Quantum entangled photon pair generation device

Abstract

This quantum entangled photon pair generation device (1) comprises: a resonance part (11) having a first optical path having a loop-shape and a generation element which is disposed on the first optical path and generates a two-photon state; an interference part (12) having a second optical path having a loop-shape and an input-output part which is disposed on the second optical path, is input with pump light, and outputs a quantum entangled photon pair, the interference part generating one or more photon pairs in a quantum entangled state from a plurality of two-photon states generated by the generation element; and a shared connection part (E2) which forms a loop-shaped optical path for each of the first optical path and the second optical path and optically connects the first optical path and the second optical path.

Description

Quantum entangled photon pair generator

[0001] The present invention relates to a quantum entangled photon pair generating device for generating quantum entangled photon pairs.

[0002] In quantum communication systems using quantum entanglement, it is important to generate entangled photons with a narrow spectral linewidth so that the entangled photons generated by a quantum entanglement source can be suitably absorbed by a quantum memory. In addition, in spectroscopic systems, the signal can be enhanced by generating entangled photons with high generation brightness.

[0003] In the quantum entanglement light source for long-distance quantum communication described in Patent Document 1, entangled photons are emitted by circulating them in a resonator, which makes it possible to generate entangled photons with a spectral linewidth equal to or less than a predetermined value.

[0004] Japanese Patent Application Publication No. 2019-035892

[0005] K. Niizeki et al. “Two-photon comb with wavelength conversion and 20-km distribution for quantum communication”, Nature Communication Physics, vol. 3,138 (2020). F. Steinlechner et al. ”Efficient heralding of polarization-entangled photons from type-0 and type-II spontaneous parametric downconversion in periodically poled KTiOPO4”, Journal of Optical Society of America B 31, No. 9, pp. 2068-2076 (2014). J. Wang et al. “Universal photonic quantum interface for a quantum network,” Physical review applied, vol. 10,054036 (2018).

[0006] However, conventional configurations have a problem in that it is difficult to improve the generated brightness of quantum entangled light for reasons described in detail below. Here, the generated brightness (also referred to as spectral brightness) of quantum entangled light in this case is defined as follows: Namely, the generated brightness (spectral brightness) is the generation probability of quantum entangled light per unit optical frequency, and is defined as including the loss when it is output to the outside for practical use. If the generated brightness is low, the influence of dark counts from the detector becomes large, which increases the time required to obtain a sufficient signal and reduces the probability of successful communication.

[0007] One aspect of the present invention has been made in view of the above-mentioned problems, and aims to improve the brightness of generated quantum entangled light.

[0008] In order to solve the above problems, a quantum entangled photon pair generating device according to one aspect of the present invention includes a resonator having a loop-shaped first optical path and a generation element arranged on the first optical path for generating two-photon states, a loop-shaped second optical path and an input / output element arranged on the second optical path for inputting pump light and outputting quantum entangled photon pairs, an interference element for generating one or more quantum entangled photon pairs from the two-photon states generated by the generation element, and a shared connection element for forming loop-shaped optical paths for each of the first optical path and the second optical path and optically connecting the first optical path and the second optical path. The two-photon states generated by the generation element in the resonator satisfy a resonance condition in the resonator.

[0009] According to one aspect of the present invention, the brightness of the generated quantum entangled light can be improved.

[0010] FIG. 1 is a diagram showing a configuration of a quantum entangled photon pair generating device according to embodiment 1 of the present invention. FIG. 2 is a diagram showing a specific configuration example 1 of the quantum entangled photon pair generating device according to embodiment 1 of the present invention. FIG. 3 is a diagram showing a modified example of specific configuration example 2 of the quantum entangled photon pair generating device according to embodiment 1 of the present invention. FIG. 4 is a diagram showing a second modified example of specific configuration example 2 of the quantum entangled photon pair generating device according to embodiment 1 of the present invention. FIG. 5 is a diagram showing a specific configuration example 6 of the quantum entangled photon pair generating device according to embodiment 1 of the present invention. FIG. 6 is a diagram showing a specific configuration example 7 of the quantum entangled photon pair generating device according to embodiment 1 of the present invention. FIG. 7 is a diagram showing a specific configuration example 8 of the quantum entangled photon pair generating device according to embodiment 1 of the present invention. FIG. 8 is a diagram showing a specific configuration example 9 of the quantum entangled photon pair generating device according to embodiment 1 of the present invention.

[0011] [Embodiment 1] An embodiment of the present invention will be described in detail below. Fig. 1 is a diagram showing the configuration of a quantum entangled photon pair generating device 1 according to this embodiment. The quantum entangled photon pair generating device 1 is, as an example, a generating device that generates photon pairs each composed of two photons that are quantum entangled with each other. The quantum entangled photon pair generating device 1 can also be simply called a quantum entangled light generating device, a quantum entangled light source, a quantum light source, an entanglement light source, or the like.

[0012] The quantum entangled photon pair generating device 1 can be used, for example, in a quantum communication system that performs communication using photons in a quantum entangled state, but this does not limit the present embodiment.

[0013] (Configuration of quantum entangled photon pair generating device) The configuration of the quantum entangled photon pair generating device 1 according to this embodiment will be described in more detail below. Fig. 1 is a diagram showing the configuration of the quantum entangled photon pair generating device 1 according to this embodiment. As shown in Fig. 1, the quantum entangled photon pair generating device 1 includes a resonator 11, an interference unit 12, and a shared connection unit E2.

[0014] (Resonator) As shown in Fig. 1, the resonator 11 has an optical path P1, an optical path P2, a generating element E1, and a shared connection E2. As shown in Fig. 1, one end of each of the optical paths P1 and P2 is optically connected by the shared connection E2 to form a loop-shaped optical path. Here, the optical path formed by the optical paths P1 and P2 may also be referred to as a first optical path. Specific configuration examples of the first optical path do not limit the present embodiment, but examples include: an optical path in a vacuum or air formed in a loop by multiple mirrors; an optical path within one or more fibers formed in a loop by the fibers, etc.

[0015] The generating element E1 is a generating element disposed on the first optical path, and is, for example, an optical generating element that generates a two-photon state. The specific configuration of the generating element E1 does not limit the present embodiment, but can be realized, for example, by a nonlinear optical medium such as a nonlinear optical crystal that causes second-order spontaneous parametric down-conversion, an optical fiber that causes four-wave mixing, or an optical fiber that causes third-order spontaneous parametric down-conversion.

[0016] (Interference Unit) The interference unit 12 is configured to generate one or more photon pairs in a quantum entangled state from the multiple two-photon states generated by the generation element E1.

[0017] As shown in Fig. 1, the interference unit 12 includes an optical path P3, an optical path P4, a shared connection unit E2, and an input / output unit E3. As shown in Fig. 1, one end of each of the optical paths P3 and P4 is optically connected via the shared connection unit E2 to form a loop-shaped optical path. Here, the optical path formed by the optical paths P3 and P4 may also be referred to as a second optical path. Specific configuration examples of the second optical path do not limit the present embodiment, but examples include: an optical path in a vacuum or air formed in a loop shape by multiple mirrors; an optical path within one or more fibers formed in a loop shape by the fibers, etc.

[0018] The input / output unit E3 is disposed on the second optical path and is configured to allow pump light PL generated by a pump light generating device (not shown) to flow into the second optical path and to emit quantum entangled photon pairs from the interference unit 12. The specific configuration of the input / output unit E3 is not limited to this embodiment, and can be realized by, for example, a beam splitter, a polarizing beam splitter, or a polarization separation fiber coupler. The input / output unit E3 splits the pump light PL into two, directs one of the split light beams to circulate clockwise through the second optical path, and directs the other split light beam to circulate counterclockwise through the second optical path.

[0019] As will be described later, in the second optical path of the interference unit 12, a portion of the light propagates clockwise, and another portion of the light propagates counterclockwise. The portion of the light and the other portion of the light are then combined (synthesized) by the input / output unit E3 and output from the input / output unit E3. Therefore, each component or part of the components of the interference unit 12 can also be described as constituting a Sagnac interferometer or a Sagnac interferometer. The quantum entangled photon pair generating device 1 according to this embodiment can also be described as a configuration that combines a Sagnac interferometer (interference unit 12) and a resonator (resonator unit 11), which was previously difficult to achieve. However, these expressions do not limit this embodiment.

[0020] (Shared Connection Portion) The shared connection portion E2 forms a loop-shaped optical path for each of the first optical path and the second optical path, and is configured to optically connect the first optical path and the second optical path, and is shared by the resonator 11 and the interference portion 12 as described above. The specific configuration of the shared connection portion E2 is not limited to this embodiment, but can be realized, for example, by including: a dichroic mirror; and a polarization-maintaining fiber coupler. For example, the shared connection portion E2 has reflectance and transmittance for optically coupling the interference portion 12 to the resonator 11.

[0021] (Operation of Quantum Entangled Photon Pair Generating Device) The operation of the quantum entangled photon pair generating device 1 will now be described. First, pump light (pump laser) PL generated by a pump light generating device (not shown) is incident on the input / output unit E3 as shown in FIG. 1 . The incident pump light is split into two by the input / output unit E3. One of the split light circulates clockwise along the second optical path composed of optical paths P3 and P4, and the other split light circulates counterclockwise along the second optical path.

[0022] At least a portion of the light circulating through the second optical path of the interference unit 12 flows into the resonator 11 via the shared connector E2. The light that has flowed into the resonator 11 circulates through the first optical path composed of optical paths P1 and P2. As an example, a portion of the light that has circulated clockwise through the second optical path of the interference unit 12 flows into the first optical path of the resonator 11 via the shared connector E2 and circulates counterclockwise through the first optical path. Furthermore, a portion of the light that has circulated counterclockwise through the second optical path of the interference unit 12 flows into the first optical path of the resonator 11 via the shared connector E2 and circulates clockwise through the first optical path.

[0023] The light circulating along the first optical path is incident on the generating element E1. A one-photon state constituting at least a portion of the light incident on the generating element E1 is converted to a two-photon state, for example, by spontaneous parametric down-conversion caused by the generating element E1. In other words, the generating element E1 generates a two-photon state from a one-photon state (pump photon) constituting at least a portion of the light circulating along the first optical path.

[0024] Incidentally, by providing a dichroic mirror as the shared connection part E2 that transmits the pump light PL and reflects the two-photon state, a resonance process is established for the two-photon state in the resonator part 11 even if the pump light PL is not resonated in the resonator part 11. However, as described above, the specific configuration of the shared connection part E2 does not limit this embodiment.

[0025] Furthermore, the resonance process in the resonator 11 does not depend on the configuration requirements of the interference unit 12 or the phase matching conditions in the generation element E1, but is determined by the wavelength and polarization of the two-photon state circulating in the resonator 11. On the other hand, as will be described in detail later, even in a two-photon state generated under phase matching conditions in which the polarized light circulating in the resonator 11 is the same, it is possible to generate a polarization-entangled state in which two-photon states with orthogonal polarizations are entangled with each other due to the action of the interference unit 12.

[0026] The two-photon state generated by the generation element E1 flows into the interference unit 12 via the shared connection E2 and reaches the input / output unit E3 via the second optical path. More specifically, as an example, a portion of the two-photon state propagating clockwise along the first optical path of the resonator 11 flows into the interference unit 12 via the shared connection E2 and propagates counterclockwise along the first optical path of the interference unit 12 to reach the input / output unit E3. Furthermore, a portion of the two-photon state propagating counterclockwise along the first optical path of the resonator 11 flows into the interference unit 12 via the shared connection E2 and propagates clockwise along the first optical path of the interference unit 12 to reach the input / output unit E3.

[0027] The input / output unit E3 generates a quantum entangled photon pair EPP composed of two photons in a quantum entangled relationship by combining the two-photon state propagated clockwise through the first optical path with the two-photon state propagated counterclockwise through the first optical path. The generated quantum entangled photon pair EPP is output to the outside of the interference unit 12 via the input / output unit E3. The output quantum entangled photon pair EPP is guided by an optical path (not shown) and is used, for example, in a quantum communication system.

[0028] If the interference unit 12 is configured to include an adjusted polarization rotation element on at least one of the optical paths P3 and P4, quantum entangled photon pairs can be generated more effectively at the input / output unit E3, although this configuration does not limit the present embodiment.

[0029] (Explanation of Effects) The following describes the effects achieved by the quantum entangled photon pair generating device 1 of the present invention. To explain the effects, first, a configuration according to a comparative example, which is a conventional configuration, will be described.

[0030] The following comparative examples are possible configurations for generating quantum entangled light, but the configurations according to any of the comparative examples have the problem that it is difficult to improve the brightness of the generated quantum entangled light.

[0031] (Comparative Example 1) Non-Patent Document 1 describes a technique for generating entanglement by arranging two periodically poled lithium niobite crystals in series, which satisfy the Type-0 phase matching condition and are capable of generating two photons of the same polarization. In this technique, two photons can be generated in one crystal for one component (one photon) of pump light, while the other crystal cannot. Therefore, the loss in the resonator increases for the crystals that cannot generate two photons, and the finesse decreases, thereby reducing the enhancement effect of the resonator. Therefore, it is difficult to improve the brightness of the generated quantum entangled light with the configuration of this comparative example.

[0032] (Comparative Example 2) Non-Patent Document 2 describes a method for generating quantum entangled light using a single crystal under the so-called Type-II phase matching condition. In the Type-II method, the nonlinear optical constant itself, which determines the generated brightness of the entire phase matching cone, is inherently lower than that of Type-0 or Type-I. Furthermore, entanglement is achieved by spatially extracting a portion of the cone, which increases loss and results in a lower generation rate. Therefore, it is difficult to improve the generated brightness of quantum entangled light with the configuration of this comparative example.

[0033] (Comparative Example 3) Non-Patent Document 3 describes a method for improving the generated brightness by using a configuration for coupling with a resonator or forming a resonator. On the other hand, with the configuration described in Non-Patent Document 3, when generating narrow-linewidth quantum entangled light, it becomes necessary to control with high precision the operating temperature of the nonlinear optical crystal, the pump light wavelength, and the quantum entangled light wavelength. This problem arises, particularly when the wavelengths of the two photons are non-degenerate, as it is necessary to search for the conditions for quadruple resonance, which complicates the driving conditions and requires additional optical components to satisfy the conditions, resulting in problems such as increased loss and reduced generated brightness.

[0034] (Effects of the quantum entangled photon pair generating device 1) On the other hand, according to the quantum entangled photon pair generating device 1 of this embodiment, the resonance process in the resonator 11 is determined by the wavelength and polarization of the two-photon state circulating in the resonator 11, and does not depend on the configuration requirements of the interference unit 12 or the phase matching conditions in the generating element E1.

[0035] Furthermore, as described above, the resonator 11 of the quantum entangled photon pair generating device 1 has the first optical path in a loop shape composed of the optical path P1, the optical path P2, and the generating element E1. This makes it possible to achieve both the resonance process by the resonator 11 and the generation of quantum entangled light by the interference unit 12 while suppressing loss. Furthermore, with the quantum entangled photon pair generating device 1, it is only necessary for the wavelength of the generated two-photon state to satisfy the resonance condition for a specific polarization. In other words, there is no need to establish quadruple resonance of polarization and wavelength, which was necessary in conventional methods, or even quintuple resonance including the pump laser.

[0036] Furthermore, generally, increasing the length of the crystal in the resonator increases the effect of chromatic dispersion, significantly changing the effective resonator length, which depends on the wavelength of the two photons. In spontaneous parametric down-conversion, which is the mechanism of two-photon generation, not only is the phase-matching wavelength width limited by the long crystal length, but the law of energy conservation calculated from the wavelength must also be satisfied, resulting in a cluster effect that actively filters the number of resonating wavelength modes. The quantum entangled photon pair generating device 1 according to this embodiment can roughly halve the loss in the resonator compared to entanglement generation methods using two optical crystals, thereby enabling both a reduction in the number of modes and high brightness.

[0037] As described above, the quantum entangled photon pair generating device 1 according to this embodiment can generate quantum entangled light with improved brightness compared to conventional configurations.

[0038] (Specific Configuration Example 1 of Quantum Entangled Photon Pair Generating Device 1) Hereinafter, a more specific configuration example (Configuration Example 1) of the quantum entangled photon pair generating device 1 will be described with reference to Fig. 2. For convenience of explanation, members having the same functions as members already described will be denoted by the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0039] 2 is a diagram showing the configuration of a specific configuration example 1 of the quantum entangled photon pair generating device 1 according to this configuration example. As shown in FIG. 2, the quantum entangled photon pair generating device 1 according to this configuration example includes a resonator 11, an interference unit 12, and a pump light generating device 13.

[0040] (Resonator 11) The resonator 11 is configured to resonate light propagating within the resonator. As shown in Fig. 2, the resonator 11 includes an optical path P11, an optical path P12, an optical path P21, an optical path P22, a mirror M11, a mirror M12, a generating element E1, and a shared connection E2.

[0041] Optical path P11 is an optical path formed between the generating element E1 and the mirror M11. Optical path P12 is an optical path formed between the mirror M11 and the shared connection E2. Optical path P21 is an optical path formed between the generating element E1 and the mirror M12. Optical path P22 is an optical path formed between the mirror M12 and the shared connection E2. Optical path P11, optical path P12, and the mirror M11 form optical path P1, and optical path P21, optical path P22, and the mirror M21 form optical path P2. As a result, a loop-shaped first optical path is formed by optical path P11, optical path P12, optical path P21, optical path P22, mirror M11, mirror M12, generating element E1, and shared connection E2. As will be described later, the resonator section 11 may be configured to include one or more other mirrors in addition to the mirrors M11 and M12 to form a loop-shaped first optical path, and in that case, the effect of increasing the resonator length can also be expected, as will be described later.

[0042] The generating element E1 is a generating element disposed on the first optical path, and in this example, is configured by a single nonlinear optical medium, such as a single nonlinear optical crystal, that induces spontaneous parametric down-conversion. The generating element E1 may have any phase matching condition or two-photon wavelength combination. This allows for optimal reduction of internal loss within the resonator 11.

[0043] The mirrors M11 and M21 are configured to direct the light propagating through the first optical path so that the first optical path becomes a loop-shaped optical path. As an example, at least one of the mirrors M11 and M21 may be a concave mirror. By using a concave mirror as at least one of the mirrors M11 and M21, loss within the resonator 11 can be suitably reduced.

[0044] At least one of the mirrors M11 and M21 may be configured to have a toroidal shape whose curvature differs between the horizontal and vertical directions by the square of cosθ. By adopting this configuration, aberrations due to a non-zero incident angle θ can be suppressed, and the shape of the output beam can be made closer to a perfect circle.

[0045] The first optical path of the resonator 11 may further include one or more lenses as a mechanism for converging light rays onto the mirrors M11 and M12, the generating element E1, the dichroic mirror, etc.

[0046] Furthermore, the length of the resonator 11 (more specifically, the length of the first optical path of the resonator 11) can be suitably determined depending on the required linewidth, the number of frequency modes, the mode spacing, etc. Furthermore, even in the case of non-degenerate wavelengths, quadruple resonance of two polarizations and two wavelengths is not necessary, and it is sufficient to satisfy double resonance at two wavelengths.

[0047] (Interference unit 12) The interference unit 12 is configured to supply pump light for generating a desired two-photon state to the generation element E1, and to generate and output one or more photon pairs in a quantum entangled state from the multiple two-photon states generated by the generation element E1. As shown in Figure 2, the interference unit 12 includes an optical path P31, an optical path P32, an optical path P41, an optical path P42, a mirror M21, a mirror M22, a polarization rotation element PP, a shared connection unit E2, and an input / output unit E3.

[0048] Optical path P31 is an optical path formed between the shared connection portion E2 and mirror M21. Optical path P32 is an optical path formed between mirror M21 and input / output portion E3. Optical path P41 is an optical path formed between the shared connection portion E2 and mirror M22. Optical path P42 is an optical path formed between mirror M22 and input / output portion E3. Optical paths P31, P32, P41, P42, mirror M21, mirror M22, input / output portion E3, and shared connection portion E2 form a loop-shaped second optical path. Note that the interference portion 12 may be configured to include one or more other mirrors in addition to mirrors M21 and M22 to form the loop-shaped second optical path; this is a design matter.

[0049] The polarization rotation element PP is an optical element disposed on the second optical path and rotates the polarization of light propagating through the second optical path. While the specific configuration of the polarization rotation element PP is not limited to this configuration example, as an example, the polarization rotation element PP may be configured to include any of a retardation plate, a Fresnel rhomb, and a periscope structure with three-dimensional reflection. When a configuration including a periscope structure is adopted, the polarization rotation element PP and the mirror M22 can be integrated. In other words, the periscope structure makes it possible to integrate the polarization rotation function and the ray angle shift function. Furthermore, since refractive index adjustment in the polarization rotation element is not required, the polarization rotation element is inexpensive.

[0050] By including the polarization rotation element PP in the interference unit 12, the clockwise path (propagation path) and the counterclockwise path (propagation path) in the second optical path can be associated with the polarization state of the photon. Therefore, the direction output from the second optical path can be encoded as the polarization of the photon. Furthermore, in this configuration example, since the clockwise path and the counterclockwise path in the second optical path have the same length, the relative phase of the generated quantum entangled photon pair can be automatically stabilized.

[0051] The input / output unit E3 is configured to allow the pump light PL generated by the pump light generating device 13 to flow into the second optical path and to emit quantum entangled photon pairs from the interference unit 12. In this configuration example, the input / output unit E3 is configured by a beam splitter or a polarizing beam splitter. The following description will be given assuming that the input / output unit E3 is a polarizing beam splitter. The input / output unit E3 splits the pump light PL into two, and directs one of the split lights to circulate clockwise through the second optical path, and directs the other split light to circulate counterclockwise through the second optical path.

[0052] In addition, the input / output unit E3 combines a two-photon state propagating clockwise in the second optical path with a two-photon state propagating counterclockwise in the second optical path, and emits the combined light in one direction as polarized entangled light (polarized quantum entangled photon pairs).

[0053] (Shared Connection Portion) The shared connection portion E2 is configured to optically connect the first optical path and the second optical path, and is shared by the resonator portion 11 and the interference portion 12 as described above. In this configuration example 1, the shared connection portion E2 is configured by a dichroic mirror E21. Here, the dichroic mirror has reflectivity and transmittance for optically coupling the interference portion 12 to the resonator portion 11. Furthermore, by providing the dichroic mirror with a dichroic mirror that transmits the pump light PL and reflects the two-photon state, as described above, the pump light PL does not resonate in the resonator portion 11, while a resonance process can be established in the resonator portion 11 for the two-photon state. In the example shown in Figure 2, the pump light input from optical paths P31 and P41 is transmitted to optical paths P22 and P12, respectively, while the two-photon states input from optical paths P12 and P22 are mostly reflected to optical paths P22 and P12, respectively, and some of them flow into optical paths P41 and P31, respectively.

[0054] Furthermore, in this configuration example, with regard to the reflectivity for two-photon states, it is desirable that the dichroic mirror E21 constituting the shared connection portion E2 has a reflectivity (e.g., 95%) lower than the reflectivity (e.g., 99%) of the mirrors M11 and M12 included in the resonator 11 described above. In other words, it is desirable that the resonator 11 described above includes a plurality of mirrors M11 and M12 for forming the first optical path, and that the plurality of mirrors M11 and M12 have a reflectivity relatively higher than that of the dichroic mirror. This configuration makes it easier for light from the resonator 11 to flow into the interference portion 12, thereby favorably improving the brightness of quantum entangled photon pairs generated by the interference portion 12. Note that the reflectivity setting of each mirror, etc. is a design matter, and the effects of the present invention can be obtained regardless of the above numerical conditions.

[0055] (Explanation of Operation and Photon State of Quantum Entangled Photon Pair Generating Device 1 According to Configuration Example 1) The operation and photon state of the quantum entangled photon pair generating device 1 according to this configuration example will be described below. First, pump light (pump laser) PL generated by the pump light generating device 13 is incident on the input / output unit E3 as shown in FIG. 2 . The incident pump light is split into two by the input / output unit E3. One of the split light circulates clockwise along the second optical path composed of optical paths P31, P32, P41, and P42, and the other split light circulates counterclockwise along the second optical path.

[0056] The pump light (photons of the pump laser) is expressed as follows, for example, immediately after being split by the polarized beam splitter (input / output section E3). Here, "H" on the right side indicates that the state is horizontally polarized, "V" indicates that the state is vertically polarized, and the subscript "clock" on the right side indicates that the state is propagating clockwise through the second optical path, and "anticlock" indicates that the state is propagating counterclockwise through the second optical path.

[0057] Next, the photon state |V> anticlock After passing through the polarization rotation element PP, just before the shared connection E2, the photons of the pump laser are expressed in the following state: where θ １ (theta subscript 1) is the relative phase imparted by the polarization rotation element PP to the counterclockwise path with respect to the clockwise path.

[0058] At least a portion of the light circulating through the second optical path of the interference unit 12 flows into the resonator 11 via the shared connection E2. The light that flows into the resonator 11 circulates through the first optical path composed of optical paths P1 and P2. The light circulating through the first optical path then enters the generator E1. A one-photon state constituting at least a portion of the light that enters the generator E1 is converted into a two-photon state by, for example, spontaneous parametric down-conversion caused by the generator E1. In other words, the generator E1 generates a two-photon state from a one-photon state constituting at least a portion of the light circulating through the first optical path.

[0059] Here, if the length of one circuit of the first optical path in the resonator 11 is L_cav and the coordinate at which spontaneous parametric down-conversion occurs is z, the photon of the pump laser at that position z is expressed in the following state. Here, k denotes the wave number of the pump laser.

[0060] The two-photon state generated by the generating element E1 flows into the interference unit 12 via the shared connection E2. Here, assuming that a two-photon state from |H> to |H'H'> is generated by parametric down-conversion by the generating element E1, and the wave number of the two photons is denoted as k', the light is expressed in the following state immediately before the shared mirror (shared connection E2): The same state is obtained even when the photons after circulation are superimposed, taking into account the resonance conditions in the resonator 11 (however, loss is ignored). The light that has passed through the shared connector E2 reaches the input / output section E3 via the second optical path of the interference section 12.

[0061] Here, the two-photon state |H'H'> that has flowed into the interference unit 12 clock is rotated by the polarization rotation element PP, clock The input / output unit E3 converts the two-photon state |V'V'> that has propagated clockwise through the first optical path and has been polarization-rotated by the polarization rotation element PP into clock and the two-photon state |H'H'> that has propagated counterclockwise along the first optical path. anticlock By combining and , a quantum entangled photon pair EPP consisting of two photons that are quantum entangled with each other is generated, and the generated quantum entangled photon pair EPP is output to the outside of the interference unit 12. The output quantum entangled photon pair EPP is guided by an optical path (not shown) and is used, for example, in a quantum communication system.

[0062] Here, the two-photon state EPP combined by the input / output unit E3 is expressed as follows, for example: where θ ２The subscript 2 (2) on θ indicates the relative phase imparted to the clockwise path relative to the counterclockwise path in the second optical path. In the interference unit 12, the clockwise path and the counterclockwise path in the second optical path are on the same optical path, and therefore these two paths cannot be distinguished from each other. Therefore, the above-mentioned two-photon state EPP combined by the input / output unit E3 is a photon state representing a photon pair composed of two photons that are quantum entangled with each other.

[0063] (Effects of the quantum entangled photon pair generating device 1 according to configuration example 1) According to the quantum entangled photon pair generating device 1 according to this configuration example, the resonance process in the resonator 11 is determined by the wavelength and polarization of the two-photon state circulating in the resonator 11, and does not depend on the configuration requirements of the interference unit 12 or the phase matching condition in the generator E1. Therefore, it becomes possible to change the two-photon wavelength or improve the degree of entanglement while maintaining the phase matching condition in the generator E1, without significantly readjusting the resonator.

[0064] Furthermore, the resonator 11 of the quantum entangled photon pair generating device 1 has the first optical path in a loop shape, which is configured by the optical path P1, the optical path P2, and the generating element E1, as described above. Therefore, it is possible to achieve both the resonance process by the resonator 11 and the generation of quantum entangled light by the interference unit 12 while suppressing loss. Furthermore, with the quantum entangled photon pair generating device 1, it is only necessary for the wavelength of the generated two-photon state to satisfy the resonance condition for a specific polarization. In other words, it is not necessary to establish quadruple resonance of polarization and wavelength or quintuple resonance including the pump laser, which were required in conventional methods.

[0065] Furthermore, even when the wavelength of the pump laser or the two-photon state changes significantly, or when the wavelengths of the two photons are different, that is, when the so-called non-degenerate state is present, the resonant / non-resonant conditions (with respect to the pump laser) in the resonant cavity 11 of the quantum entangled photon pair generating device 1 can be realized by designing an appropriate resonant cavity 11. In other words, the effects of the present invention can be obtained for any combination of pump laser / two-photon state wavelength by designing an appropriate resonant cavity 11 and selecting an appropriate generating element E1 in accordance with the wavelength of the pump laser or the two-photon state.

[0066] In addition, generally, increasing the length of the crystal in the resonator increases the effect of chromatic dispersion, significantly changing the effective resonator length, which depends on the wavelength of the two photons. In spontaneous parametric down-conversion, which is the mechanism of two-photon generation, not only is the phase-matching wavelength width limited by the long crystal length, but the law of energy conservation calculated from the wavelength must also be satisfied, resulting in a cluster effect that actively filters the number of resonating wavelength modes. The quantum entangled photon pair generating device 1 according to this configuration example can roughly halve the loss in the resonator compared to entanglement generation methods using two optical crystals, thereby enabling both a reduction in the number of modes and high brightness.

[0067] In this way, the quantum entangled photon pair generating device 1 according to this configuration example can generate quantum entangled light with improved brightness compared to conventional configurations.

[0068] (Specific Configuration Example 2 of Quantum Entangled Photon Pair Generating Device 1) Hereinafter, with reference to Fig. 3, a description will be given of another specific configuration example (Configuration Example 2) of the quantum entangled photon pair generating device 1. For convenience of explanation, members having the same functions as members already explained will be assigned the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0069] In the specific configuration example 2 of the quantum entangled photon pair generating device 1 according to this configuration example, the quantum entangled photon pair generating device 1 also includes a resonator 11, an interference unit 12, and a pump light generating device 13. In the specific configuration example 2 of this case, in addition to the configuration of the specific configuration example 1 shown in Fig. 2 , the mirrors M11 and M12 are arranged to be line-symmetric with respect to the shared connector E2 and the generating element E1 as the symmetry axis.

[0070] Furthermore, in a preferred modification of the specific configuration example 2 of the quantum entangled photon pair generating device 1 according to this configuration example, the optical path P1 and the optical path P2 of the resonator unit each include m mirrors (m is an integer greater than or equal to 2). Furthermore, the shared connector E2 and the multiple mirrors of the resonator unit form the first optical path on the diagonal of an n-gon, and are formed in line symmetry, with the shared connector E2 and the generating element E1 disposed on the axis of symmetry. Here, n = 2m + 1, where n is an odd number greater than or equal to 5. That is, in this configuration example, the first optical path of the resonator unit is formed by the shared connector E2 and 2m mirrors. FIG. 3 shows a configuration example where n = 5 (m = 2).

[0071] The configuration in which one mirror is included in the optical path P1 and the optical path P2 of the resonator is the same as in the specific configuration example 1 shown in Fig. 2, so a detailed description will be omitted here. Below, the configuration in the case of n = 5 (m = 2) shown in Fig. 3 will be described.

[0072] (Resonator unit 11) The resonator unit 11 includes mirrors M31, M32, M33, M34, a generating element E1, and a shared connector E2. Here, the shared connector E2 is formed by a dichroic mirror E21, similar to the case of specific configuration example 1. The dichroic mirror has a reflectance and a transmittance for optically coupling the interference unit 12 to the resonator unit 11, similar to the case of specific configuration example 1.

[0073] An optical path P13 is formed between the generating element E1 and mirror M31. An optical path P14 is formed between mirror M31 and mirror M32. An optical path P15 is formed between mirror M32 and shared connection E2. An optical path P23 is formed between generating element E1 and mirror M33. An optical path P24 is formed between mirror M33 and mirror M34. An optical path P25 is formed between mirror M34 and shared connection E2. The optical paths P13, P14, P15, mirror M31, and mirror M32 form the optical path P1, and the optical paths P23, P24, P25, mirror M33, and mirror M34 form the optical path P2, resulting in the formation of a loop-shaped first optical path.

[0074] Mirrors M31, M32, M33, M34, and the dichroic mirror E21, which is the shared connecting part, are configured to direct the light propagating through the optical path so that the optical path becomes a loop-shaped optical path within the resonator 11. For the same reasons as described above, at least one of mirrors M31, M32, M33, and M34 may be a concave mirror or may be configured to have a toroidal shape, and any suitable mirror may be used.

[0075] The generating element E1 is disposed between the mirrors M31 and M33 and generates a two-photon state by inducing spontaneous parametric down-conversion.

[0076] The configuration and operation of the interference section 12 and the loop-shaped second optical path are the same as those in the specific configuration example 1 shown in FIG. 2, and therefore a detailed description thereof will be omitted here.

[0077] In this embodiment, the dichroic mirror E21, which is the shared connection part E2, has an incident angle of 45 degrees between the first optical path and the second optical path, which allows the use of mass-produced mirrors.

[0078] (Explanation of Operation and Photon States of Quantum Entangled Photon Pair Generating Device 1 According to Configuration Example 2) In this embodiment, the optical path P1 and the optical path P2 constituting the resonator each include m mirrors (m is an integer equal to or greater than 2). The first optical path constituted by the shared connector E2 and the multiple mirrors of the resonator is formed on the diagonal of an n-gon, and the multiple mirrors are arranged to be line-symmetric with respect to the shared connector E2 and the generating element E1 as the axis of symmetry. As shown in FIG. 3 , when n = 5, mirror M31, mirror M32, mirror M33, mirror M34, and shared connector E2 form the shape of a pentagram as the first optical path. The shared connector E2 and generating element E1 are arranged on the axis of symmetry.

[0079] According to the above configuration, the light traveling from the shared connection E2 along the first optical path (optical path P15 → optical path P14 → optical path P13) and the light traveling from the shared connection E2 along the second optical path (optical path P25 → optical path P24 → optical path P23), which is the opposite direction to the first rotation, travel linearly symmetrically to reach the generating element E1. Therefore, the optical path length of the light traveling from the shared connection E2 along the first rotation to the generating element E1 is equal to the optical path length of the light traveling from the shared connection E2 along the second rotation to the generating element E1.

[0080] The above effect can also be obtained in the case of specific configuration example 1 having a triangular configuration as shown in Figure 2 by arranging the mirrors M11 and M12 so that they are symmetrical with respect to the shared connection part E2 and the generating element E1, with the axis of symmetry being the axis of symmetry.

[0081] The two-photon state generated in the first revolution of the first optical path of the resonator and the two-photon state generated in the second revolution of the first optical path generate quantum entangled light by the operation of the interference unit 12, due to the same effect as described in the specific configuration example 1 of the present invention.

[0082] On the other hand, in the case of the specific configuration example 2 of the present invention, if the length of one circuit of the first optical path in the resonator 11 is L_cav and the coordinate at which spontaneous parametric down-conversion occurs is z, then z = L_cav / 2. At this time, from equation (5), when θ1 = θ2, the phase difference between the two-photon states is zero, and therefore, a quantum entangled photon pair in a Bell state can be generated.

[0083] The above-described effect can also be achieved in a configuration in which one mirror is included in the optical path P1 and the optical path P2 of the resonator as shown in FIG.

[0084] Furthermore, the inventors of the present application have found that in the case of the multiple mirror configuration shown in FIG. 3, the optical path length of the first optical path can be made longer compared to the single mirror configuration shown in FIG. 2, even if the occupied area of ​​the resonator is the same.

[0085] Now, when the resonator unit 11 has a regular n-gon configuration and its area is S, the length L of one side of the regular n-gon is given by the following equation. In this case, if we calculate the longest path to go around a regular n-gon by passing through each vertex once, we get the following for a regular quadrilateral: In the case of a regular pentagon In the case of a regular hexagon When it is a regular heptagon That is, for the same area S, the longest path length increases as n increases.

[0086] Therefore, by arranging multiple mirrors in the resonator 11 so as to obtain this longest path length, it is possible to increase the optical path length of the first optical path while maintaining the same occupied area of ​​the resonator 11. As a result, it is possible to narrow the spectral linewidth and achieve high multiplexing of frequency modes.

[0087] The value of n, the location of the mirrors, etc. are design matters and can be flexibly designed depending on the application.

[0088] 4 is a diagram showing a second modified example of specific configuration example 2 when n = 7. In this configuration example, optical path P1 is configured to include mirrors M41, M42, and M43, and optical path P2 is configured to include mirrors M44, M45, and M46. These mirrors are formed on the diagonals of a heptagon and are formed line-symmetrically with respect to shared connection part E2 and generation element E1.

[0089] (Effects of the quantum entangled photon pair generating device 1 according to Configuration Example 2) The quantum entangled photon pair generating device 1 according to Configuration Example 2 can obtain the following effect in addition to the effect of Specific Configuration Example 1 shown in Fig. 2 . That is, quantum entangled light in a Bell state can be obtained. Furthermore, even if the occupied area of ​​the resonator is the same, the resonator length in the resonator can be increased, and as a result, the spectral linewidth can be narrowed and a high degree of multiplexing of frequency modes can be achieved.

[0090] (Specific Configuration Example 3 of Quantum Entangled Photon Pair Generating Device 1) Hereinafter, with reference to Fig. 5 , a more specific configuration example (Configuration Example 3) of the quantum entangled photon pair generating device 1 will be described. For convenience of explanation, members having the same functions as members already described will be denoted by the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0091] In the specific configuration example 3 of the quantum entangled photon pair generating device 1 according to this configuration example, the quantum entangled photon pair generating device 1 also includes a resonator 11, an interference unit 12, and a pump light generating device 13. In the specific configuration example 3 of this case, unlike the configurations of the specific configuration examples 1 and 2 shown in FIGS.

[0092] (Resonator 11) The resonator 11 includes a mirror M51, a mirror M52, a generating element E1, and a shared connector E2. Here, the shared connector E2 is configured with two dichroic mirrors similar to those in Specific Configuration Example 1, as described above. The two dichroic mirrors have reflectances and transmittances for optically coupling the interference section 12 to the resonator 11, similar to those in Specific Configuration Example 1. Furthermore, the two dichroic mirrors also include dichroic mirrors that transmit the pump light PL and reflect the two-photon state, similar to those in Specific Configuration Example 1. As described above, by providing these two dichroic mirrors, the pump light PL does not resonate in the resonator 11, while allowing a resonance process to be established in the resonator 11 for the two-photon state. In the example shown in Figure 5, the pump light input from optical paths P31 and P41 is transmitted to optical paths P17 and P27, respectively, while the two-photon states input from optical paths P17 and P27 are both mostly reflected and output in opposite directions to optical path P18, with portions of them flowing into optical paths P31 and P41, respectively.

[0093] An optical path P16 is formed between the generating element E1 and mirror M51. An optical path P17 is formed between mirror M51 and second dichroic mirror E23. An optical path P18 is formed between first dichroic mirror E22 and second dichroic mirror E23. An optical path P26 is formed between generating element E1 and mirror M52. An optical path P27 is formed between mirror M52 and first dichroic mirror E22. The first dichroic mirror E22, the second dichroic mirror E23, and the optical path P18 optically connecting them constitute a shared connection part E2. The optical path P16, the optical path P17, and mirror M51 form the optical path P1, and the optical path P26, the optical path P27, and mirror M52 form the optical path P2, resulting in the formation of a loop-shaped first optical path.

[0094] The mirror M51, the mirror M52, and the first dichroic mirror E22 and the second dichroic mirror E23, which are the shared connecting parts, are configured to direct the light propagating through the optical path so that the optical path becomes a loop-shaped optical path within the resonator 11. For the same reasons as described above, at least one of these mirrors may be a concave mirror or may be configured to have a toroidal shape, and any suitable mirror may be used.

[0095] The generating element E1 is disposed between the mirrors M51 and M52 and generates a two-photon state by inducing spontaneous parametric down-conversion.

[0096] The configuration and operation of the interference section 12 and the loop-shaped second optical path are the same as those in the specific configuration example 1 shown in FIG. 2, and therefore a detailed description thereof will be omitted here.

[0097] (Explanation of Operation and Photon States of Quantum Entangled Photon Pair Generating Device 1 According to Configuration Example 3) In this embodiment, the resonator has a so-called bowtie resonator configuration. According to the above configuration, clockwise pump light input from optical path P31 is input to the resonator 11 via the second dichroic mirror E23, travels along optical path P17, mirror M51, and optical path P16, and is input to the generating element E1. The pump light input to the generating element E1 generates a two-photon state by spontaneous parametric down-conversion or the like. The generated two-photon state light travels along optical path P26, mirror M52, and optical path P27 and reaches the first dichroic mirror E22. A portion of the two-photon state light input to the first dichroic mirror E22 is output to optical path P41 and input to the interference unit 12, and the remaining majority is output to optical path P18. The two-photon state light output to optical path P18 reaches the second dichroic mirror E23. A portion of the two-photon state light input to the second dichroic mirror E23 is output to a port not shown in the figure, and the majority of the remaining light is output to optical path P17, and then passes through optical path P17 → mirror M51 → optical path P16 and is input again to the generation element E1. As a result, the two-photon state generated in the generation element E1 resonates in the resonator 11. Hereinafter, this optical path will be referred to as the first loop optical path of the first optical path.

[0098] On the other hand, the counterclockwise pump light input from optical path P41 is input to the resonator 11 via the first dichroic mirror E22, travels along optical path P27, mirror M52, and optical path P26, and is input to the generator E1. The pump light input to the generator E1 generates a two-photon state by spontaneous parametric down-conversion or the like. The generated two-photon state light travels along optical path P16, mirror M51, and optical path P17, and reaches the second dichroic mirror E23. A portion of the two-photon state light input to the second dichroic mirror E23 is output to optical path P31 and input to the interference unit 12, while the remaining majority is output to optical path P18. The two-photon state light output to optical path P18 reaches the first dichroic mirror E22. A portion of the two-photon state light input to the first dichroic mirror E22 is output to a port not shown in the figure, and the remaining majority is output to optical path P27, and then passes through optical path P27 → mirror M52 → optical path P26 and is input again to the generation element E1. As a result, the two-photon state generated in the generation element E1 resonates in the resonator 11. Hereinafter, this optical path will be referred to as the second loop optical path of the first optical path.

[0099] The two-photon state generated in the first revolution of the first optical path of the resonator and the two-photon state generated in the second revolution of the first optical path are output to optical paths P41 and P31, respectively, and are then input to interference unit 12. Furthermore, the polarization state of the two-photon state output to optical path P41 is rotated by 90° by passing through polarization rotation element PP, and therefore quantum entangled light is generated in this configuration example by the operation of interference unit 12, due to the same effect as described in specific configuration example 1 of the present invention.

[0100] (Effects of the quantum entangled photon pair generating device 1 according to configuration example 3) The quantum entangled photon pair generating device 1 according to this configuration example can provide the following effects. That is, the effects of the present invention can also be applied to a bowtie resonator, which is a general resonator configuration.

[0101] (Specific Configuration Example 4 of Quantum Entangled Photon Pair Generating Device 1) Hereinafter, with reference to Fig. 6, a more specific configuration example (Configuration Example 4) of the quantum entangled photon pair generating device 1 will be described. For convenience of explanation, members having the same functions as members already described will be denoted by the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0102] 6 is a diagram showing the configuration of a specific configuration example 4 of the quantum entangled photon pair generating device 1 according to this configuration example. As shown in FIG. 6 , in the specific configuration example 4 of the quantum entangled photon pair generating device 1 according to this configuration example, the quantum entangled photon pair generating device 1 also includes a resonator 11, an interference unit 12, and a pump light generating device 13.

[0103] (Resonator 11) The resonator 11 is configured to resonate light propagating within the resonator. As shown in Fig. 6, the resonator 11 includes an optical path P1, an optical path P2, a generating element E1, and a shared connection E2. The optical path P1 and the optical path P2 form a loop-shaped first optical path. In this configuration example, the optical path P1 and the optical path P2 are configured by optical fibers. As will be described in detail later, in this configuration example, the optical path P1 and the optical path P2 correspond to input and output fibers of a polarization-maintaining fiber coupler, and their lengths are typically at most several meters.

[0104] The generating element E1 is a generating element disposed on the first optical path, and in this configuration example, the generating element E1 is configured by a polarization-maintaining fiber having a sufficient length, as shown in FIG. 6 . Here, the specific length of the polarization-maintaining fiber serving as the generating element E1 is not limited to this configuration example, but as an example, a configuration having a length of 10 times or more the length of the first optical path can be adopted. The length of the polarization-maintaining fiber is a value that directly affects not only the brightness but also the resonator length. The longer the fiber, the greater the loss, but the shorter the free spectral range, making it possible to aim for a narrower linewidth. The maximum length is limited by the beat length of the polarization-maintaining fiber.

[0105] Furthermore, the polarization-maintaining fiber induces a four-wave mixing process and third-order parametric down-conversion utilizing the nonlinear optical effect of the fiber. Furthermore, by using a polarization-maintaining fiber with such a sufficient length as the generating element E1, the coincidence accidental ratio in the resonator 11 can be increased.

[0106] Furthermore, the length of the resonator 11 (more specifically, the total length of the optical path of the resonator 11) can be suitably determined depending on the required linewidth, the number of frequency modes, the mode spacing, etc. Furthermore, even in the case of non-degenerate wavelengths, quadruple resonance of two polarizations and two wavelengths is not necessary, and it is sufficient to satisfy double resonance at two wavelengths.

[0107] (Interference unit 12) The interference unit 12 is configured to generate one or more photon pairs in a quantum entangled state from the multiple two-photon states generated by the generation element E1. As shown in Fig. 6, the interference unit 12 includes an optical path P3, an optical path P41, an optical path P42, a polarization rotation element PP, a shared connection unit E2, and an input / output unit E3.

[0108] The optical path P41 is an optical path between the shared connection unit E2 and the polarization rotation element PP. The optical path P42 is an optical path between the polarization rotation element PP and the input / output unit E3. In this configuration example, the optical path P3, the optical path P41, and the optical path P42 are configured by optical fibers and form a loop-shaped second optical path.

[0109] The polarization rotation element PP is an optical element disposed on the second optical path, and rotates the polarization of the light propagating through the second optical path. In this configuration example, the polarization rotation element PP can be configured as a single twisted fiber in which the input key and the output key are orthogonal to each other.

[0110] By including the polarization rotation element PP in the interference unit 12, the clockwise path (propagation path) and the counterclockwise path (propagation path) of the second optical path can be associated with the polarization state of the photon. Therefore, the direction output from the second optical path can be encoded as the polarization of the photon. Furthermore, in this configuration example, since the clockwise path and the counterclockwise path of the second optical path have the same length, the relative phase of the generated quantum entangled photon pair can be automatically stabilized.

[0111] The input / output unit E3 is configured to allow the pump light PL generated by the pump light generating device 13 to flow into the second optical path and to emit quantum entangled photon pairs from the interference unit 12. In this configuration example, the input / output unit E3 is configured by a polarization splitting fiber coupler. The input / output unit E3 splits the pump light PL into two beams, directs one of the split beams to circulate clockwise through the second optical path, and directs the other of the split beams to circulate counterclockwise through the second optical path.

[0112] In addition, the input / output unit E3 combines a two-photon state propagating clockwise in the second optical path with a two-photon state propagating counterclockwise in the second optical path, and emits the combined light in one direction as polarized entangled light (polarized quantum entangled photon pairs).

[0113] (Shared Connection Portion) The shared connection portion E2 is configured to optically connect the first optical path and the second optical path, and as an example, is shared by the resonator 11 and the interference portion 12 as described above. In this configuration example, the shared connection portion E2 is configured by a polarization-maintaining fiber coupler E24 having four ports (2:2 ports). Two of the four ports are connected to the optical paths P3 and P41, respectively, as shown in FIG. 3, and the other two of the four ports (optical paths P1 and P2) are connected to both ends of the polarization-maintaining fiber serving as the generation element E1, as shown in FIG.

[0114] The quantum entangled photon pair generating device 1 configured as described above can also achieve the same effect as that described in specific configuration example 1, by generating a plurality of two-photon states from the pump light generated by the pump light generating device 13 using the generating element E1 of the resonator 11, and generating quantum entangled photon pairs from the generated plurality of two-photon states using the interference unit 12.

[0115] Also in the quantum entangled photon pair generating device 1 according to this configuration example, the resonance process in the resonator 11 is determined by the wavelength and polarization of the two-photon state circulating in the resonator 11, and does not depend on the configuration requirements of the interference unit 12 or the phase matching condition in the generating element E1. As in the first configuration example, it is only necessary that the wavelength of the two-photon state to be generated satisfies the resonance condition for a specific polarization.

[0116] As described above, the resonator 11 of the quantum entangled photon pair generating device 1 has the first loop-shaped optical path composed of the optical paths P1 and P2, which makes it possible to suppress loss while simultaneously achieving both the resonance process by the resonator 11 and the generation of quantum entangled light by the interference unit 12.

[0117] Therefore, the quantum entangled photon pair generating device 1 according to this configuration example can improve the brightness of the generated quantum entangled light compared to the conventional configuration. Furthermore, compared to the specific configuration example 1 of the present invention, a long optical fiber can be used as the generating element E1, and the resonator 11 can be made long, thereby enabling the linewidth of the generated quantum entangled light to be further narrowed.

[0118] As is well known, a polarization-maintaining fiber can be wound multiple times around a cylindrical bobbin having a diameter of, for example, several centimeters to several tens of centimeters without deteriorating its propagation loss or polarization-maintaining properties. Therefore, even if a long polarization-maintaining fiber, about 100 meters long, is used as the generating element E1 in order to shorten the free spectral range and narrow the linewidth, it is possible to avoid increasing the size of the device.

[0119] (Specific Configuration Example 5 of Quantum Entangled Photon Pair Generating Device 1) Hereinafter, with reference to Fig. 7 , another more specific configuration example (Configuration Example 5) of the quantum entangled photon pair generating device 1 will be described. For convenience of explanation, members having the same functions as members already described will be denoted by the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0120] 7 is a diagram showing the configuration of a specific configuration example 5 of the quantum entangled photon pair generating device 1 according to this configuration example. As shown in FIG. 7, in the specific configuration example 5 of the quantum entangled photon pair generating device 1 according to this configuration example, the quantum entangled photon pair generating device 1 also includes a resonator 11, an interference unit 12, and a pump light generating device 13. While the configuration examples described above have mainly been exemplified as being axisymmetric with respect to the shared connector E2 and the generating element E1 as the axis of symmetry, this configuration example is an asymmetric configuration. Furthermore, this configuration example is an implementation example with a bow-tie structure.

[0121] Specific Configuration Example 5 further includes mirrors M23 and M15 in addition to the configuration of Specific Configuration Example 1 shown in FIG. 7. As shown in FIG. 7, the presence of mirror M23 forms: an optical path P311 between mirror M21 and mirror M23; and an optical path P312 between mirror M23 and shared connection portion E2 (dichroic mirror E21). The combined optical path of optical paths P311 and P312 may correspond to optical path P31 in Specific Configuration Example 1 shown in FIG. 2. Furthermore, as shown in FIG. 7, the presence of mirror M15 forms: an optical path P221 between shared connection portion E2 (dichroic mirror E21) and mirror M15; and an optical path P222 between mirror M15 and mirror M12. The combined optical path of optical paths P221 and P222 may correspond to optical path P22 (or optical path P12) in Specific Configuration Example 1 shown in FIG. 2.

[0122] In the photon pair generating device 1 according to this configuration example, the presence of the mirror M23 makes it possible to correct "diffraction broadening" that occurs due to the difference in path length between clockwise and counterclockwise directions in the resonator unit 11. Generally, when there is a path length difference, the beam systems of the two two-photon states that ultimately flow into the input / output unit E3 due to diffraction are different, which can result in a phenomenon in which the fidelity of the quantum entangled state decreases due to a decrease in spatial overlap. In the photon pair generating device 1 according to this configuration example, by correcting the path length difference, it is possible to correct the beam diameter non-uniformity that occurs due to diffraction broadening, thereby achieving the effect of improving fidelity.

[0123] (Specific Configuration Example 6 of Quantum Entangled Photon Pair Generating Device 1) Hereinafter, with reference to Fig. 8 , a more specific configuration example (Configuration Example 6) of the quantum entangled photon pair generating device 1 will be described. For convenience of explanation, members having the same functions as members already described will be denoted by the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0124] 8 is a diagram showing the configuration of a specific configuration example 6 of the quantum entangled photon pair generating device 1 according to this configuration example. As shown in FIG. 8 , in the specific configuration example 6 of the quantum entangled photon pair generating device 1 according to this configuration example, the quantum entangled photon pair generating device 1 also includes a resonator 11, an interference unit 12, and a pump light generating device 13.

[0125] Specific Configuration Example 6 further includes a mirror M24 in addition to the configuration of Specific Configuration Example 5 shown in Fig. 7. As shown in Fig. 8, the presence of mirror M24 forms: an optical path P3111 between mirror M21 and mirror M24, and an optical path P3112 between mirror M24 and mirror M23. The optical path combining optical paths P3111 and P3112 can correspond to optical path P311 in Specific Configuration Example 5 shown in Fig. 7.

[0126] In the photon pair generating device 1 according to this configuration example, the presence of the mirror M24 allows the orientation of the images of the clockwise path and the counterclockwise path to be aligned, thereby achieving the effect of further improving the fidelity of the quantum entangled state.

[0127] To explain the alignment of image orientations, photons of two wavelengths are generated in the generation element (nonlinear crystal) E1 (assuming a nondegenerate state in which the wavelengths of the two photons are different). In this case, due to refraction, the two photons may be spatially separated, such as when a green wavelength propagates to the front right after emission and when a yellow wavelength propagates to the front left after emission. In this example, the green image appears on the right and the yellow image appears on the left. If the image orientations are not aligned, the two photons become spatially distinguishable, which can result in a decrease in fidelity. In the photon pair generation device 1 according to this configuration example, as described above, the presence of the mirror M24 allows the image orientations of the clockwise and counterclockwise paths to be aligned, thereby improving the fidelity of the quantum entangled state.

[0128] (Specific Configuration Example 7 of Quantum Entangled Photon Pair Generating Device 1) Hereinafter, with reference to Fig. 9 , another more specific configuration example (Configuration Example 7) of the quantum entangled photon pair generating device 1 will be described. For convenience of explanation, members having the same functions as members already described will be denoted by the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0129] 9 is a diagram showing the configuration of a specific configuration example 7 of the quantum entangled photon pair generating device 1 according to this configuration example. As shown in FIG. 9 , in the specific configuration example 7 of the quantum entangled photon pair generating device 1 according to this configuration example as well, the quantum entangled photon pair generating device 1 includes a resonator 11, an interference unit 12, and a pump light generating device 13.

[0130] In this specific configuration example 7, the configuration of the interference unit 12 is different from that of the specific configuration example 5 shown in FIG. 7 or the specific configuration example 6 shown in FIG. 8. More specifically, in this configuration example, as shown in FIG. 9, a polarization rotation element PP is arranged on an optical path P42 formed between the mirror M22 and the input / output unit E3. Also, in this configuration example, as shown in FIG. 9, an optical path P411 is formed between the mirror M22 and the mirror M21, and an optical path P412 is formed between the mirror M21 and the shared connection unit E2 (dichroic mirror E21). The optical path combining the optical paths P411 and P412 may correspond to the optical path P41 in the specific configuration example 5 shown in FIG. 7. However, in this configuration example, as described above, the polarization rotation element PP is arranged on the optical path P42.

[0131] In this configuration example, an optical path P51 is formed between the input / output unit E3 and the shared connection unit E2 (dichroic mirror E21) as shown in Fig. 9. The optical path P51 may correspond to the optical path formed by combining the optical paths P32, P311, and P312 in the specific configuration example 5 shown in Fig. 7.

[0132] In the photon pair generating device 1 according to this configuration example, the number of mirrors included in the interference unit 12 can be reduced due to the above configuration, and therefore, the effect of reducing costs can be obtained.

[0133] (Specific Configuration Example 8 of Quantum Entangled Photon Pair Generating Device 1) Hereinafter, with reference to Fig. 10 , another more specific configuration example (Configuration Example 8) of the quantum entangled photon pair generating device 1 will be described. For convenience of explanation, members having the same functions as members already described will be denoted by the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0134] 10 is a diagram showing the configuration of a specific example 8 of the quantum entangled photon pair generating device 1 according to this configuration example. As shown in Fig. 10 , in the specific example 8 of the quantum entangled photon pair generating device 1 according to this configuration example, the quantum entangled photon pair generating device 1 also includes a resonator 11, an interference unit 12, and a pump light generating device 13.

[0135] In this specific configuration example 8, the configuration of the interference unit 12 is different from that of the specific configuration example 7 shown in Figure 9. In this configuration example, a mirror M21 is arranged so that the optical path P42 and the optical path P411 in the specific configuration example 7 shown in Figure 9 overlap. Furthermore, in this configuration example, the polarization rotation element PP is configured to have the same function as a λ / 4 wave plate (a quarter wave plate) rather than a λ / 2 wave plate (a half wave plate). Furthermore, in this configuration example, the mirror M22 is arranged so that light from the input / output unit E3 is incident and reflected perpendicularly, and is configured to be movable along the optical path direction, thereby enabling continuous path length correction.

[0136] As described above, in this configuration example, optical path P42 and optical path P411 are configured to overlap, so that light from the input / output unit E3 travels back and forth through the polarization rotation element PP (λ / 4 wave plate). Therefore, the polarization rotation element PP in this configuration example essentially operates as a λ / 2 wave plate. Furthermore, in this configuration example, light from the input / output unit E3 enters and reflects perpendicularly to the mirror M22. Therefore, even when the path length is changed by moving the mirror M22 along the optical path direction, there is no need to adjust the angle of other optical paths.

[0137] (Specific Configuration Example 9 of Quantum Entangled Photon Pair Generating Device 1) Hereinafter, with reference to Fig. 11 , another more specific configuration example (Configuration Example 9) of the quantum entangled photon pair generating device 1 will be described. For convenience of explanation, members having the same functions as members already described will be denoted by the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0138] 11 is a diagram showing the configuration of a specific configuration example 9 of the quantum entangled photon pair generating device 1 according to this configuration example. As shown in FIG. 11 , in this specific configuration example 9 of the quantum entangled photon pair generating device 1 according to this configuration example, the quantum entangled photon pair generating device 1 also includes a resonator 11, an interference unit 12, and a pump light generating device 13.

[0139] As shown in Fig. 11 , in this Specific Configuration Example 9, the functions of the mirror M22 and the polarization rotation element PP provided in the Specific Configuration Example 5 shown in Fig. 7 are realized by a periscope PS. Also, as shown in Fig. 11 , the resonator unit 11 according to this configuration example further includes mirrors M13 and M14 in addition to the configuration of the Specific Configuration Example 5 shown in Fig. 7. By including mirror M13, the optical path P222 in the Specific Configuration Example 5 shown in Fig. 7 is divided into: an optical path P2221 between mirror M15 and mirror M14, an optical path P2222 between mirror M14 and mirror M13, and an optical path P2223 between mirror M13 and mirror M12.

[0140] Furthermore, by providing the mirror M14, the optical path P12 in the specific configuration example 5 shown in Figure 7 is divided into: an optical path P121 between the shared connection part E2 (dichroic mirror E21) and the mirror M13, an optical path P122 between the mirror M13 and the mirror M14, and an optical path P123 between the mirror M14 and the mirror M11.

[0141] In the photon pair generating device 1 according to this configuration example, the presence of the mirror M23 makes it possible to correct the “diffraction spread” that occurs due to the difference in path length between the clockwise and counterclockwise directions in the resonator unit 11, as in the specific configuration example 5 shown in FIG.

[0142] 11 , in the photon pair generating device 1 according to this configuration example, a photon enters and is reflected twice at different positions on the mirror M13. Similarly, a photon enters and is reflected twice at different positions on the mirror M14. With this configuration, the photon pair generating device 1 according to this configuration example can offset angular instability in the resonator 11, making it possible to form a more stable resonator 11.

[0143] The present invention 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 invention.

[0144] (Additional Notes) The configuration described in this specification can also be expressed as follows.

[0145] (Aspect 1) A quantum entangled photon pair generating device comprising: a resonator having a loop-shaped first optical path and a generation element arranged on the first optical path for generating a two-photon state; an interference unit having a loop-shaped second optical path and an input / output unit arranged on the second optical path for inputting pump light and outputting quantum entangled photon pairs, and generating one or more photon pairs in a quantum entangled state from the multiple two-photon states generated by the generation element; and a shared connection unit forming a loop-shaped optical path for each of the first optical path and the second optical path, and optically connecting the first optical path and the second optical path.

[0146] (Aspect 2) The quantum entangled photon pair generating device according to Aspect 1, wherein in the resonator, the plurality of two-photon states generated by the generating element satisfy a resonance condition in the resonator.

[0147] (Aspect 3) The quantum entangled photon pair generating device according to Aspect 1 or 2, wherein the shared connection part includes a dichroic mirror.

[0148] (Aspect 4) The quantum entangled photon pair generating device according to Aspect 3, wherein the resonator includes a plurality of mirrors for forming the first optical path, the plurality of mirrors having a reflectance relatively higher than that of the dichroic mirror.

[0149] (Aspect 5) The quantum entangled photon pair generating device according to Aspect 1 or 2, wherein the shared connection section includes a plurality of dichroic mirrors.

[0150] (Aspect 6) The quantum entangled photon pair generating device according to Aspect 5, wherein the resonator includes a plurality of mirrors for forming the first optical path, the plurality of mirrors having a reflectivity relatively higher than that of the plurality of dichroic mirrors.

[0151] (Aspect 7) The quantum entangled photon pair generating device according to Aspect 3 or 4, wherein a plurality of mirrors for forming the first optical path are arranged in line symmetry with respect to an axis of symmetry of the dichroic mirror and the generating element.

[0152] (Aspect 8) The quantum entangled photon pair generating device according to any one of Aspects 3, 4, and 7, wherein the number of mirrors for forming the first optical path is 2m, and the first optical path is formed on a diagonal of an n-gon, where m is an integer of 2 or more, and n=2m+1.

[0153] (Aspect 9) The quantum entangled photon pair generating device according to aspect 8, wherein the n-gon is a pentagon, and the first optical path is a pentagram-shaped optical path.

[0154] (Aspect 10) The quantum entangled photon pair generating device according to any one of Aspects 3, 4, and 7 to 9, wherein the shared connection portion has an incident angle of 45 degrees between the first optical path and the second optical path.

[0155] (Aspect 11) The quantum entangled photon pair generating device according to any one of Aspects 3 to 10, wherein the plurality of mirrors includes a concave mirror.

[0156] (Aspect 12) The quantum entangled photon pair generating device according to any one of Aspects 3 to 10, wherein the plurality of mirrors includes a toroidal mirror.

[0157] (Aspect 13) The quantum entangled photon pair generating device according to any one of Aspects 3 to 12, further comprising a lens having a mechanism for converging a light beam, in the first optical path of the resonator.

[0158] (Aspect 14) The quantum entangled photon pair generating device according to any one of Aspects 1 to 13, wherein the interference unit includes a polarization rotation element disposed on the second optical path.

[0159] (Aspect 15) The quantum entangled photon pair generating device according to Aspect 14, wherein the polarization rotation element includes any one of a half-wave plate, a Fresnel rhomb, and a periscope structure.

[0160] (Aspect 16) The quantum entangled photon pair generating device according to any one of Aspects 1 to 15, wherein the input / output unit includes a beam splitter.

[0161] (Aspect 17) The quantum entangled photon pair generating device according to any one of Aspects 1 to 16, wherein the generating element is a nonlinear optical medium that causes second-order spontaneous parametric down-conversion.

[0162] (Aspect 18) The quantum entangled photon pair generating device according to any one of Aspects 1 and 2, wherein the shared connection section includes a polarization-maintaining fiber coupler.

[0163] (Aspect 19) The quantum entangled photon pair generating device according to any one of Aspects 1, 2, and 18, wherein the input / output unit includes a polarization splitting fiber coupler.

[0164] (Aspect 20) The quantum entangled photon pair generating device according to any one of Aspects 1, 2, and 18, wherein the generating element is a polarization-maintaining fiber.

[0165] 1 Quantum entangled photon pair generating device 11 Resonator 12 Interference unit 13 Pump light generating device E1 Generator E2 Shared connection unit E3 Input / output unit PL Pump light EPP Quantum entangled light PP Polarization rotation element P1 to P4 Optical paths P31, P32, P41, P42, P311, P312, P3111, P3112, P411, P412, P51 Optical paths of interference unit 12 P11 to P18, P21 to P27, P221, P222, P121 to P123, P2221 to P2223 Optical paths of resonator unit 11 M11, M12, M21, M22, M31 to M34, M41 to M46, M15, M23, M24, Mirror M51, M52 Mirrors E21, E22, E23 Dichroic Mirrors E24 Polarization-Maintaining Fiber Coupler PS Periscope

Claims

1. A quantum entangled photon pair generating device comprising: a resonator having a loop-shaped first optical path and a generating element arranged on the first optical path for generating a two-photon state; an interference unit having a loop-shaped second optical path and an input / output unit arranged on the second optical path for inputting pump light and outputting quantum entangled photon pairs, and generating one or more photon pairs in a quantum entangled state from the multiple two-photon states generated by the generating element; and a shared connection unit that forms a loop-shaped optical path for each of the first optical path and the second optical path, and optically connects the first optical path and the second optical path.

2. A quantum entangled photon pair generating device as set forth in claim 1, wherein in said resonator, a plurality of two-photon states generated by said generating element satisfy a resonance condition in said resonator.

3. The quantum entangled photon pair generating device according to claim 1 or 2, wherein the shared connection portion is provided with a dichroic mirror.

4. The quantum entangled photon pair generating device according to claim 3, wherein the resonator comprises a plurality of mirrors for forming the first optical path, the mirrors having a relatively higher reflectance than the dichroic mirror.

5. A quantum entangled photon pair generating device according to claim 1 or 2, wherein the shared connection portion comprises a plurality of dichroic mirrors.

6. The quantum entangled photon pair generating device according to claim 5, wherein the resonator comprises a plurality of mirrors for forming the first optical path, the plurality of mirrors having a relatively higher reflectivity than the plurality of dichroic mirrors.

7. A quantum entangled photon pair generating device as described in claim 3, wherein a plurality of mirrors for forming the first optical path are arranged symmetrically with respect to an axis of symmetry of the dichroic mirror and the generating element.

8. A quantum entangled photon pair generating device as described in claim 3, wherein the number of mirrors for forming the first optical path is 2m, the first optical path is formed on a diagonal of an n-sided polygon, m is an integer of 2 or more, and n = 2m + 1.

9. The quantum entangled photon pair generating device according to claim 8, wherein the n-gon is a pentagon, and the first optical path is a pentagram-shaped optical path.

10. The quantum entangled photon pair generating device according to claim 3, wherein the shared connection portion has an incident angle of 45 degrees for the first optical path and the second optical path.

11. The quantum entangled photon pair generating device according to claim 4, wherein the plurality of mirrors includes a concave mirror.

12. The quantum entangled photon pair generating device according to claim 4, wherein the plurality of mirrors includes a toroidal shaped mirror.

13. The quantum entangled photon pair generating device according to claim 3, further comprising a lens having a mechanism for converging a light beam in the first optical path of the resonator.

14. The quantum entangled photon pair generating device according to claim 1 or 2, wherein the interference section includes a polarization rotation element disposed on the second optical path.

15. The quantum entangled photon pair generating device according to claim 14, wherein the polarization rotation element comprises any one of a half-wave plate, a Fresnel rhomb, and a periscope structure.

16. The quantum entangled photon pair generating device according to claim 1 or 2, wherein the input / output section is provided with a beam splitter.

17. A quantum entangled photon pair generating device according to claim 1 or 2, wherein the generating element is a nonlinear optical medium that induces second-order spontaneous parametric down-conversion.

18. The quantum entangled photon pair generating device according to claim 1 or 2, wherein the shared connection portion comprises a polarization-maintaining fiber coupler.

19. The quantum entangled photon pair generating device according to claim 1 or 2, wherein the input / output section is provided with a polarization splitting fiber coupler.

20. A quantum entangled photon pair generating device according to claim 1 or 2, wherein the generating element is a polarization maintaining fiber.

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