirror 12 and incident on the quantum device X. The first pump light PL1 from the resonator structure 60 is reflected by the dichroic mirror 12, incident on the optical terminator 14, and is lost.

[0105] The quantum wavelength converter 58 of this embodiment converts the wavelength of the pump light from the laser light source 6 to generate output photons OF having a fourth resonant frequency ω4, which can be input to the quantum device X. In particular, because the fourth resonant frequency ω4 is twice the first resonant frequency ω1, the parametric up-conversion occurring in the resonator structure 60 is so-called second-harmonic generation. In other words, the quantum wavelength converter 58 functions as a second-harmonic generator, generating output photons OF having a frequency twice the frequency of the pump light from the laser light source 6.

[0106] Furthermore, compared to the resonator structure 4 of the quantum wavelength converter 2, the resonator structure 60 of the quantum wavelength converter 58 does not include the second resonator 36 and the third resonator 38, and therefore includes two fewer resonators. Therefore, compared to the quantum wavelength converter 2, the resonator structure 60 of the quantum wavelength converter 58 can be configured more simply and the design of the resonator structure 60 is easier.

[0107] [Embodiment 6] <Single photon source with messenger> By using a quantum wavelength converter including a resonator structure according to any of the above-described embodiments to convert the wavelength of pump light from a laser light source, it is possible to generate photon pairs and heralded single photons. A heralded single photon source according to this embodiment, which includes a quantum wavelength converter, will now be described. Figure 12 is a schematic plan view of a heralded single photon source 62 according to this embodiment.

[0108] 12, the herald-equipped single-photon source 62 includes a quantum wavelength converter 64, a dichroic mirror 66, a single-photon detector 68, a first output optical fiber 70, and a second output optical fiber 72. The quantum wavelength converter 64 differs in configuration from the quantum wavelength converter 2 according to the first embodiment only in that it does not include the single-photon source 10.

[0109] However, in this embodiment, the dichroic mirror 12 transmits photons having the third resonant frequency ω3 in addition to photons having the fourth resonant frequency ω4. Furthermore, the dichroic mirror 12 according to this embodiment reflects photons having the first resonant frequency ω1 and the second resonant frequency ω2. Therefore, the photons having the first resonant frequency ω1 and the second resonant frequency ω2 reflected by the dichroic mirror 12 are incident on the optical terminator 14.

[0110] The dichroic mirror 66 differs from the dichroic mirror 12 in configuration only in that it transmits photons having the fourth resonant frequency ω4 and reflects photons having the third resonant frequency ω3. The dichroic mirror 66 is positioned so that photons that have transmitted through the dichroic mirror 12 are incident on it.

[0111] The single-photon detector 68 is an element for detecting incident single photons. The single-photon detector 68 is positioned so that photons reflected by the dichroic mirror 66 are incident on the single-photon detector 68. Therefore, the single-photon detector 68 detects photons having the third resonant frequency ω3 that are reflected by the dichroic mirror 66.

[0112] In this embodiment, quantum device X is arranged so that photons that have passed through dichroic mirror 66 are incident on it. Therefore, photons having the fourth resonant frequency ω4 that have passed through dichroic mirror 12 are incident on single-photon detector 68.

[0113] The first output optical fiber 70 and the second output optical fiber 72 are single-mode optical fibers and may have the same configuration as the optical fiber 8. The first output optical fiber 70 is arranged so that photons reflected by the dichroic mirror 66 are incident thereon, and the second output optical fiber 72 is arranged so that photons transmitted through the dichroic mirror 66 are incident thereon.

[0114] Therefore, the first output optical fiber 70 propagates the photons having the third resonant frequency ω3 that have been reflected by the dichroic mirror 66 to the single-photon detector 68. The second output optical fiber 72 propagates the photons having the fourth resonant frequency ω4 that have been transmitted through the dichroic mirror 66 to the quantum device X.

[0115] In this embodiment, the laser light source 6 inputs the first pump light PL1 having the first resonant frequency ω1 and the second pump light PL2 having the second resonant frequency ω2 into the resonator structure 4.

[0116] In this embodiment, the resonator structure 4 generates a first output photon OF1 having a third resonant frequency ω3 and a second output photon OF2 having a fourth resonant frequency ω4 through a four-wave mixing process using the input pump light, as described below. In other words, the resonator structure 4 generates photon pairs with different wavelengths from the input pump light. The first pump light PL1 and the second pump light PL2 emitted by the laser light source 6 according to this embodiment can be regarded as successive pulse waves with pulse lengths equal to the photon lifetimes of the respective resonators of the resonator structure 4. Here, in this embodiment, the probability that one photon pair is generated from one pulse wave of each of the first pump light PL1 and the second pump light PL2 is set to p, which is sufficiently smaller than 1. Note that the laser light source 6 may intermittently emit pulse waves having the respective resonant frequencies as the first pump light PL1 and the second pump light PL2.

[0117] The generation of photon pairs in the resonator structure 4 will be described in more detail with reference to Fig. 13. Fig. 13 shows a schematic diagram 4B for explaining the input and output of photons in the resonator structure 4 and an energy diagram D6 showing state transitions occurring in the resonator structure 4.

[0118] 13, a first pump light PL1 having a first resonant frequency ω1 and a second pump light PL2 having a second resonant frequency ω2 are input to the resonator structure 4. As a result, as shown in schematic diagram 4B, a first output photon OF1 having a third resonant frequency ω3 and a second output photon OF2 having a fourth resonant frequency ω4 are output from the resonator structure 4 by a method described later.

[0119] In this embodiment, the above-described formulas (1) and (2) hold for the first resonant frequency ω1, the second resonant frequency ω2, the third resonant frequency ω3, and the fourth resonant frequency ω4. In this embodiment, a virtual ground level g and an excited level e are set in the nano-optical fiber section 16. In this embodiment, as shown in the energy diagram D6 in FIG. 13, the energy difference between the ground level g and the excited level e is set to be approximately equal to the sum of the energy of photons having the first resonant frequency ω1 and the energy of photons having the second resonant frequency ω2. Since ω1 + ω2 = ω3 + ω4 holds true from the above formula (1), the energy difference between the ground level g and the excited level e is also approximately equal to the sum of the energy of photons having the third resonant frequency ω3 and the energy of photons having the fourth resonant frequency ω4.

[0120] When the above two formulas are satisfied, the first pump light PL1 and the second pump light PL2 cause a state transition shown in the energy diagram D6 in the nano optical fiber portion 16. Specifically, the first pump light PL1 and the second pump light PL2 cause excitation from the ground level g to the excited level e.

[0121] In addition, in conjunction with this excitation, a state transition occurs from the excited level e to the ground level g. Here, the resonator structure 4 has a third resonator 38 having a third resonant frequency ω3 as a resonant frequency, and a fourth resonator 40 having a fourth resonant frequency ω4 as a resonant frequency. Therefore, as shown in the energy diagram D6, in the nano optical fiber section 16, a state transition first occurs from the excited level e to the ground level g by the energy of the photon having the third resonant frequency ω3. Furthermore, a state transition occurs by the energy of the photon having the fourth resonant frequency ω4, resulting in a state transition from the excited level e to the ground level g.

[0122] Therefore, in the nano optical fiber portion 16 according to this embodiment, a four-wave mixing process occurs significantly, as shown in the energy diagram D6. As a result of the above-described four-wave mixing process, photons having a third resonant frequency ω3 are generated in the third resonator 38 of the resonator structure 4 and coupled with the third resonator 38. Furthermore, as a result of the above-described four-wave mixing process, photons having a fourth resonant frequency ω4 are generated in the fourth resonator 40 of the resonator structure 4 at approximately the same time as the photons having the third resonant frequency ω3.

[0123] Therefore, when the above-described four-wave mixing process occurs in the resonator structure 4, a first output photon OF1 is emitted from the third resonator 38, and a second output photon OF2 is emitted from the fourth resonator 40. Referring back to Fig. 12, the first output photon OF1 emitted from the resonator structure 4 passes through the dichroic mirror 12 and is reflected by the dichroic mirror 66, thereby propagating through the first output optical fiber 70 and being detected by the single-photon detector 68. The second output photon OF2 emitted from the resonator structure 4 passes through both the dichroic mirror 12 and the dichroic mirror 66, thereby propagating through the second output optical fiber 72 and entering the quantum device X.

[0124] As described above, the probability that one photon pair is generated from one pulse wave of each pump light by the four-wave mixing process is p. Therefore, the probability that two photon pairs are simultaneously generated from each pump light by the four-wave mixing process is p squared. Therefore, when the single-photon detector 68 detects a photon, the probability that two photons are incident on the quantum device X is sufficiently smaller than 1. In other words, when the single-photon detector 68 detects a photon, the photon incident on the quantum device X is a single photon with a probability sufficiently close to 1. Specifically, when the single-photon detector 68 detects the first output photon OF1, the probability that the second output photon OF2 includes multiple photons is sufficiently smaller than 1. In other words, when the single-photon detector 68 detects the first output photon OF1, the probability that the second output photon OF2 is a single photon with a probability sufficiently close to 1.

[0125] For the reasons described above, in this embodiment, when the single-photon detector 68 detects the first output photon OF1, it is almost certain that the second output photon OF2 has been incident on the quantum device X. In addition, the second output photon OF2 is a single photon with a probability sufficiently close to 1. Therefore, the heralded single-photon source 62 functions as a heralded single-photon source that uses the detection of the first output photon OF1, which is an idler photon, as a herald to generate the second output photon OF2, which is a signal photon. The quantum device X may be, for example, a device including a quantum communication device that requires a heralded single photon, such as a quantum cryptography communication device.

[0126] The heralded single photon source 62 according to this embodiment uses a quantum wavelength converter 64 including a resonator structure 4 to generate a first output photon OF1, which is an idler photon, and a second output photon OF2, which is a signal photon. For the same reason as the quantum wavelength converter 2, the heralded single photon source 62 reduces the complexity of the structure of a quantum device including an optical fiber even when incorporated into the quantum device.

[0127] [Embodiment 7] <Messenger-assisted single-photon source using degenerate four-wave mixing process> 14 is a schematic plan view of a herald-equipped single-photon source 74 according to this embodiment. As shown in FIG. 12, the herald-equipped single-photon source 74 differs from the herald-equipped single-photon source 62 in that it includes the quantum wavelength converter 54 according to the fourth embodiment instead of the quantum wavelength converter 64. However, in this embodiment, the dichroic mirror 12 included in the quantum wavelength converter 54 transmits photons having the third resonant frequency ω3 in addition to the photons having the fourth resonant frequency ω4, and reflects photons having the first resonant frequency ω1 and the second resonant frequency ω2.

[0128] In this embodiment, the laser light source 6 inputs a first pump light PL1 having a first resonant frequency ω1 to the resonator structure 4. In this embodiment, the resonator structure 4 generates a first output photon OF1 having a third resonant frequency ω3 and a second output photon OF2 having a fourth resonant frequency ω4 by a four-wave mixing process using the input pump light, using a method described below.

[0129] Therefore, in this embodiment, the second resonant frequency ω2 can be regarded as being the same as the first resonant frequency ω1 in the previous embodiment. Also, the herald-equipped single-photon source 74 according to this embodiment corresponds to the herald-equipped single-photon source 62 in which the second resonator 36 is the same as the first resonator 34, and the second pump light PL2 is the same as the first pump light PL1.

[0130] The generation of photon pairs in the resonator structure 56 will be described in more detail with reference to Fig. 15. Fig. 15 shows a schematic diagram 56B for explaining the input and output of photons in the resonator structure 56 and an energy diagram D7 showing state transitions occurring in the resonator structure 56.

[0131] 15, a first pump light PL1 having a first resonant frequency ω1 is input to the resonator structure 56. As a result, as shown in schematic diagram 56B, a first output photon OF1 having a third resonant frequency ω3 and a second output photon OF2 having a fourth resonant frequency ω4 are output from the resonator structure 4 by a method described later.

[0132] In this embodiment, the above-described formulas (1) and (2) are satisfied with ω1=ω2 for the first resonant frequency ω1, the third resonant frequency ω3, and the fourth resonant frequency ω4. In this embodiment, the energy difference between the ground level g and the excited level e in the nano optical fiber section 16 is set to be approximately equal to twice the energy of a photon having the first resonant frequency ω1. Since 2ω1=ω3+ω4 holds true from the above formula (1), the energy difference between the ground level g and the excited level e is also approximately equal to the sum of the energy of a photon having the third resonant frequency ω3 and the energy of a photon having the fourth resonant frequency ω4.

[0133] When the above two formulas are satisfied, the first pump light PL1 causes a state transition shown in energy diagram D7 in the nano optical fiber portion 16. Specifically, the first pump light PL1 causes excitation from the ground level g to an energy level that is half the energy difference between the ground level g and the excited level e. Here, if the intensity of the first pump light PL1 is sufficiently strong, further excitation by the first pump light PL1 occurs continuously. Therefore, the first pump light PL1 causes excitation from the ground level g to the excited level e.

[0134] In addition, along with this excitation, a state transition occurs from the excited level e to the ground level g. In this embodiment, too, for the same reasons as those described in the previous embodiment, a four-wave mixing process occurs significantly, as shown in energy diagram D7. The four-wave mixing process occurring in the nanooptical fiber section 16 according to this embodiment corresponds to the degenerate four-wave mixing process in which ω1 = ω2 in the four-wave mixing process occurring in the nanooptical fiber section 16 according to the previous embodiment. As a result of this degenerate four-wave mixing process, photons having a third resonant frequency ω3 are generated in the third resonator 38 of the resonator structure 4, and photons having a fourth resonant frequency ω4 are generated in the fourth resonator 40 of the resonator structure 4 at approximately the same time.

[0135] The first pump light PL1 can be regarded as a series of pulse waves, each having a pulse length equal to the photon lifetime of each resonator in the resonator structure 4. In this embodiment, the probability that one photon pair is generated from one pulse wave of the first pump light PL1 is set to p, which is sufficiently smaller than 1. In this case, also in this embodiment, the probability that two photon pairs are simultaneously generated from the first pump light PL1 through a four-wave mixing process is p squared. Therefore, when the single-photon detector 68 detects a photon, the probability that two photons are incident on the quantum device X is sufficiently smaller than 1. In other words, also in this embodiment, when the single-photon detector 68 detects a photon, the photon incident on the quantum device X is a single photon with a probability sufficiently close to 1. Specifically, when the single-photon detector 68 detects the first output photon OF1, the probability that the second output photon OF2 includes multiple photons is sufficiently smaller than 1. In other words, when single-photon detector 68 detects the first output photon OF1, the second output photon OF2 is a single photon with a probability sufficiently close to 1.

[0136] Therefore, when the above-described four-wave mixing process occurs in the resonator structure 4, a first output photon OF1 is emitted from the third resonator 38, and a second output photon OF2 is emitted from the fourth resonator 40. Referring back to Fig. 14, the first output photon OF1 emitted from the resonator structure 56 is detected by the single-photon detector 68, and the second output photon OF2 emitted from the resonator structure 56 is incident on the quantum device X.

[0137] In this embodiment as well, for the same reasons as those explained in the previous embodiment, when a photon pair is generated from the resonator structure 56 and the single-photon detector 68 detects the first output photon OF1, the second output photon OF2 is almost certainly input to the quantum device X. In addition, in this embodiment as well, the second output photon OF2 is a single photon with a probability sufficiently close to 1. Therefore, the herald single-photon source 74 functions as a herald single-photon source that generates the second output photon OF2, which is a signal photon, using the detection of the first output photon OF1, which is an idler photon, as a herald.

[0138] The heralded single photon source 74 according to this embodiment uses a quantum wavelength converter 54 including a resonator structure 56 to generate a first output photon OF1, which is an idler photon, and a second output photon OF2, which is a signal photon. For the same reason as the quantum wavelength converter 2, the heralded single photon source 74 reduces the complexity of the structure of a quantum device including an optical fiber even when incorporated into the quantum device.

[0139] Furthermore, compared to the resonator structure 4 of the herald single photon source 62, the resonator structure 56 of the herald single photon source 74 does not include the second resonator 36, and therefore includes one less resonator. Therefore, compared to the herald single photon source 62, the resonator structure 56 of the herald single photon source 74 can be configured more simply, and the design of the resonator structure 56 is also easier.

[0140] [Embodiment 8] <Messenger-assisted single photon source using parametric down-conversion> 16 is a schematic plan view of a herald-equipped single-photon source 76 according to this embodiment. As shown in FIG. 16, the herald-equipped single-photon source 76 differs in configuration from the herald-equipped single-photon source 62 in that it includes a quantum wavelength converter 78 instead of the quantum wavelength converter 64. The quantum wavelength converter 78 differs in configuration from the quantum wavelength converter 46 according to the second embodiment only in that it does not include the single-photon source 10. However, in this embodiment, the dichroic mirror 12 included in the quantum wavelength converter 78 transmits photons having the third resonant frequency ω3 in addition to photons having the fourth resonant frequency ω4, and reflects photons having the first resonant frequency ω1 and the second resonant frequency ω2.

[0141] In this embodiment, the laser light source 6 inputs a first pump light PL1 having a first resonant frequency ω1 to the resonator structure 4. In this embodiment, the resonator structure 48 generates a first output photon OF1 having a third resonant frequency ω3 and a second output photon OF2 having a fourth resonant frequency ω4 by parametric down-conversion using the input pump light, using a method described below.

[0142] The generation of photon pairs in the resonator structure 48 will be described in more detail with reference to Fig. 17. Fig. 17 shows a schematic diagram 48B for explaining the input and output of photons in the resonator structure 48 and an energy diagram D8 showing state transitions occurring in the resonator structure 48.

[0143] 17, a first pump light PL1 having a first resonant frequency ω1 is input to the resonator structure 48. As a result, as shown in schematic diagram 48B, a first output photon OF1 having a third resonant frequency ω3 and a second output photon OF2 having a fourth resonant frequency ω4 are output from the resonator structure 48 by a method described later.

[0144] In this embodiment, the above-described formulas (3) and (4) hold for the first resonant frequency ω1, the third resonant frequency ω3, and the fourth resonant frequency ω4. In this embodiment, the energy difference between the ground level g and the excited level e in the nanooptical fiber section 16 is set to be approximately the same as the energy of a photon having the first resonant frequency ω1. From the above formula (3), ω1 = ω3 + ω4 holds, and therefore the energy difference between the ground level g and the excited level e is also approximately the same as the sum of the energy of a photon having the third resonant frequency ω3 and the energy of a photon having the fourth resonant frequency ω4.

[0145] When the above two formulas are satisfied, the first pump light PL1 causes a state transition shown in the energy diagram D7 in the nano optical fiber portion 16. Specifically, the first pump light PL1 causes excitation from the ground level g to the excited level e.

[0146] In addition, a state transition from the excited level e to the ground level g occurs in conjunction with this excitation. For the same reasons as those described in the previous embodiment, parametric down-conversion also occurs significantly in this embodiment, as shown in energy diagram D8. Accompanying the above-described parametric down-conversion, photons having the third resonant frequency ω3 are generated in the third resonator 38 of the resonator structure 4, and photons having the fourth resonant frequency ω4 are generated in the fourth resonator 40 of the resonator structure 4 at approximately the same time.

[0147] The first pump light PL1 can be regarded as a series of pulse waves, each having a pulse length equal to the photon lifetime of each resonator in the resonator structure 4. In this embodiment, the probability that one photon pair is generated from one pulse wave of the first pump light PL1 is set to p, which is sufficiently smaller than 1. In this case, also in this embodiment, the probability that two photon pairs are simultaneously generated from the first pump light PL1 by parametric down-conversion is p squared. Therefore, when the single-photon detector 68 detects a photon, the probability that two photons are incident on the quantum device X is sufficiently smaller than 1. In other words, also in this embodiment, when the single-photon detector 68 detects a photon, the photon incident on the quantum device X is a single photon with a probability sufficiently close to 1. Specifically, when the single-photon detector 68 detects the first output photon OF1, the probability that the second output photon OF2 includes multiple photons is sufficiently smaller than 1. In other words, when single-photon detector 68 detects the first output photon OF1, the second output photon OF2 is a single photon with a probability sufficiently close to 1.

[0148] Therefore, when the above-described parametric down-conversion occurs in the resonator structure 4, a first output photon OF1 is emitted from the third resonator 38, and a second output photon OF2 is emitted from the fourth resonator 40. Referring back to Fig. 16, the first output photon OF1 emitted from the resonator structure 48 is detected by the single-photon detector 68, and the second output photon OF2 emitted from the resonator structure 48 is incident on the quantum device X.

[0149] In this embodiment as well, for the same reasons as those explained in the previous embodiment, when a photon pair is generated from the resonator structure 56 and the single-photon detector 68 detects the first output photon OF1, the second output photon OF2 is almost certainly input to the quantum device X. In addition, in this embodiment as well, the second output photon OF2 is a single photon with a probability sufficiently close to 1. Therefore, the herald single-photon source 76 functions as a herald single-photon source that generates the second output photon OF2, which is a signal photon, using the detection of the first output photon OF1, which is an idler photon, as a herald.

[0150] The heralded single photon source 76 according to this embodiment uses a quantum wavelength converter 78 including a resonator structure 48 to generate a first output photon OF1, which is an idler photon, and a second output photon OF2, which is a signal photon. For the same reason as the quantum wavelength converter 2, the heralded single photon source 76 reduces the complexity of the structure of a quantum device including an optical fiber even when incorporated into the quantum device.

[0151] Furthermore, compared to the resonator structure 4 of the herald single photon source 62, the resonator structure 48 of the herald single photon source 76 does not include the second resonator 36, and therefore includes one less resonator. Therefore, compared to the herald single photon source 62, the resonator structure 48 of the herald single photon source 76 can be configured more simply, and the design of the resonator structure 48 is also easier.

[0152] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment. [Explanation of symbols]

[0153] 2. Quantum wavelength converter 4 Resonator structure 6 Laser light source 8. Optical Fiber 16 Nano-Optical Fiber Section 18 Tapered section 20 common FBG 22 1st FBG 24 2nd FBG 26 3rd FBG 28 4th FBG 34 1st resonator 36 Second resonator 38 Third resonator 40 4th resonator 62 Single-photon source with messenger 68 Single Photon Detector 70 First output optical fiber 72 Second output optical fiber

Claims

1. an optical fiber including a plurality of resonators therein, each of which has a common resonant optical path in at least a portion thereof; a laser light source connected to the optical fiber and configured to input pump light having at least one frequency into the optical fiber; The plurality of resonators have a first resonant frequency ω １ a first resonator having a resonant frequency ω ２ a second resonator having a resonant frequency ω ３ a third resonator having a resonant frequency of the first resonant frequency ω １ , the second resonant frequency ω ２ , and the third resonant frequency ω ３ The fourth resonance frequency ω is different from any of the above. ４ a fourth resonator having a resonant frequency of n ｉ is the i-th resonance frequency ω ｉ is the effective refractive index within the optical fiber for light of (a), c is the speed of light, γ is the nonlinear coefficient of the optical fiber, and P is the intensity of the pump light. [Equation 1] [Equation 2] A quantum wavelength converter in which the above formula (1) and formula (2) are satisfied.

2. The optical fiber is connected to the plurality of resonators, and the first resonant frequency ω １ and a single-photon source for inputting input photons having the formula: The laser light source has the second resonance frequency ω ２ and the third resonant frequency ω ３ and a second pump light having the following formula: from the fourth resonator to the fourth resonant frequency ω ４ 10. The quantum wavelength converter of claim 1, wherein the quantum wavelength converter emits output photons having a wavelength of 1000 .mu.m.

3. the first resonant frequency ω １ is the second resonant frequency ω ２ is identical to the first resonator is identical to the second resonator; The laser light source has the first resonance frequency ω １ and the third resonant frequency ω ３ and a second pump light having the following formula: from the fourth resonator to the fourth resonant frequency ω ４ 10. The quantum wavelength converter of claim 1, wherein the quantum wavelength converter emits output photons having a wavelength of 1000 .mu.m.

4. an optical fiber including a plurality of resonators therein, each of which has a common resonant optical path in at least a portion thereof; a laser light source connected to the optical fiber and configured to input pump light into the optical fiber; The plurality of resonators have a first resonant frequency ω １ a first resonator having a resonant frequency of ω ３ a third resonator having a resonant frequency of the first resonant frequency ω １ and the third resonant frequency ω ３ The fourth resonance frequency ω is different from both ４ a fourth resonator having a resonant frequency of n ｉ is the i-th resonance frequency ω ｉ is the effective refractive index within the optical fiber for light of (a), c is the speed of light, γ is the nonlinear coefficient of the optical fiber, and P is the intensity of the pump light. [Equation 3] [Equation 4] A quantum wavelength converter in which the above formula (3) and formula (4) are satisfied.

5. The optical fiber is connected to the plurality of resonators, and the first resonant frequency ω １ and a single-photon source for inputting input photons having the formula: The laser light source has the third resonance frequency ω ３ inputting the pump light having the formula: from the fourth resonator to the fourth resonant frequency ω ４ 5. The quantum wavelength converter of claim 4, which emits output photons having the following formula:

6. an optical fiber including a plurality of resonators therein, each of which has a common resonant optical path in at least a portion thereof; a laser light source connected to the optical fiber and configured to input pump light into the optical fiber; The plurality of resonators have a first resonant frequency ω １ a first resonator having a resonant frequency ω ２ a second resonator having a resonant frequency ω １ and the second resonant frequency ω ２ The fourth resonance frequency ω is different from both ４ a fourth resonator having a resonant frequency of n ｉ is the i-th resonance frequency ω ｉ is the effective refractive index within the optical fiber for light of (a), c is the speed of light, γ is the nonlinear coefficient of the optical fiber, and P is the intensity of the pump light. [Equation 5] [Equation 6] A quantum wavelength converter in which the above formulas (5) and (6) hold.

7. The optical fiber is connected to the plurality of resonators, and the first resonant frequency ω １ a single-photon source for inputting input photons having the formula: The laser light source has the second resonance frequency ω ２ inputting the pump light having the formula: from the fourth resonator to the fourth resonant frequency ω ４ 7. The quantum wavelength converter of claim 6, which emits output photons having the following formula:

8. the first resonant frequency ω １ is the second resonant frequency ω ２ is identical to the first resonator is identical to the second resonator; The laser light source has the first resonance frequency ω １ inputting a first pump light having a first wavelength into the optical fiber; from the fourth resonator to the fourth resonant frequency ω ４ 7. The quantum wavelength converter of claim 6, comprising:

9. A quantum wavelength converter described in any one of claims 1 to 8, wherein the optical fiber includes a nano-optical fiber portion and two end portions having a larger diameter than the nano-optical fiber portion and connected to both end portions of the nano-optical fiber portion via tapered portions, and at least a portion of the resonant optical path is located in the nano-optical fiber portion.

10. 9. The quantum wavelength converter according to claim 1, wherein each of the plurality of resonators has a pair of fiber Bragg gratings whose reflection bands include the respective resonant frequencies.

11. 11. The quantum wavelength converter of claim 10, wherein at least one of the fiber Bragg gratings is identical to any other of the fiber Bragg gratings.

12. The quantum wavelength converter of claim 1 ; The third resonant frequency ω output from the third resonator connected to the optical fiber ３ a first output optical fiber that propagates a first single output photon having The fourth resonant frequency ω output from the fourth resonator connected to the optical fiber ４ a second output optical fiber that propagates a second single output photon having a single-photon detector connected to the first output optical fiber and configured to detect the first single output photon; The laser light source has the first resonance frequency ω １ and the second resonant frequency ω ２ and a second pump light having the same wavelength as the first pump light, and a heralded single photon source for inputting the second pump light having the same wavelength as the first pump light into the optical fiber.

13. the first resonant frequency ω １ is the second resonant frequency ω ２ is identical to the first resonator is identical to the second resonator; 13. The heralded single photon source of claim 12, wherein the second pump light is the same as the first pump light.

14. The quantum wavelength converter according to claim 4; The third resonant frequency ω output from the third resonator connected to the optical fiber ３ a first output optical fiber that propagates a first single output photon having The fourth resonant frequency ω output from the fourth resonator connected to the optical fiber ４ a second output optical fiber that propagates a second single output photon having a single-photon detector connected to the first output optical fiber and configured to detect the first single output photon; The laser light source has the first resonance frequency ω １ a heralded single photon source for inputting the pump light having the following structure into the optical fiber:

15. The optical fiber includes a nano-optical fiber portion and two end portions having a diameter larger than that of the nano-optical fiber portion and connected to both end portions of the nano-optical fiber portion via tapered portions, and at least a portion of the resonant optical path is located in the nano-optical fiber portion.

16. 15. A heralded single photon source according to claim 12, wherein each of the plurality of resonators has a pair of fiber Bragg gratings whose reflection bands include the respective resonant frequencies.

17. 17. The heralded single photon source of claim 16, wherein at least one of the fiber Bragg gratings is identical to any other of the fiber Bragg gratings.

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