Quantum entanglement light source device

Abstract

The present invention provides a quantum entanglement light source device that takes advantage of the benefits of using optical fibers and can compensate for the decrease in polarization extinction ratio with a simple configuration. [Solution] A quantum entanglement light source device according to one embodiment of the present invention comprises a wavelength conversion unit capable of generating polarized light with a wavelength longer than the wavelength of the pump light, and a polarization control unit provided midway through optical fibers provided on both end faces of the wavelength conversion unit, capable of controlling the polarization direction of the pump light in the wavelength range.

Description

Technical Field

[0001] The present invention relates to a quantum entanglement light source device.

Background Art

[0002] In recent years, research on new information processing technologies (hereinafter referred to as "quantum information processing technologies") based on the principles of quantum cryptography has been actively conducted. As one of the basic technologies for realizing this quantum information processing technology, entangled quantum photon pairs are known. For the generation of entangled quantum photon pairs, for example, spontaneous parametric down-conversion (SPDC) using a second-order nonlinear optical medium is used. When pump light is input into the second-order nonlinear optical medium, one pump photon is annihilated by SPDC, and signal photons and idler photons are generated. There is a quantum mechanical correlation between these two photons with respect to the polarization state and the generation time. Therefore, these two photons are called entangled quantum photon pairs. The generation of entangled quantum photon pairs is performed by utilizing the generation process of these entangled quantum photon pairs.

[0003] Various methods can be used for generating the above-mentioned entangled quantum photon pairs. For example, Patent Document 1 discloses a method of generating SPDC by propagating pump light in a polarization-maintaining optical fiber (PMF) and inputting it into a nonlinear optical medium. Further, for example, Non-Patent Document 1 discloses a method of propagating pump light in space without using an optical fiber and inputting it into a nonlinear optical medium.

[0004] As shown in Non-Patent Document 1, when propagating pump light in space, optical components such as mirrors and filters are generally used to adjust the optical path. However, these optical components require highly precise adjustment of their position and angle. Furthermore, using such optical components results in low resistance to disturbances. These problems lead to instability of characteristics when modularizing optical components. Therefore, as shown in Patent Document 1, a configuration that eliminates the need for optical path adjustment by using optical fibers as much as possible is desirable. However, in a typical quantum entangled photon pair, the wavelength λp of the pump light and the wavelengths λs and λi of the signal light / idler light have a relationship such that λs=λi=2λp. Therefore, the wavelengths of the pump light and the signal light / idler light differ significantly.

[0005] Since no conventional polarization-retaining optical fiber offers such a wide bandwidth, the pump light will have a wavelength shorter than the cutoff wavelength of the polarization-retaining optical fiber. Therefore, the pump light cannot propagate in single mode and must propagate in multimode. Generally, polarization-retaining optical fibers are designed for single-mode propagation, so multimode propagation results in a decrease in the polarization extinction ratio. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2011-013348 [Non-patent literature]

[0007] [Non-Patent Document 1] BS Shi, A. Tomita, “Generation of a pulsed polarization entangled photon pair using a Sagnac interferometer”, Phys. Rev. A 69 (2004) 013803. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] As described above, when light waves with a wavelength shorter than the cutoff wavelength are propagated in a polarization-retaining optical fiber, the polarization extinction ratio decreases due to multimode propagation. This decrease in the polarization extinction ratio causes a decrease in the efficiency of the SPDC. In Patent Document 1, in order to avoid multimode propagation, the frequency of the pump light is upconverted in a PPLN waveguide, and the resulting light is used as the new pump light in an SPDC to generate signal light and idler light. This makes it possible to make all the pump light, signal light, and idler light propagating in the polarization-retaining optical fiber wavelengths that can be propagated in single mode. However, in order to achieve this, it is necessary to use a special PPLN waveguide as described above. Therefore, it is desirable to provide a quantum entanglement light source device that can enjoy the advantages of using optical fibers and compensate for the decrease in the polarization extinction ratio with a simple configuration. [Means for solving the problem]

[0009] A quantum entanglement light source device according to one embodiment of the present invention is a device capable of generating quantum entanglement light using pump light. This quantum entanglement light source device comprises a pump light generation unit, a polarization separation unit, a wavelength conversion unit, a first optical fiber, a second optical fiber, a first polarization control unit, and a second polarization control unit. The pump light generation unit is capable of generating pump light. The polarization separation unit is capable of generating first polarized light and second polarized light with mutually different polarization directions by polarization separation of the pump light. The wavelength conversion unit has a first end face and a second end face that face each other. The first optical fiber is arranged between the third end face of the polarization separation unit from which the first polarized light is emitted and the first end face of the wavelength conversion unit. The second optical fiber is arranged between the fourth end face of the polarization separation unit from which the second polarized light is emitted and the second end face of the wavelength conversion unit. The first polarization control unit is provided in the middle of the first optical fiber and is capable of controlling the polarization direction of the pump light in the wavelength range. The second polarization control unit is located in the middle of the second optical fiber and is capable of controlling the polarization direction of the pump light in the wavelength range. The wavelength conversion unit is configured such that when light from the pump light that has passed through the polarization separation unit and the first polarization control unit is incident on the first end face, and light from the pump light that has passed through the polarization separation unit and the second polarization control unit is incident on the second end face, it emits third-polarized light, which is the signal wave and idler wave and has a longer wavelength than the pump light, from the first end face, and emits fourth-polarized light, which is the signal wave and idler wave and has a longer wavelength than the pump light, from the second end face. [Effects of the Invention]

[0010] In a quantum entanglement light source device according to one embodiment of the present invention, a polarization control unit capable of controlling the polarization direction in the wavelength range of the pump light is provided in the middle of each optical fiber provided on both end faces of a wavelength conversion unit capable of generating polarized light with a wavelength longer than the wavelength of the pump light. As a result, the polarization control unit can suppress the decrease in the polarization extinction ratio of the pump light propagating through the optical fiber. The polarization control unit can be realized, for example, by applying stress to the optical fiber, and can be realized, for example, by winding a part of the optical fiber multiple times. In this way, the decrease in the polarization extinction ratio of the pump light propagating through the optical fiber can be suppressed without using a special PPLN waveguide. Therefore, the advantages of using optical fibers can be enjoyed, and the decrease in the polarization extinction ratio can be compensated for with a simple configuration. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a diagram showing a schematic configuration example of a quantum entanglement light source device according to one embodiment of the present invention. [Figure 2] Figure 2 is a diagram illustrating the quantum entanglement light output process in the quantum entanglement light source device shown in Figure 1. [Figure 3] Figure 3 shows an example of polarized light input and output to the wavelength conversion element shown in Figures 1 and 2. [Figure 4] Figure 4 shows an example of the input and output of polarized light to the polarizing beam splitter shown in Figures 1 and 2. [Figure 5] Figure 5(A) is a diagram showing a schematic configuration example of the polarization control unit shown in Figures 1 and 2. Figure 5(B) is a diagram showing the adjustment of the paddle rotation angle in the polarization control unit shown in Figure 5(A). Figure 5(C) is a diagram explaining the paddle rotation angle in the polarization control unit shown in Figures 5(A) and 5(B). [Figure 6] Figure 6 shows an example of a device that measures light output from a polarization-maintaining optical fiber via the polarization control unit shown in Figures 1 and 2. [Figure 7] Figure 7 shows an example of the dependence of the paddle rotation angle of the light output from the polarization-maintaining optical fiber via the polarization control unit shown in Figures 1 and 2. [Figure 8] Figure 8 shows a modified example of the schematic configuration of the quantum entanglement light source device shown in Figure 1. [Modes for carrying out the invention]

[0012] The embodiments for carrying out the present invention will be described in detail below with reference to the drawings. The following description is one specific example of the present invention, and the present invention is not limited to the following embodiments. Furthermore, the present invention is not limited to the arrangement, dimensions, dimensional ratios, etc., of each component shown in each figure.

[0013] <1. Background> In recent years, research into new information processing technologies based on the principles of quantum cryptography (hereinafter referred to as "quantum information processing technologies") has been actively conducted. One of the fundamental technologies for realizing these quantum information processing technologies is the generation of entangled photon pairs. For example, spontaneous parametric down-conversion (SPDC) using a second-order nonlinear optical medium is used to generate entangled photon pairs. When pump light is input to a second-order nonlinear optical medium, the SPDC causes one pump photon to disappear, and a signal photon and an idler photon are generated. There is a quantum mechanical correlation between these two photons in terms of polarization state and generation time. Therefore, these two photons are called a quantum correlated photon pair. The generation of entangled photon pairs is carried out by utilizing the generation process of this quantum correlated photon pair.

[0014] Various methods can be used to generate the above-mentioned entangled photon pairs. For example, Patent Document 1 discloses a method in which a pump light is propagated through a polarization-retaining optical fiber and input into a nonlinear optical medium to generate SPDC. Also, for example, Non-Patent Document 1 discloses a method in which a pump light is propagated in space without using an optical fiber and input into a nonlinear optical medium.

[0015] When propagating pump light in space as shown in Non-Patent Document 1, optical components such as mirrors and prisms are generally used for adjusting the optical path. However, these adjustments require techniques and labor for fine adjustment. Therefore, as shown in Patent Document 1, a configuration that does not require adjustment of the optical path by using an optical fiber as much as possible is desirable. However, in a typical entangled photon pair, there is a relationship such as λs = λi = 2λp between the wavelength λp of the pump light and the wavelengths λs and λi of the signal light and idler light. Therefore, the wavelengths of the pump light and the signal light / idler light are significantly different.

[0016] Since there is no product corresponding to such a wide band in a general polarization-maintaining optical fiber, the pump light has a wavelength shorter than the cut-off wavelength of the polarization-maintaining optical fiber. For this reason, the pump light cannot propagate in single mode and becomes multi-mode propagation. Generally, since a polarization-maintaining optical fiber is designed on the premise of single-mode propagation, the polarization extinction ratio decreases for multi-mode propagation.

[0017] As described above, when propagating a light wave with a wavelength shorter than the cut-off wavelength in a polarization-maintaining optical fiber, the polarization extinction ratio decreases due to multi-mode propagation. This decrease in the polarization extinction ratio causes a decrease in the efficiency of SPDC. In Patent Document 1, in order to avoid multi-mode propagation, the frequency of the pump light is up-converted in a PPLN waveguide, and signal light and idler light are generated by SPDC using the obtained light as a new pump light. Thereby, it is realized that all of the pump light, signal light, and idler light propagating in the polarization-maintaining optical fiber have wavelengths that enable single-mode propagation. However, in order to achieve this, it is necessary to use a special PPLN waveguide as described above. Therefore, as a result of intensive studies, the inventor of the present application came up with an invention that can enjoy the merits of using an optical fiber and compensate for the decrease in the polarization extinction ratio with a simple configuration.

[0018] Hereinafter, embodiments for implementing the present invention will be described in detail with reference to the drawings. The following description is a specific example of the present invention, and the present invention is not limited to the following aspects. Also, the present invention is not limited to the arrangement, dimensions, dimensional ratios, etc. of each component shown in each figure.

[0019] <2. Embodiment> [Configuration] The configuration of the entangled photon source device 100 according to an embodiment of the present invention will be described. FIGS. 1 and 2 show schematic configuration examples of the entangled photon source device 100. The entangled photon source device 100 is a device capable of generating entangled photons L10 using pump light L1. The entangled photon source device 100 includes, for example, a pump light generation unit 110, a wavelength conversion element 120, and a polarization beam splitter 130, as shown in FIGS. 1 and 2.

[0020] The entangled photon source device 100 corresponds to a specific example of the "entangled photon source device" according to an embodiment of the present invention. The pump light generation unit 110 corresponds to a specific example of the "pump light generation unit" according to an embodiment of the present invention. The wavelength conversion element 120 corresponds to a specific example of the "wavelength conversion unit" according to an embodiment of the present invention. The polarization beam splitter 130 corresponds to a specific example of the "polarization separation unit" according to an embodiment of the present invention. The pump light L1 corresponds to a specific example of the "pump light" according to an embodiment of the present invention.

[0021] The pump light generation unit 110 can generate pump light L1 with an optical frequency fp and an optical wavelength λp (= 1 / fp) and make it incident on the polarization beam splitter 130. The pump light generation unit 110 includes, for example, a semiconductor laser element capable of generating laser light as the pump light L1. The pump light L1 is linearly polarized light in a diagonal direction with respect to the polarization beam splitter 130. By making the pump light L1 incident on the polarization beam splitter 130 in the diagonal direction, the pump light L1 is separated into pump lights (polarized lights L2, L3) with polarization components in the horizontal and vertical directions.

[0022] The wavelength conversion element 120 is composed of a second-order nonlinear optical medium. The second-order nonlinear optical medium is provided with a first end face S1 and a second end face S2 facing each other, as shown in Figure 3, for example. The first end face S1 corresponds to a specific example of the "first end face" according to one embodiment of the present invention. The second end face S2 corresponds to a specific example of the "second end face" according to one embodiment of the present invention. An example of the second-order nonlinear optical medium is periodically polled lithium niobate (PPLN). When pump light with optical frequency fp and optical wavelength λp is input to both the first end face S1 and the second end face S2 of the PPLN, the SPDC causes one pump photon to disappear, and a signal photon (signal wave) with optical frequency fs and optical wavelength λs (=1 / fs), and an idler photon (idler wave) with frequency fi and optical wavelength λs (=1 / fi) are simultaneously generated from both the first end face S1 and the second end face S2.

[0023] Here, the optical frequencies fs and fi satisfy fp = fs + fi. The optical wavelengths λs and λi satisfy 1 / λp = 1 / λs + 1 / λi. The optical wavelength λp is, for example, 0.755 μm. The optical wavelengths λs and λi are each longer wavelengths than the optical wavelength λp, for example, 1.55 μm. The pump light incident on the first end face S1 is vertically polarized light, which corresponds to the linearly polarized light L4 shown in Figures 1 and 3. The pump light incident on the second end face S2 is vertically polarized light, which corresponds to the linearly polarized light L5 shown in Figures 1 and 3. The polarization direction of the pump light incident on the first end face S1 (polarized light L4) and the polarization direction of the pump light incident on the second end face S2 (polarized light L5) are equal to each other.

[0024] The signal photons (signal waves) and idler photons (idler waves) emitted from the first end face S1 are vertically polarized light, and are linearly polarized, corresponding to the polarized light L6 shown in Figures 2 and 3. The signal photons (signal waves) and idler photons (idler waves) emitted from the second end face S2 are vertically polarized light, and are linearly polarized, corresponding to the polarized light L7 shown in Figures 2 and 3. The polarization direction of the polarized light L6 emitted from the first end face S1 and the polarization direction of the polarized light L7 emitted from the second end face S2 are equal to each other.

[0025] When the wavelength conversion element 120 receives light from the first end face S1 that has been transmitted through the polarizing beam splitter 130 (polarized light L2) and light that has passed through the polarization control unit 170 (polarized light L4), which will be described later, and when the wavelength conversion element 120 receives light from the first end face S1 that has been transmitted through the polarizing beam splitter 130 (polarized light L3) and light that has passed through the polarization control unit 180 (polarized light L5), which will be described later, the SPDC emits polarized light L6, which is a signal wave and idler wave and has a longer wavelength than the pump light L1, from the first end face S1, and polarized light L7, which is a signal wave and idler wave and has a longer wavelength than the pump light L1, from the second end face S2.

[0026] The polarizing beam splitter 130 is a cube-shaped beam splitter composed of two right-angle prisms. A thin optical film capable of transmitting p-polarized light and reflecting s-polarized light is deposited on the slanted surface of one of the two right-angle prisms, and the slanted surfaces of the two right-angle prisms are joined together to obtain the cube-shaped beam splitter. The polarizing beam splitter 130 is arranged so that the pump light L1 is incident on the slanted surface inside the polarizing beam splitter 130 at an incident angle of 45°.

[0027] When pump light L1 is input to the polarizing beam splitter 130, it is capable of transmitting the component of the input pump light L1 that vibrates in a direction intersecting the inclined surface within the polarizing beam splitter 130, and reflecting the component of the input pump light L1 that vibrates parallel to the inclined surface within the polarizing beam splitter 130. The polarizing beam splitter 130 is provided with a third end face S3 from which light transmitted through the polarizing beam splitter 130 (polarized light L2) is emitted, and a fourth end face S4 from which light reflected by the polarizing beam splitter 130 (polarized light L3) is emitted. Polarized light L2 is the horizontally polarized component contained in the pump light L1 and is emitted from the third end face S3 in the same direction as the direction of propagation of the input pump light L1. Polarized light L3 is the vertically polarized component contained in the pump light L1 and is emitted from the fourth end face S4 in a direction 90° to the direction of propagation of the input pump light L1. The third end face S3 corresponds to a specific example of the "third end face" according to one embodiment of the present invention. The fourth end face S4 corresponds to a specific example of the "fourth end face" according to one embodiment of the present invention.

[0028] The polarizing beam splitter 130 is capable of generating quantum entangled light L10 by combining polarizing light L8 and polarizing light L9 when polarizing light L8 is incident on the third end face S3 and polarizing light L9 is incident on the fourth end face S4. Polarizing light L8 is horizontally polarized light output from the polarization control unit 170, which will be described later. Polarizing light L9 is vertically polarized light output from the polarization control unit 180, which will be described later.

[0029] The quantum entanglement light source device 100 includes, for example, a polarization-retaining optical fiber (PMF) 150 in the first optical path OP1 from the third end face S3 of the polarizing beam splitter 130 to the first end face S1 of the wavelength conversion element 120, as shown in Figures 1 and 2. In other words, the polarization-retaining optical fiber 150 is positioned between the third end face S3 of the polarizing beam splitter 130 and the first end face S1 of the wavelength conversion element 120. A focusing collimator 151 is connected to one end of the polarization-retaining optical fiber 150, and a focusing collimator 152 is connected to the other end of the polarization-retaining optical fiber 150. The polarization-retaining optical fiber 150 corresponds to a specific example of the "first optical fiber" according to one embodiment of the present invention.

[0030] The polarization-retaining optical fiber 150 is a fiber that enhances the polarization-maintaining characteristics of transmitted light by utilizing photoelastic effects and structural changes to create birefringence, where the effective refractive index differs in the longitudinal and transverse directions of the core. In the polarization-retaining optical fiber 150, the cutoff wavelength is longer than the wavelength of the pump light L1 and shorter than the wavelengths of the polarized light L6 and polarized light L7. The focusing collimator 151 is positioned close to the third end face S3 of the polarizing beam splitter 130. The focusing collimator 151 has the function of converting the polarized light L2 emitted from the third end face S3 of the polarizing beam splitter 130 into a predetermined beam diameter and propagating it within the polarization-retaining optical fiber 150. The focusing collimator 152 is positioned close to the first end face S1 of the wavelength conversion element 120. The focusing collimator 152 has the function of converting the polarized light L6 emitted from the first end face S1 of the wavelength conversion element 120 into a predetermined beam diameter and propagating it within the polarization-retaining optical fiber 150.

[0031] The quantum entanglement light source device 100 further includes a polarization-retaining optical fiber (PMF) 160 in the second optical path OP2 from the fourth end face S4 of the polarizing beam splitter 130 to the second end face S2 of the wavelength conversion element 120, as shown in Figures 1 and 2. In other words, the polarization-retaining optical fiber 160 is positioned between the fourth end face S4 of the polarizing beam splitter 130 and the second end face S2 of the wavelength conversion element 120. A focusing collimator 161 is connected to one end of the polarization-retaining optical fiber 160, and a focusing collimator 162 is connected to the other end of the polarization-retaining optical fiber 160. The polarization-retaining optical fiber 160 corresponds to a specific example of the "second optical fiber" according to one embodiment of the present invention. The optical path length of the second optical path OP2 is equal to the optical path length of the first optical path OP1.

[0032] The polarization-retaining optical fiber 160 is a fiber that enhances the polarization-maintaining characteristics of transmitted light by utilizing photoelastic effects and structural changes to create birefringence, where the effective refractive index differs in the longitudinal and transverse directions of the core. In the polarization-retaining optical fiber 160, the cutoff wavelength is longer than the wavelength of the pump light L1 and shorter than the wavelengths of the polarized light L6 and polarized light L7. The focusing collimator 161 is positioned close to the fourth end face S4 of the polarizing beam splitter 130. The focusing collimator 161 has the function of converting the polarized light L3 emitted from the fourth end face S4 of the polarizing beam splitter 130 into a predetermined beam diameter and propagating it through the polarization-retaining optical fiber 160. The focusing collimator 162 is positioned close to the second end face S2 of the wavelength conversion element 120. The focusing collimator 162 has the function of converting the polarized light L7 emitted from the second end face S2 of the wavelength conversion element 120 into a predetermined beam diameter and propagating it through the polarization-retaining optical fiber 160.

[0033] The quantum entanglement light source device 100 further includes, for example, a polarization control unit 170 provided in the middle of the polarization-retaining optical fiber 150 and a polarization control unit 180 provided in the middle of the polarization-retaining optical fiber 160, as shown in Figures 1 and 2. The polarization control unit 170 corresponds to a specific example of the "first polarization control unit" according to one embodiment of the present invention. The polarization control unit 180 corresponds to a specific example of the "second polarization control unit" according to one embodiment of the present invention. The polarization control units 170 and 180 are capable of controlling the polarization direction of the pump light L1 in the wavelength range.

[0034] The polarization control units 170 and 180 are configured to include three paddles Pdl1, Pdl2, and Pdl3, as shown in Figure 5(A), for example. Paddle Pdl1 has a configuration in which multiple portions of a polarization-retaining optical fiber 150 or a polarization-retaining optical fiber 160 (fiber F1) are wound around a spool Sp1, and an angle adjustment unit ST1 that can adjust the angle of spool Sp1. Paddle Pdl2 has a configuration in which multiple portions of a polarization-retaining optical fiber 150 or a polarization-retaining optical fiber 160 (fiber F2) are wound around a spool Sp2, and an angle adjustment unit ST2 that can adjust the angle of spool Sp2. Paddle Pdl3 has a configuration in which multiple portions of a polarization-retaining optical fiber 150 or a polarization-retaining optical fiber 160 (fiber F3) are wound around a spool Sp3, and an angle adjustment unit ST3 that can adjust the angle of spool Sp3. Spools Sp1, Sp2, and Sp3 correspond to specific examples of the "first spool" and "second spool" according to one embodiment of the present invention. Angle adjustment parts ST1, ST2, and ST3 correspond to specific examples of the "first angle adjustment part" and "second angle adjustment part" according to one embodiment of the present invention.

[0035] In paddle Pdl1, for example, fiber F1 has 4 turns. In paddle Pdl2, for example, fiber F2 has 3 turns. In paddle Pdl3, for example, fiber F3 has 4 turns. In paddles Pdl1, Pdl2, and Pdl3, the number of turns of fibers F1, F2, and F3 may be equal to each other or different from each other. In paddles Pdl1, Pdl2, and Pdl3, the winding diameters of fibers F1, F2, and F3 may be equal to each other or different from each other. The number of paddles included in polarization control units 170 and 180 may be equal to each other or different from each other.

[0036] The angle adjustment unit ST1 is a mechanism that can adjust the rotation angle τ of the paddle Pdl1 (see Figure 5(C)). By adjusting the rotation angle τ of the paddle Pdl1 with the angle adjustment unit ST1, the paddle Pdl1 can be shifted, for example, from the position shown in Figure 5(A) to the position shown in Figure 5(B). The angle adjustment unit ST2 is a mechanism that can adjust the rotation angle τ of the paddle Pdl2. The angle adjustment unit ST3 is a mechanism that can adjust the rotation angle τ of the paddle Pdl3.

[0037] In the polarization control unit 170, the rotation angle τ is adjusted by the angle adjustment units ST1, ST2, and ST3 so that polarized light L2 propagating through the polarization-retaining optical fiber 150 can be converted into polarized light L4. Polarized light L4 is polarized light perpendicular to the polarization direction of polarized light L2 (vertically polarized light). In the polarization control unit 170, the rotation angle τ is adjusted by the angle adjustment units ST1, ST2, and ST3 so that polarized light L6 propagating through the polarization-retaining optical fiber 150 can be converted into polarized light L8. Polarized light L8 is polarized light perpendicular to the polarization direction of polarized light L6 (horizontally polarized light). In the polarization control unit 180, the rotation angle τ is adjusted by the angle adjustment units ST1, ST2, and ST3 so that polarized light L3 propagating through the polarization-retaining optical fiber 160 can be converted into polarized light L5. Polarized light L5 is polarized light parallel to the polarization direction of polarized light L3 (vertically polarized light). In the polarization control unit 180, the rotation angle τ is adjusted by the angle adjustment units ST1, ST2, and ST3 so that the polarized light L7 propagating through the polarization-retaining optical fiber 160 can be converted into polarized light L9. Polarized light L9 is polarized light parallel to the polarization direction of polarized light L7 (perpendicularly polarized light).

[0038] By applying stress to fibers F1, F2, and F3 through bending or twisting, birefringence is induced in fibers F1, F2, and F3. This birefringence makes it possible to create any desired polarization state at the paddle output. It is known that the retardation obtained by this birefringence is proportional to the number of turns and inversely proportional to the winding diameter, and that the polarization rotation angle is proportional to the paddle rotation angle τ. Furthermore, Non-Patent Literature 2 discloses that, under normal operating conditions, the effect on polarization crosstalk (polarization extinction ratio) is minimal even when the optical fiber is bent or twisted, for light propagating in single mode through a polarization-retaining optical fiber. Based on these two findings, the present invention provides a polarization control unit 170 in the middle of the polarization-retaining optical fiber 150 and a polarization control unit 180 in the middle of the polarization-retaining optical fiber 160. The following describes experiments conducted to verify the effects of providing the polarization control units 170 and 180.

[0039] Non-patent document 2: Shinichi Arai, Hirofumi Saito, et al., "Polarization-maintaining optical fiber," Furukawa Electric Times, No. 109 (2002)

[0040] Figure 6 shows an example of a device for measuring light output from a polarization-maintaining optical fiber 150 via a polarization control unit 170. A Thorlabs Inc. PM980-XP was used as the polarization-maintaining optical fiber 150. The polarization control unit 170 has three paddles Pdl1, Pdl2, and Pdl3. Fiber F1 is wound four times on paddle Pdl1. Fiber F2 is wound three times on paddle Pdl2. Fiber F3 is wound four times on paddle Pdl3. The diameters of fibers F1, F2, and F3 are 30 mm. In this case, paddle Pdl1 has a function similar to a λ / 4 wave plate. Paddle Pdl2 has a function similar to a λ / 2 wave plate. Paddle Pdl3 has a function similar to a λ / 4 wave plate.

[0041] The light source 230 that supplies light to the polarization-maintaining optical fiber 150 is a semiconductor laser capable of emitting laser light with a wavelength of 786 nm or 1550 nm. The light source 230 is optically coupled by a focusing collimator 171 connected to one end of the polarization-maintaining optical fiber 150. Light with a wavelength of 786 nm propagates through the polarization-maintaining optical fiber 150 in multimode, and light with a wavelength of 1550 nm propagates through the polarization-maintaining optical fiber 150 in single mode. The light that has propagated through the polarization-maintaining optical fiber 150 is output into free space by a focusing collimator 172 connected to the other end of the polarization-maintaining optical fiber 150 and passes through the analyzer 210. Only the polarization component in a specific direction is extracted by the analyzer 210, and the power of the light that has passed through the analyzer 210 is measured by the sensor 220.

[0042] In the apparatus shown in Figure 6, with a laser beam of either 786 nm or 1550 nm emitted from the light source 230, the rotation angle τ of the paddle P3 was rotated, and the polarization extinction ratio was measured for each rotation angle τ. The results are shown in Figure 7. The waveform shown as "PMF Single" in Figure 7 is the result obtained when a laser beam of 1550 nm was emitted from the light source 230, and the waveform shown as "PMF Multi" in Figure 7 is the result obtained when a laser beam of 786 nm was emitted from the light source 230. As a comparative example, a standard single-mode optical fiber (SMF) was used instead of the polarization-maintaining optical fiber 150, and with a laser beam of either 786 nm or 1550 nm emitted from the light source 230, the rotation angle τ of the paddle P3 was rotated, and the polarization extinction ratio was measured for each rotation angle τ. The results are shown in Figure 7. The waveform shown in Figure 7 as "SMF Single" is the result obtained when laser light with a wavelength of 1550 nm is emitted from the light source 230, and the waveform shown in Figure 7 as "SMF Multi" is the result obtained when laser light with a wavelength of 786 nm is emitted from the light source 230.

[0043] Figure 7 shows that when the polarization-maintaining optical fiber 150 propagates in single mode, a constant characteristic is obtained regardless of the rotation angle τ. Also, Figure 7 shows that when the polarization-maintaining optical fiber 150 propagates in multimode, when a single-mode optical fiber (SMF) propagates in single mode, or when a single-mode optical fiber (SMF) propagates in multimode, a periodic change in characteristics is observed, where the characteristics deteriorate with increasing or decreasing rotation angle τ, and then improve again with further increases or decreases in rotation angle τ, relative to the angle showing the maximum characteristics. Furthermore, Figure 7 shows that the characteristics deteriorate the most when propagating in multimode through a single-mode optical fiber (SMF), and the deviation in the polarization extinction ratio is small when propagating in multimode through the polarization-maintaining optical fiber 150.

[0044] From the above, it can be seen that when the polarization-maintaining optical fiber 150 propagates in single mode, the polarization control unit 170 has no effect, while when the polarization-maintaining optical fiber 150 propagates in multimode, the deviation in the polarization extinction ratio can be reduced by controlling the rotation angle τ in the polarization control unit 170. Therefore, the experimental results show that it is possible to maintain the polarization state of single-mode propagating light while simultaneously controlling the polarization state of multi-mode propagating light.

[0045] The quantum entanglement light source device 100 further includes a wavelength separation mirror 140 capable of outputting the quantum entangled light L10 generated by the polarizing beam splitter 130 to the outside, as shown in Figures 1 and 2. The wavelength separation mirror 140 is positioned between the pump light generation unit 110 and the polarizing beam splitter 130. The wavelength separation mirror 140 is capable of transmitting the pump light L1 and reflecting the quantum entangled light L10.

[0046] [effect] Next, the effects of the quantum entanglement light source device 100 according to this embodiment will be described.

[0047] In this embodiment, polarization control units 170 and 180 capable of controlling the polarization direction in the wavelength range of the pump light L1 are provided midway through the polarization-retaining optical fibers 150 and 160, which are located on both end faces of the wavelength conversion unit 120 capable of generating polarized light L6 and L7 with wavelengths longer than the wavelength of the pump light L1. This allows the polarization control units 170 and 180 to suppress the decrease in the polarization extinction ratio of the pump light propagating through the polarization-retaining optical fibers 150 and 160. The polarization control units 170 and 180 can be implemented, for example, by applying stress to the polarization-retaining optical fibers 150 and 160, and can be implemented, for example, by winding a portion of the polarization-retaining optical fibers 150 and 160 multiple times. In this way, the decrease in the polarization extinction ratio of the pump light propagating through the polarization-retaining optical fibers 150 and 160 can be suppressed without using a special PPLN waveguide. Therefore, the benefits of using the polarization-retaining optical fibers 150 and 160 can be enjoyed while compensating for the decrease in the polarization extinction ratio with a simple configuration.

[0048] In this embodiment, the polarization control units 170 and 180 are configured to include three paddles Pdl1, Pdl2, and Pdl3. Each paddle Pdl1, Pdl2, and Pdl3 is provided with a configuration in which multiple portions of a polarization-retaining optical fiber 150 or a polarization-retaining optical fiber 160 are wound around a spool, and an angle adjustment unit that allows the angle of the spool to be adjusted. As a result, the polarization control units 170 and 180 can maintain the polarization state of single-mode propagating light while simultaneously compensating for the decrease in the polarization extinction ratio of multi-mode propagating light.

[0049] In this embodiment, the cutoff wavelengths of the polarization-retaining optical fibers 150 and 160 are longer than the wavelength of the pump light L1 and shorter than the wavelengths of the polarized light L6 and polarized light L7. However, since the polarization control units 170 and 180 are provided in the middle of the polarization-retaining optical fibers 150 and 160, it is possible to maintain the polarization state of single-mode propagating light while simultaneously compensating for the decrease in the polarization extinction ratio of multi-mode propagating light.

[0050] <3. Variant> In the above embodiment, the quantum entanglement light source device 100 may, for example, include a variable optical delay line 190 in the middle of the polarization-retaining optical fiber 150, as shown in Figure 8. The variable optical delay line 190 has a configuration in which a dispersive medium is sandwiched between two wavelength converters. The configuration of the variable optical delay line 190 is not limited to the above configuration. The variable optical delay line 190 may be provided only in the middle of the polarization-retaining optical fiber 160, or it may be provided in the middle of both the polarization-retaining optical fiber 150 and the polarization-retaining optical fiber 160.

[0051] In this modified example, a variable optical delay line 190 is provided in the middle of at least one of the polarization-retaining optical fibers 150 and 160. By adjusting the variable optical delay line 190, the optical path length of the first optical path OP1 and the optical path length of the second optical path OP2 can be made equal to each other.

[0052] In the above embodiment and its modifications, single-mode optical fibers may be used instead of polarization-retaining optical fibers 150 and 160. In this case, the single-mode optical fiber (SMF) propagates in single mode and multimode while searching for the rotation angle τ that minimizes the polarization extinction ratio deviation. Therefore, adjusting the rotation angle τ is not as easy as in the above embodiment. However, since it is possible to find a better rotation angle τ, it is possible to obtain the same effects as in the above embodiment even when single-mode optical fibers are used instead of polarization-retaining optical fibers 150 and 160.

[0053] The effects described herein are for illustrative purposes only. The effects of this disclosure are not limited to those described herein. This disclosure may have effects other than those described herein.

[0054] Furthermore, for example, this disclosure can take the following configuration. <1> A quantum entanglement light source device (100) capable of generating quantum entanglement light (L10) using pump light (L1), A pump light generating unit (110) capable of generating the aforementioned pump light (L1), A polarization separation unit (130) capable of generating first polarized light (L2) and second polarized light (L3) having different polarization directions by polarizing the pump light (L1), A wavelength conversion unit (120) having a first end face (S1) and a second end face (S2) facing each other, Of the polarization separation section (130), a first optical fiber (150) is disposed between the third end face (S3) from which the first polarized light (L2) is emitted and the first end face (S1) of the wavelength conversion section. Of the polarization separation section (130), a second optical fiber (160) is disposed between the fourth end face (S4) from which the second polarized light (L3) is emitted and the second end face (S2) of the wavelength conversion section. A first polarization control unit (170) is provided in the middle of the first optical fiber (150) and is capable of controlling the polarization direction of the wavelength range of the pump light (L1), A second polarization control unit (180) is provided in the middle of the second optical fiber (160) and is capable of controlling the polarization direction of the wavelength range of the pump light (L1). Equipped with, The wavelength conversion unit (120) is configured such that when light from the pump light (L1) that has passed through the polarization separation unit (130) and the first polarization control unit (170) is incident on the first end face (S1), and when light from the pump light (L1) that has passed through the polarization separation unit (130) and the second polarization control unit (180) is incident on the second end face (S2), it emits third-polarized light (L6), which is a signal wave and idler wave, and has a longer wavelength than the pump light (L1), from the first end face (S1), and emits fourth-polarized light (L7), which is a signal wave and idler wave, and has a longer wavelength than the pump light (L1), from the second end face (S2). Quantum entanglement light source device (100). <2> The first optical fiber (150) and the second optical fiber (160) are polarization-retaining optical fibers. <1> The quantum entanglement light source device (100) described above. <3> The first polarization control unit (170) is configured to include one or more first paddles (pdl1, pdl2, pdl3), The above or a plurality of first paddles (pdl1, pdl2, pdl3) have a configuration in which a portion of the first optical fiber (150) is wound multiple times around a first spool (Sp1, Sp2, Sp3), and have first angle adjustment parts (ST1, ST2, ST3) that can adjust the angle of the first spool (Sp1, Sp2, Sp3). The second polarization control unit (180) is configured to include one or more second paddles (pdl1, pdl2, pdl3), The above or a plurality of second paddles (pdl1, pdl2, pdl3) have a configuration in which a portion of the second optical fiber (160) is wound multiple times around a second spool (Sp1, Sp2, Sp3), and have second angle adjustment parts (ST1, ST2, ST3) that can adjust the angle of the second spool (Sp1, Sp2, Sp3). <1> or <2> The quantum entanglement light source device (100) described above. <4> The optical delay line (190) is further provided in the middle of at least one of the first optical fiber (150) and the second optical fiber (160). <1> or <3> A quantum entanglement light source device (100) as described in any one of the following. <5> The cutoff wavelengths of the first optical fiber (150) and the second optical fiber (160) are longer than the wavelength of the pump light (L1) and shorter than the wavelengths of the third polarized light (L6) and the fourth polarized light (L7). <1> or <4> A quantum entanglement light source device (100) as described in any one of the following. [Explanation of Symbols]

[0055] 100...Quantum entanglement light source device, 110...Pump light generator, 120...Wavelength conversion element, 130...Polarization beam splitter, 140...Wavelength separation mirror, 150,160...Polarization-retaining optical fiber, 151,152,161,162...Collimator, 170,180...Polarization control unit, 190...Variable optical delay line, 210...Analyzer, 220...Sensor, L1...Pump light, L2, L3, L4, L5, L6, L7, L8, L9... Polarized light, L10... Entangled light, F1, F2, F3... Fiber, OP1... First optical path, OP2... Second optical path, Ppdl1, Pdl2, Pdl3... Paddle, Sp1, Sp2, Sp3... Spool, S1... First end face, S2... Second end face, S3... Third end face, S4... Fourth end face, ST1, ST2, ST3... Angle adjustment section, τ... Rotation angle.

Claims

1. A quantum entanglement light source device capable of generating quantum entangled light using pump light, A pump light generating unit capable of generating the aforementioned pump light, A polarization separation unit capable of generating first polarized light and second polarized light having different polarization directions by polarizing the pump light, A wavelength conversion unit having a first end face and a second end face facing each other, Of the polarization separation section, a first optical fiber is disposed between the third end face from which the first polarized light is emitted and the first end face of the wavelength conversion section, A second optical fiber is disposed between the fourth end face from which the second polarized light is emitted and the second end face of the wavelength conversion unit, A first polarization control unit is provided in the middle of the first optical fiber, which is capable of controlling the polarization direction of the wavelength range of the pump light, A second polarization control unit is provided in the middle of the second optical fiber, which is capable of controlling the polarization direction of the wavelength range of the pump light. Equipped with, The wavelength conversion unit is configured such that, when light from the pump light that has passed through the polarization separation unit and the first polarization control unit is incident on the first end face, and light from the pump light that has passed through the polarization separation unit and the second polarization control unit is incident on the second end face, it emits third-polarized light, which is a signal wave and idler wave, with a wavelength longer than the wavelength of the pump light, from the first end face, and fourth-polarized light, which is a signal wave and idler wave, with a wavelength longer than the wavelength of the pump light, from the second end face. Quantum entanglement light source device.

2. The first optical fiber and the second optical fiber are polarization-retaining optical fibers. The quantum entanglement light source device according to claim 1.

3. The first polarization control unit is configured to include one or more first paddles, The above or a plurality of first paddles have a configuration in which a portion of the first optical fiber is wound multiple times around a first spool, and a first angle adjustment unit that can adjust the angle of the first spool. The second polarization control unit is configured to include one or more second paddles, The above or a plurality of second paddles have a configuration in which a portion of the second optical fiber is wound multiple times around a second spool, and a second angle adjustment unit that can adjust the angle of the second spool. A quantum entanglement light source device according to claim 1 or claim 2.

4. The first optical fiber and the second optical fiber are further provided with an optical delay line in the middle of at least one of them. A quantum entanglement light source device according to claim 1 or claim 2.

5. The cutoff wavelengths of the first and second optical fibers are longer than the wavelength of the pump light and shorter than the wavelengths of the third and fourth polarized light. A quantum entanglement light source device according to claim 1 or claim 2.

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