Quantum computing unit, single photon source, quantum computing device, and quantum computing method

Abstract

In order to deterministically operate a quantum computing unit (13-m) in which a plurality of quantum systems are trapped, quantum computing is performed using a quantum computing unit including: a nano optical fiber (131-m) which is optically connected by way of a tapered portion to an optical fiber (12) through which incident photons propagate; and the plurality of quantum systems (132-m), which are arranged outside the nano optical fiber, and which are arranged spaced apart in the longitudinal direction of the nano optical fiber. However, at least any one of the quantum systems functions as a quantum bit that interacts with photons.

Description

[Technical field]

[0001] The present invention relates to quantum computing technology, and as an example, to distributed quantum computing technology. [Background technology]

[0002] Quantum computing, a new information processing technology based on quantum mechanics, enables high-speed calculations that were previously impossible. Large-scale quantum computing requires the implementation of a large number of quantum bits, but there is a limit to the number of quantum bits that can be implemented in one unit. One way to overcome this limit and realize large-scale quantum computing is the "distributed quantum computing" technique, which connects and networks a large number of small-scale quantum computing units. As a specific implementation method of distributed quantum computing, Monroe et al. have shown a method in which ion traps are used as quantum computing units and these are connected by photons (see, for example, Non-Patent Document 1, etc.). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] C. Monroe et al., "Large-scale modular quantum-computer architecture with atomic memory and photonic interconnects", Phys. Rev. A 89, 022317 (2014). Summary of the Invention [Problem to be solved by the invention]

[0004] However, with conventional implementation methods, it has been difficult to operate a quantum computing unit that traps multiple atoms deterministically.

[0005] First, the method of Non-Patent Document 1 does not allow multiple quantum computing units to be connected and operated deterministically. In contrast, it is possible to operate deterministically by using a cavity quantum electrodynamics (CQED) system in the optical domain. However, when a conventional free-space resonator is used as a quantum computing unit to achieve this, atoms must be placed inside a photon beam narrowed to have a small mode cross-sectional area in the free-space resonator. However, when a photon beam is narrowed in free space, the area with a mode cross-sectional area suitable for the arrangement of atoms becomes extremely narrow, making it difficult to place multiple atoms while maintaining the necessary coupling with the resonator. In addition, since free-space resonators have poor compatibility with optical fibers and require extremely complex and precise adjustment and control, it is extremely difficult to connect multiple quantum computing units and use them simultaneously while keeping losses low. Such problems arise not only when atoms are used as a quantum system that functions as a quantum bit that interacts with the state of photons, but also when a quantum system that shows the same optical response as an atom is used.

[0006] The present invention has been made in view of the above-mentioned points, and has as its object to operate a quantum computing unit in which a plurality of quantum systems are arranged in a deterministic manner. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, a quantum computing unit for quantum computing is provided, which includes a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons, and a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along the longitudinal direction of the nano-optical fiber, where at least one of the quantum systems functions as a quantum bit that interacts with the state of the photon. Effect of the Invention

[0008] This makes it possible to operate a quantum computing unit in which multiple quantum systems are arranged deterministically. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram illustrating the configuration of a quantum computing device according to the first embodiment. [Diagram 2] FIG. 2 is a diagram illustrating a configuration of a quantum computing unit according to an embodiment. [Diagram 3] FIG. 3 is a schematic diagram of a Λ-type three-level atom. [Figure 4] FIG. 4 is a diagram illustrating the configuration of a single photon source according to an embodiment. [Diagram 5] Figure 5A is a model of a photon entering a quantum computing unit and then exiting, and Figure 5B is a model of a photon entering a half-wave plate and then exiting. [Figure 6] FIG. 6 is an optical circuit diagram illustrating a method for realizing a quantum gate using two quantum computing units. [Figure 7] FIG. 7 is a diagram illustrating the configuration of a quantum computing device according to the second embodiment. [Figure 8] FIG. 8 is an optical circuit diagram illustrating a method for realizing a quantum gate using two quantum computing units. [Figure 9] FIG. 9 is an optical circuit diagram illustrating a method for realizing a quantum gate using three quantum computing units. [Figure 10] FIG. 10 is a diagram illustrating the configuration of a quantum computing device according to the third embodiment. [Figure 11] FIG. 11 is an optical circuit diagram illustrating a method for realizing a quantum gate using three quantum computing units. [Figure 12] FIG. 12 is a diagram illustrating the configuration of a quantum computing device according to the fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0011] [First embodiment] First, a first embodiment of the present invention will be described.

[0012] As illustrated in Fig. 1, the quantum computing device 1 of this embodiment includes a single photon source 11 for quantum computing, M quantum computing units 13-1 to 13-M, M optical switches 14-1 to 14-M, half-wave plates 15-1 to 15-(M+1), a polarization state measuring device 16, and an optical fiber 12. Here, M is an integer of 2 or more. Note that the quantum computing device 1 can be used for distributed quantum computing as an example, but this is not intended to limit the present embodiment. The quantum computing device 1 can also be used for non-distributed quantum computing.

[0013] <Optical fiber 12> The optical fiber 12 optically connects the single photon source 11, the quantum computing units 13-1 to 13-M, the optical switches 14-1 to 14-M, and the polarization state measuring device 16, and propagates the incident photons. In this embodiment, one end of the single photon source 11 is optically connected to one end of the optical switch 14-1 via the optical fiber 12, the optical switches 14-i (where i=1, ..., (M-1)) are optically connected to the optical switch 14-(i+1) and one end of the quantum computing unit 13-i via the optical fiber 12, and the optical switch 14-M is optically connected to the polarization state measuring device 16 and one end of the quantum computing unit 13-M via the optical fiber 12. The optical fiber 12 may be a single-mode optical fiber or a multimode optical fiber. However, when the optical fiber 12 is a multimode optical fiber, it is necessary to make the photons stay in one mode. Therefore, it is preferable that the optical fiber 12 is a single-mode optical fiber.

[0014] <Quantum Computing Unit 13-m> As illustrated in FIG. 2, the quantum computing unit 13-m of this embodiment includes a nano-optical fiber 131-m, an end 121a-m of the optical fiber 12, an end 121b-m of the optical fiber, tapered portions 122a-m and 122b-m, K(m) atoms 132-m (quantum systems functioning as quantum bits that interact with the state of photons), and a vacuum layer 133-m that houses them in a vacuum state. Here, m=1,...,M, and M is an integer of 2 or more. K(m) is an integer of 2 or more, and the K(m) atoms 132-m are a1-m,...,a K(m) An example of an atom 132-m is a laser-cooled atom, although this is not a limitation of the present invention.

[0015] The nano-optical fiber 131-m is an extremely thin optical fiber with a diameter equal to or smaller than the wavelength of light, with the entire fiber functioning as the core and the vacuum surrounding it functioning as the cladding. One end of the nano-optical fiber 131-m is optically connected to the end 121a-m of the optical fiber 12 via the tapered portion 122a-m. Similarly, the other end of the nano-optical fiber 131-m is optically connected to the end 121b-m of the optical fiber 12 via the tapered portion 122b-m. That is, in terms of structure, the ratio of the core diameter to the cladding diameter is constant or approximately constant throughout the entire fiber, including the tapered portions 122a-m and 122b-m, from the end 121a-m of the optical fiber 12 to the nano-optical fiber 131-m and further to the end 121b-m of the optical fiber 12. The cores of the ends 121a-m and 121b-m are connected to the core of the nano-optical fiber 131-m via the cores of the tapered portions 122a-m and 122b-m, respectively. The clads of the ends 121a-m and 121b-m are connected to the clad of the nano-optical fiber 131-m via the clads of the tapered portions 122a-m and 122b-m, respectively. For example, the core of the nano-optical fiber 131-m is integral with the cores of the tapered portions 122a-m and 122b-m and the ends 121a-m and 121b-m. The clad of the nano-optical fiber 131-m is integral with the clads of the tapered portions 122a-m and 122b-m and the ends 121a-m and 121b-m. Here, the diameters of the core and clad at end 121a-m are larger than those of the nano optical fiber 131-m, and the diameters of the core and clad at tapered portion 122a-m gradually increase from the nano optical fiber 131-m side toward end 121a-m. Similarly, the diameters of the core and clad at end 121b-m are larger than those of the nano optical fiber 131-m, and the diameters of the core and clad at tapered portion 122b-m also gradually increase from the nano optical fiber 131-m side toward end 121b-m.With this structure, in optical fiber 12, light is confined by the refractive index difference between the core and clad, and in nano optical fiber 131-m, light is confined by the refractive index difference between the clad and vacuum, and in tapered sections 122a-m and 122b-m that connect them, all three layers of the core, clad, and vacuum contribute to the confinement of light.

[0016] Nano-optical fibers are also described in References 1 and 2, which are incorporated herein by reference.

[0017] Reference 1: S. Kato et al., "Observation of dressed states of distant atoms with delocalized photons in coupled-cavities quantum electrodynamics", Nature Communications volume 10, Article number: 1160 (2019). Reference 2: S. Kato and T. Aoki, "Strong coupling between a trapped single atom and an all-fiber cavity", Physical Review Letters 115, 9 (2015). The cores of the ends 121a-m and 121b-m are provided with fiber Bragg gratings (FBGs) 123a-m and 123b-m, respectively, whose reflection bands include the wavelengths of photons propagating through the optical fiber 12. The fiber Bragg gratings 123a-m and 123b-m function as mirrors that reflect photons.

[0018] Fiber Bragg gratings 123a-m and 123b-m are provided in the cores of ends 121a-m and 121b-m, respectively, and by adjusting the transmittance and reflectance of each fiber Bragg grating, photons incident on the nano optical fiber 131-m from the end 121a-m side can be emitted from the end 121a-m side.

[0019] K(m) atoms 132-m (multiple atoms a1-m, …, a K(m) K(m) atoms 132-m (a plurality of atoms a1-m, ..., a1-m) are disposed on the exterior of the nano-optical fiber 131-m and spaced apart along the length of the nano-optical fiber 131-m by known trapping techniques. K(m) The atoms a1-m,...,a1-m) are aligned along the longitudinal direction of the nano-optical fiber 131-m. K(m) It is preferable that the atoms a1-m, ..., a1-m are arranged at positions corresponding to the antinodes or the vicinity of the antinodes of the standing waves generated in the nano-optical fiber 131-m. However, depending on the setting of the standing waves, the atoms a1-m, ..., a1-m, ..., a1-m, ... K(m) If sufficient coupling between the nano-optical fiber resonator QED (quantum electrodynamics) system (hereinafter simply referred to as the "resonator") consisting of the quantum computing unit 13-m and the standing wave is obtained, atoms a1-m, ..., a1-m, ..., a2-m, ..., a3-m, ..., a4-m, ..., a5-m, ..., a6-m, ..., a7-m, ..., a8-m, ..., a9-m, ..., a1 ...2-m, ... K(m) -m may be arranged. The atom 132-m may be a neutral atom or an ion. The atom 132-m is, for example, a cesium atom. For example, when the atom 132-m is a neutral atom, the atom 132-m is trapped by a known dipole trap or the like, and when the atom 132-m is an ion, the atom 132-m is trapped by a known ion trap method.

[0020] Figure 3 shows the interaction of each atom a with a photon propagating through the nano-optical fiber 131-m. k Here we show a schematic diagram of a Λ-type three-level atom in the CQED system of -m (where k = 1, ..., K(m)). k-m has a complex energy level structure, but only a few levels are actually involved in the dynamics of the CQED system (for example, the resonant frequency of the cavity or the external excitation of atom a k -m, and satisfies the selection rule for the transition). Here, a model in which the energy level structure of an atom is limited to three specific levels is called a three-level atom, and a three-level atom like the one in Figure 3 is called a Λ-type three-level atom. As shown in Figure 3, a Λ-type three-level atom has two lower levels |u〉 and |g〉, which are the ground state, and one upper level |e〉, which is the excited state. In the transition |g〉→|e〉 from |g〉 to |e〉, the atom a k -m is coupled to the cavity with coupling rate g, while in the transition from |u〉 to |e〉 (because the polarization of the transition is orthogonal to the polarization of the cavity mode, or because it is sufficiently non-resonant with the cavity that coupling can be effectively neglected), atom a k -m is not coupled to the resonator. However, the transition |u〉→|e〉 can be driven by control light irradiated from outside the resonator by a laser irradiation device (not shown), and the amplitude of the control light is expressed in Rabi frequency as Ω. Similar to the Λ-type three-level atom that interacts with photons in free space, the Λ-type three-level atom that interacts with photons propagating through the nano-optical fiber 131-m also reacts to the control light Θ that resonates with the transitions |g〉→|e〉 and |u〉→|e〉, respectively. g ,Θ u However, atoms a are emitted from outside the resonator by a laser irradiation device (not shown). k When irradiated with the -m wavelength, coherent Raman transitions can cause Rabi oscillations between the two ground state levels |g〉 and |u〉, or an arbitrary superposition of these (ρ g |g〉+ρ u |u〉), where ρ g and ρ u is |ρ g | 2 +|ρ u | 2 = 1, the complex coefficient (|ρ g | 2 and |ρ u | 2represents the probability).

[0021] Now consider the case without control light (Ω=0). Atom a k When -m is in the ground level |g〉, the optical response of the system is the same as that of a two-level atom with a lower level |g〉 and an upper level |e〉. If the cooperation coefficient C is sufficiently larger than 1, the incident photon is reflected with almost no loss and its phase is not shifted (amplitude reflectivity is 1). Here, the cooperation coefficient C of the resonator is C=g 2 / 2κγ, where κ is the amplitude relaxation rate due to external and internal losses of the resonator, and γ=Γ / 2, where Γ is the relaxation rate of energy due to spontaneous emission from the resonator. On the other hand, atom a k When -m is in another ground level |u〉, atom a k Since -m is not coupled to the cavity, the optical response of the system is the same as that without the atom (empty cavity), and the incident photon is reflected with almost no loss and its phase is shifted by π (amplitude reflectivity is -1). That is, atom a k By controlling the -m state (quantum state), the optical response of the system can be switched. Furthermore, when an atom is in a superposition state (ρ g |g〉+ρ u In the case of |u〉), the system shows a "superposition response" of the two responses mentioned above. In other words, the state after the incident photon interacts with the CQED system is a superposition of "a state in which the atom is in the ground level |g〉 and the incident photon is reflected with an amplitude reflectivity of 1" and "a state in which the atom is in the ground level |u〉 and the incident photon is reflected with an amplitude reflectivity of -1." In other words, just by reflecting a photon into the CQED system, the atom a k -m and photon entanglement can be generated. This response can realize a quantum gate.

[0022] The above-mentioned matters regarding the case without control light (Ω=0) are solved by the Hamiltonian of the system being

number

number

[0023] Here, ω in Eq. A is the transition frequency (resonance frequency) of the atom, and σ eg and σ with a hat ge is the raising and lowering operator for the atomic level |e〉|g〉. Also, ω in Eq. C is the resonant frequency, and a with hat and dagger c , and a with a hat c is the creation and annihilation operator of the cavity photons, and g is the coupling rate between the atom and the cavity, as mentioned above.

[0024] In addition, κ in Eq. ex is the amplitude relaxation rate due to external losses, κ is the amplitude relaxation rate due to external and internal losses of the resonator as described above, and Δ p is the frequency ω of the incident photon p and the atomic resonance frequency ω A This is the difference between...

[0025] When g is not zero, from Eq. 2, at the resonance of the atom, the amplitude reflection coefficient is

number

[0026] On the other hand, when g is 0 (no atoms (empty cavity)), the amplitude reflection coefficient is given by Eq.

number

number

[0027] The explanation for the above "case of no control light (Ω=0)" is as follows: If |u〉→|e〉 is an allowed optical transition (can be driven by the control light) and no control light is irradiated (Ω=0), Not only that, "In the first place, if |u〉→|e〉 is not an allowed optical transition (it cannot be driven by the control light) This explanation also applies to the

[0028] In addition, from the viewpoint of use as a single photon source, it is preferable that |u〉→|e〉 is an allowed optical transition, but from the viewpoint of use as a quantum computing unit, |u〉→|e〉 may or may not be an allowed optical transition.

[0029] Next, we consider the case with control light (Ω ≠ 0). In this case, atom a k -m only couples between pairs of levels {|u,n〉,|e,n〉,|g,n+1〉}, and the eigenstates are expressed as superpositions of these. Here, |i,n〉 is the state of the entire CQED system |i〉 A (×)|n〉 C where (×) represents the tensor product, and |i〉 A is atom a k -m represents one of the ground states |u〉 or |g〉 or the excited state |e〉, n represents the number of photons in the cavity, and |n〉 Crepresents the state of the resonator. By varying Ω from 0 to infinity slowly enough, the state of the system can be adiabatically changed from |u,n〉 to |g,n+1〉 while keeping it in the dark state (without exciting it to the upper level |e〉). The reverse time evolution is also possible by varying Ω from infinity to 0 slowly enough.

[0030] The above-mentioned matters regarding the case with control light (Ω≠0) are solved by the Hamiltonian of the system being

number

[0031] For simplicity, ω C =ω L = ω, the eigenenergy of the excited state is

number

number

[0032] In the CQED system, the state of an atom or a photon can be treated as a quantum bit. k If the state of -m is considered as a quantum bit, then atom a k -m any superposition state (ρ g |g〉+ρu In this case, the one-qubit gate is, for example, a control light Θ that resonates with the transitions |g〉→|e〉 and |u〉→|e〉, respectively, as mentioned above. g ,Θ u From outside the cavity, atom a k -m. Multi-qubit gates will be described later. When photons are used as qubits, a "polarization qubit" is a quantum bit based on two orthogonal polarization modes of a photon (for example, a superposition state ρ h |h〉+ρ v |v〉), etc., where ρ h and ρ v is |ρ h | 2 +|ρ v | 2 = 1, the complex coefficient (|ρ h | 2 and |ρ v | 2 (where represents the probability). A one-qubit gate can be easily realized using linear optical elements. For example, a polarization qubit can be transformed into any unitary form using a waveplate.

[0033] The electric field (evanescent field) of a photon propagating through the nano-optical fiber 131-m outside the nano-optical fiber 131-m is approximately expressed as follows, and decays exponentially with the decay length L. E=E0e -(r-a) / L where E0 is the electric field on the surface of the nano-optical fiber 131-m, e is the base of the natural logarithm, r is the distance from the central axis of the nano-optical fiber 131-m, and a is the radius of the nano-optical fiber 131-m. The attenuation length L depends on the refractive index, radius a, and wavelength λ of the nano-optical fiber 131-m, but is approximately a fraction of λ. Therefore, for atom a k In order to increase the coupling rate g between the -m and the resonator, atoms a are introduced from the surface of the nano-optical fiber 131-m. kThe distance to the trap position of atom a-m must be less than the evanescent decay length L, and the closer the better. k For stable trapping of the atom a, it is necessary to avoid the influence of van der Waals forces from the surface of the nano-optical fiber 131-m. k The trapping position of the atoms a1-m, ..., a1-m must be separated from the surface of the nano-optical fiber 131-m by several tens of nanometers or more. K(m) The strength of the evanescent field at the position a1-m is equal to or greater than a threshold th2 (second threshold), and multiple atoms a1-m, ..., a K(m) It is desirable that the van der Waals forces between each of the atoms a-m and the nano-optical fiber 131-m be equal to or less than a threshold th3 (third threshold). k It is desirable that -m is trapped at a position of about several tens to 200 nanometers (for example, at a position of 100 to 200 nanometers) from the nano optical fiber 131-m.

[0034] In order to realize a high-performance quantum gate when a three-level atom is used as a quantum bit, the above-mentioned cooperation coefficient C needs to be larger than a threshold TH1 (first threshold) (for example, C>1). Here, the cooperation coefficient C depends only on the mode cross-sectional area and total loss of the resonator, and the smaller the mode cross-sectional area and the smaller the total loss of the resonator, the larger the cooperation coefficient C. In this embodiment, the resonator is configured using a nano-optical fiber 131-m. Therefore, in principle, atoms a1-m, ..., a K(m) The length of the region in which the atoms a1-m, ..., a1-m are trapped can be made as long as desired. Furthermore, the nano-optical fiber 131-m is optically connected to the end 121a-m of the optical fiber 12 that propagates the incident photons via the tapered portion 122a-m, so the total loss of the resonator can be made small. Therefore, in principle, if the length of the nano-optical fiber 131-m is made longer, the resonator and each atom a1-m, ..., ... K(m) While keeping the coupling rate g with -m sufficiently large with respect to the amplitude relaxation rate κ and the relaxation rate γ, many atoms a1-m,…,a K(m)In addition, such a plurality of quantum computing units 13-m can be optically connected with low loss by optical fiber 12, so that a deterministic quantum circuit for distributed quantum computing can be configured.

[0035] The above-mentioned matters regarding the cooperation coefficient C are defined by the formula C=g 2 In / 2κγ,

number

number

[0036] <Single Photon Source 11> 4, the single photon source 11 of this embodiment includes a nano-optical fiber 111, an end 121a-0 of an optical fiber 12, an end 121b-0 of an optical fiber, tapered portions 122a-0 and 122b-0, an atom 112 (a quantum system having multiple levels), and a vacuum layer 113 that houses them in a vacuum state. One end of the nano-optical fiber 111 is optically connected to the end 121a-0 of the optical fiber 12 via the tapered portion 122a-0. Similarly, the other end of the nano-optical fiber 111 is optically connected to the end 121b-0 of the optical fiber via the tapered portion 122b-0. That is, in terms of structure, the ratio of the core diameter to the cladding diameter is constant or approximately constant throughout the entire fiber including the tapered portion 122a-0 and the tapered portion 122b-0 from the end portion 121a-0 of the optical fiber 12 to the nano optical fiber 111 and further to the end portion 121b-0 of the optical fiber 12. The cores of the ends 121a-0 and 121b-0 are connected to the core of the nano optical fiber 111 via the cores of the tapered portions 122a-0 and 122b-0, respectively. The claddings of the ends 121a-0 and 121b-0 are connected to the cladding of the nano optical fiber 111 via the claddings of the tapered portions 122a-0 and 122b-0, respectively. For example, the core of the nano optical fiber 111 is integral with the cores of the tapered portions 122a-0 and 122b-0 and the ends 121a-0 and 121b-0. The cladding of the nano optical fiber 111 is integral with the claddings of the tapered portions 122a-0 and 122b-0 and the end portions 121a-0 and 121b-0. Here, the diameters of the core and cladding of the end portion 121a-0 are larger than the diameters of the core and cladding of the nano optical fiber 131-0, respectively, and the diameters of the core and cladding at the tapered portion 122a-0 gradually increase from the nano optical fiber 111 side toward the end portion 121a-0 side. Similarly, the diameters of the core and cladding at the end portion 121b-0 are larger than the diameters of the core and cladding of the nano optical fiber 131-0, respectively, and the diameters of the core and cladding at the tapered portion 122b-0 also gradually increase from the nano optical fiber 111 side toward the end portion 121b-0 side.With this structure, light is confined in the optical fiber 12 by the refractive index difference between the core and the clad, and in the nano-optical fiber 111 by the refractive index difference between the clad and the vacuum, and in the tapered sections 122a-0 and 122b-0 connecting them, all three layers of the core, clad, and vacuum contribute to the confinement of light. The cores of the ends 121a-0 and 121b-0 are provided with fiber Bragg gratings 123a-0 and 123b-0, respectively, whose reflection bands include the wavelength of the generated photon (single photon). The fiber Bragg gratings 123a-0 and 123b-0 function as mirrors. The atoms 112 are placed outside the nano-optical fiber 111 by a known trapping method. The atoms 112 may be neutral atoms or ions. The atoms 112 are, for example, cesium atoms. The conditions of the trapping position of atom 112 are the same as those of the trapping position of atom 132-m in quantum computing unit 13-m, for example, except that atom 132-m is replaced by atom 112 and nano-optical fiber 131-m is replaced by nano-optical fiber 111.

[0037] The simplest method for realizing a deterministic single photon source using the single photon source 11 is based on the Purcell effect of a two-level atom having one lower level |g〉, which is the ground state, and one upper level |e〉, which is the excited state. That is, just by exciting the atom 112 to the upper level |g〉 by irradiating the atom 112 with control light using a laser irradiation device (not shown) in the same manner as the laser irradiation device for irradiating the above-mentioned optical shift light, the photon 111 (single photon) is spontaneously emitted from the end 121a-0 to the optical fiber 12. However, the method based on the Purcell effect of the two-level atom does not allow the waveform of the photon 111 to be controlled. By using a Λ-type three-level atom, the waveform of the photon 111 can be freely controlled. That is, when the initial state is a dark state |u,0〉 with the amplitude Ω=0 of the control light and Ω is changed sufficiently slowly from 0 to infinity, the state of the system adiabatically changes to |g,1〉 while remaining in the dark state, and a photon 111 (single photon) is generated in the single-photon source 11 and emitted from the end 121a-0 to the optical fiber 12. Here, the waveform of the photon can be controlled via the change in the amplitude Ω of the control light over time.

[0038] As will be understood from other descriptions in this specification, the term "deterministic configuration" in this specification refers to a configuration that can realize a 100% probability under ideal conditions (a situation in which experimental losses are negligible). As an example, since single-photon source 11 can generate a one-photon state with a 100% probability under ideal conditions, single-photon source 11 is called a "deterministic single-photon source."

[0039] <Optical switch 14-m> The optical switch 14-m (m=1, . . . , M) is a device that outputs an input photon to a selected optical path. The input photon and the output photon are propagated through the optical fiber 12.

[0040] <Half-wave plate 15-m'> The half-wave plate 15-m' (m'=1, ..., M+1) is a device that rotates the state of the incident photon by π / 2 and outputs it. The incident and output photons are propagated through the optical fiber 12.

[0041] <Polarization state measuring instrument 16> The polarization state measuring instrument 16 has a polarizing beam splitter 16a and photodetectors 16b and 16c. Photons incident on the polarization state measuring instrument 16 via the optical fiber 12 are incident on the polarizing beam splitter 16a, and the horizontally polarized |h〉 component of the photon emitted from the polarizing beam splitter 16a is detected by detector 16b, and the vertically polarized |v〉 component is detected by detector 16c.

[0042] <Operation of quantum computing device 1> The quantum gate of this embodiment is based on an atom-photon quantum gate. That is, as illustrated in FIG. 5A, the operation of injecting a photon 111 into a quantum computing unit 13-m (resonator QED system) corresponds to an atom-photon quantum gate. Here, if the photon 111 is set to a predetermined state (e.g., transverse polarization |h〉) and is not used as a quantum bit (e.g., measured midway), the operation on the multiple atoms trapped in the quantum computing unit 13-m is an atom-atom quantum gate. For example, x atoms (x is an integer equal to or greater than 2) to be used for quantum computing (hereinafter referred to as "selected atoms") are selected from K(m) atoms 132-m trapped in the quantum computing unit 13-m, and only the selected atoms are coupled to the resonator by irradiating the light shift light, and the photon 111 is injected into the quantum computing unit 13-m. The light shift light is light (e.g., laser light) with a frequency sufficiently separated from the resonance frequency of the transition |e>→|g>. By irradiating an atom with this optically shifted light, the energy of each level of the atom can be shifted, and this energy shift is called the "optical shift." When an atom is irradiated with optically shifted light with a frequency sufficiently different from the resonance frequency of the transition |e>→|g>, the energy of |g> and |e> shifts, and the resonance frequency of the transition |e>→|g> also changes accordingly. If the transition |e>→|g> resonates (couples) with the resonator without the optical shift, only the atoms irradiated with the optically shifted light will not resonate with the resonator in the transition |e>→|g>, so the coupling between those atoms and the resonator can be selectively "disconnected." Conversely, the resonant frequency of the resonator may be shifted from the resonance frequency of the transition |e>→|g> without the optically shifted light, and the intensity of the optically shifted light may be adjusted so that the resonance frequency of the transition |e>→|g> resonates (couples) with the resonator when the optically shifted light is irradiated.As a result, the state of the selected atom after the photon 111 is reflected from the quantum computing unit 13-m (the state representing the calculation result) is obtained by applying a controlled phase flip gate (hereinafter referred to as a "CPF gate") (when x=2) or an x-bit multiple controlled phase flip gate (a phase flip gate with x-1 quantum bits as control bits, hereinafter referred to as an "x-bit Toffoli gate") (when x≧3) to the state of the selected atom before the photon 111 is incident on the quantum computing unit 13-m (the state representing the operand) (see, for example, Reference 3, etc.).

[0043] Reference 3: L.-M. Duan, B. Wang, and HJ Kimble, "Robust quantum gates on neutral atoms with cavity-assisted photon scattering", Phys. Rev. A 72, 032333 (2015). The state of the selected atom before the photon 111 is incident on the quantum computing unit 13-m may be an initialized state (for example, a state in which the states of all selected atoms are initialized to 0), or may be a state in which a one-qubit gate operation or a multi-qubit gate operation has been performed on the initialized state. In the atom-to-atom quantum gate, the state of the photon 111 plays a role in connecting multiple quantum computing units 13-m.

[0044] 5B, passing the photon 111 through a half-wave plate 15-m with a rotation angle of θ / 2 corresponds to rotating the state of the photon 111 by θ (see, for example, Reference 3). In this embodiment, a half-wave plate 15-m with θ=π / 2 is used.

[0045] In this embodiment, a control unit (not shown) operates the optical switch 14-m (m=1,...,M) to optically connect a plurality of quantum computing units 13-m selected from M quantum computing units 13-1 to 13-M, and a photon (single photon) 111 emitted from the single photon source 11 is input to an optical circuit including the selected plurality of quantum computing units 13-m, thereby performing distributed quantum computing. For example, the optical switch 14-m (m=1,...,M) is operated to optically connect two quantum computing units 13-m1 and 13-m2 (m1,m2∈{1,...,M}) (first quantum computing unit and second quantum computing unit), and a photon 111 is input to an optical circuit including these two quantum computing units 13-m1 and 13-m2, thereby performing distributed quantum computing.

[0046] In the example of Fig. 6, quantum computing units 13-1 and 13-M are selected by operating optical switch 14-m (m = 1, ..., M). Hereinafter, the operation of this embodiment will be illustrated with reference to Fig. 6. First, a laser irradiation device (not shown) irradiates light shift light to select x1 selected atoms sa1 selected from the multiple atoms 132-1 trapped in quantum computing unit 13-1, and x1 selected atoms sa1 selected from the multiple atoms 132-M trapped in quantum computing unit 13-M. M Selected atoms sa M is coupled to each resonator. The number of qubits used in this distributed quantum computation is x1+x M x1 ≥ 1,x M ≧1, but x1≧1 and x M ≧2, or x1≧2 and x M ≧1, or x1≧2 and x M In the case where distributed quantum computation is performed using a plurality of atoms trapped in the quantum computation units 13-1 and 13-M, x1≧2, x M ≧2. x1 ≧ 1 and x M ≧2, or x1≧2 and x M ≧1 (step S10). In this state, the following series of operations are performed.

[0047] (1) The single-photon source 11 generates a photon 111 (single photon) in a predetermined state (for example, horizontal polarization |h〉) and emits it from the end 121a-0 (step S11).

[0048] (2) In step S11, the photon 111 emitted from the single photon source 11 is made incident on the half-wave plate 15-1, and the state of the photon 111 is rotated by π / 2 before being emitted (step S12).

[0049] (3) In step S12, the photons emitted from half-wave plate 15-1 are made incident on end 121a-1 of quantum computing unit 13-1 (step S13).

[0050] (4) In response to the operation of step S13, a photon emitted from end 121a-1 of quantum computing unit 13-1 is made incident on half-wave plate 15-M, and the state of the photon is rotated by π / 2 before being emitted (step S14).

[0051] (5) In step S14, the photons emitted from half-wave plate 15-M are made incident on end 121a-M of quantum computing unit 13-M (step S15).

[0052] (6) In response to the operation of step S15, a photon emitted from end 121a-M of quantum computing unit 13-M is made incident on half-wave plate 15-(M+1), and the state of the photon is rotated by π / 2 before being emitted (step S16).

[0053] (7) In step S16, the photon emitted from the half-wave plate 15-(M+1) is made incident on the polarization state measuring device 16, and the polarization state of the photon is measured. As a result, the state of the photon is contracted to a certain polarization state, and the selected atoms sa1 and sa2, which are in a state entangled with the state of this photon, are MThe state of is also contracted according to the measurement result of the polarization state of the photon. Then, a rotation operation according to the measurement result of the polarization state of the photon is performed on the selected atom sa1 trapped in the quantum computing unit 13-1. That is, if the polarization state of the measured photon is a predetermined state in step S11 (for example, horizontal polarization |h〉), a π rotation operation (Z gate operation) is performed on the selected atom sa1. This rotation operation can be realized by irradiating the selected atom sa1 with a control light. If the polarization state of the measured photon is not a predetermined state in step S11 (for example, vertical polarization |v〉), no operation is performed on the selected atom sa1 (step S17).

[0054] The selected atom sa1 and the selected atom sa2 subjected to the above-mentioned steps S11 to S17 are M The state of the selected atom sa1 and the selected atom sa before the operation is performed is M For the state of M =2) or x1+x M Toffoli gate of bits (x1+x M ≧3) is applied.

[0055] [Second embodiment] Next, a second embodiment of the present invention will be described. The following description will focus on the differences from the first embodiment, and the same reference numerals will be used to simplify the description of the matters already described.

[0056] As illustrated in FIG. 7, the quantum computing device 2 of this embodiment includes a single-photon source 11, M quantum computing units 13-1 to 13-M, an optical switch 24, half-wave plates 15-1 to 15-M, and an optical fiber 12.

[0057] <Optical fiber 12> Optical fiber 12 optically connects single-photon source 11, quantum computing units 13-1 to 13-M, and optical switch 24, and propagates incident photons.

[0058] <Optical switch 24> The optical switch 24 is a device that outputs an input photon to a selected optical path. The photon is propagated to the optical switch 24 and from the optical switch 24 to the quantum computing unit 13-m through the optical fiber 12.

[0059] <Operation of quantum computing device 2> In this embodiment, a control unit (not shown) operates the optical switch 24 to optically connect a plurality of quantum computing units 13-m selected from the M quantum computing units 13-1 to 13-M, and perform distributed quantum computing by inputting photons 111 emitted from the single-photon source 11 to an optical circuit including the selected plurality of quantum computing units 13-m. An example of the operation is shown below.

[0060] <Example 1> In the operation example 1 of this embodiment, two quantum computing units 13-m1 and 13-m2 (m1, m2 ∈ {1, ..., M}) (first quantum computing unit and second quantum computing unit) are optically connected by operating the optical switch 24, and a photon 111 is input to an optical circuit including these two quantum computing units 13-m1 and 13-m2 to perform distributed quantum computing. In the example of FIG. 8, the quantum computing units 13-1 and 13-2 are selected by operating the optical switch 24. The operation of the operation example 1 will be illustrated below with reference to FIG. 8. First, as described in the first embodiment, x1 selected atoms sa1 selected from the multiple atoms 132-1 trapped in the quantum computing unit 13-1 and x2 selected atoms sa2 selected from the multiple atoms 132-2 trapped in the quantum computing unit 13-2 are coupled to each resonator by irradiation with the optical shift light. The number of quantum bits used in this distributed quantum computing is x1 + x2. Although x1≧1, x2≧1, it may be x1≧1 and x2≧2, or x1≧2 and x2≧1, or x1≧2 and x2≧2. When performing distributed quantum computing using multiple atoms trapped in quantum computing units 13-1 and 13-2, it becomes x1≧2, x2≧2 (step S20). In this state, the following series of operations are performed.

[0061] (1) A photon 111 in a predetermined state (for example, horizontal polarization |h〉) is generated by the single photon source 11 and emitted from the end 121a-0 (step S11).

[0062] (2) In step S11, the photon 111 emitted from the single photon source 11 is made incident on the half-wave plate 15-1, and the state of the photon 111 is rotated by π / 2 before being emitted (step S12).

[0063] (3) In step S12, the photons emitted from half-wave plate 15-1 are made incident on end 121a-1 of quantum computing unit 13-1 (step S13).

[0064] (4) In response to the operation of step S13, a photon emitted from end 121a-1 of quantum computing unit 13-1 is made incident on half-wave plate 15-2, and the state of the photon is rotated by π / 2 before being emitted (step S24).

[0065] (5) In step S24, the photons emitted from half-wave plate 15-2 are made incident on end 121a-2 of quantum computing unit 13-2 (step S25).

[0066] (6) In response to the operation of step S25, a photon emitted from end 121a-2 of quantum computing unit 13-2 is made incident on half-wave plate 15-1, and the state of the photon is rotated by π / 2 before being emitted (step S26).

[0067] (7) The photon emitted from half-wave plate 15-1 in step S26 is made incident again on end 121a-1 of quantum computing unit 13-1 (step S27). In response to the operation of step S27, a photon is emitted from end 121a-1 of quantum computing unit 13-1.

[0068] The states of the selected atoms sa1 and sa2 after the operations in steps S11 to S13 and S24 to S27 described above are obtained by applying a CPF gate (when x1+x2=2) or a x1+x2-bit Toffoli gate (when x1+x2≧3) to the states of the selected atoms sa1 and sa2 before the operations. The state of the photon emitted from the single-photon source 11 remains the same as before the operations.

[0069] <Example 2> In the operation example 2 of this embodiment, the optical switch 24 is operated to optically connect three quantum computing units 13-m1, 13-m2, and 13-m3 (m1, m2, m3 ∈ {1, ..., M}) (first quantum computing unit, second quantum computing unit, and third quantum computing unit), and a photon 111 is input to an optical circuit including these three quantum computing units 13-m1, 13-m2, and 13-m3 to perform distributed quantum computing. In the example of FIG. 9, the quantum computing units 13-1, 13-2, and 13-3 are selected by operating the optical switch 24. The operation of the operation example 2 will be illustrated below with reference to FIG. 9.

[0070] First, as described in the first embodiment, x1 selected atoms sa1 selected from the multiple atoms 132-1 trapped in the quantum computing unit 13-1, x2 selected atoms sa2 selected from the multiple atoms 132-2 trapped in the quantum computing unit 13-2, and x3 selected atoms sa3 selected from the multiple atoms 132-3 trapped in the quantum computing unit 13-3 are coupled to each resonator by irradiation with the optical shift light. The number of quantum bits used in this distributed quantum computing is x1+x2+x3. x1≧1, x2≧1, x3≧1, but any of x1, x2, and x3 may be 2 or more, or x1≧2, x2≧2, and x3≧2 (step S20'). In this state, the following series of operations are performed.

[0071] (0) First, the operations of steps S11 to S13 and S24 to S27 of Operation Example 1 are performed. However, in steps S12, S24, and S26, half-wave plate 15-1 or half-wave plate 15-2 rotates the state of photon 111 by π / 4 instead of π / 2. In response to the operation of step S27, a photon is emitted from end 121a-1 of quantum computing unit 13-1. The photon emitted in step S27 is made incident on half-wave plate 15-3, and the photon state is rotated by π / 4 before being emitted (step S28).

[0072] (1) In step S28, the photons emitted from half-wave plate 15-3 are made incident on end 121a-3 of quantum computing unit 13-3 (step S29).

[0073] (2) In response to the operation of step S29, a photon emitted from end 121a-3 of quantum computing unit 13-3 is made incident on half-wave plate 15-1, and the photon is rotated by π / 4 and then emitted (step S12').

[0074] (3) In step S12', the photons emitted from half-wave plate 15-1 are made incident on end 121a-1 of quantum computing unit 13-1 (step S13').

[0075] (4) In response to the operation of step S13', a photon emitted from end 121a-1 of quantum computing unit 13-1 is made incident on half-wave plate 15-2, and the state of the photon is rotated by π / 4 before being emitted (step S24').

[0076] (5) In step S24', the photons emitted from half-wave plate 15-2 are made incident on end 121a-2 of quantum computing unit 13-2 (step S25').

[0077] (6) In response to the operation of step S25', a photon emitted from end 121a-2 of quantum computing unit 13-2 is made incident on half-wave plate 15-1, and the state of the photon is rotated by π / 4 before being emitted (step S26').

[0078] (7) The photons emitted from half-wave plate 15-1 in step S26' are made incident again on end 121a-1 of quantum computing unit 13-1 (step S27'). In response to the operation of step S27', photons are emitted from end 121a-1 of quantum computing unit 13-1.

[0079] The states of the selected atoms sa1, sa2, and sa3 after the above-mentioned operations S11 to S13, S24 to S29, S12', S13', and S24' to S27' are obtained by applying a x1+x2+x3-bit Toffoli gate to the states of the selected atoms sa1, sa2, and sa3 before the operations. The state of the photon emitted from the single-photon source 11 remains the same as before the operations.

[0080] [Third embodiment] 10, a quantum computing device 3 of this embodiment includes a single photon source 11, M quantum computing units 13-1 to 13-M, an optical switch 24, half-wave plates 15-1 to 15-(M+1), a polarization state measuring device 16, and an optical fiber 12.

[0081] <Operation of quantum computing device 3> In this embodiment, the optical switch 24 is operated by a control unit (not shown) to control the three quantum computing units 13-m1, 13-m2, and 13-m M (m1,m2,m M ∈{1,…,M}) (the first quantum computing unit, the second quantum computing unit, and the third quantum computing unit) are optically connected to each other, and these three quantum computing units 13-m1, 13-m2, 13-m M Distributed quantum computation is performed by inputting photons 111 to an optical circuit including the above-mentioned. In the example of Fig. 11, quantum computation units 13-1, 13-2, and 13-M are selected by operating optical switch 24. The operation of this embodiment will be illustrated below with reference to Fig. 11.

[0082] First, as described in the first and second embodiments, x1 selected atoms sa1, x2 selected atoms sa2, and x M Selected atoms saM is coupled to each resonator. In this state, the following series of operations are carried out.

[0083] (0) First, the operations of steps S11 to S13, S24 to S29, S12', and S13' described above are performed. However, in steps S12, S24, and S26, half-wave plate 15-1 or half-wave plate 15-2 rotates the state of photon 111 by π / 4 instead of π / 2. Also, quantum computing unit 13-M is used instead of quantum computing unit 13-3.

[0084] (4) In response to the operation of step S13', a photon emitted from end 121a-1 of quantum computing unit 13-1 is made incident on half-wave plate 15-2, and the state of the photon is rotated by π / 4 before being emitted (step S34).

[0085] (5) In step S34, the photons emitted from half-wave plate 15-2 are made incident on end 121a-2 of quantum computing unit 13-2 (step S35).

[0086] (6) In response to the operation of step S35, the photon emitted from end 121a-2 of quantum computing unit 13-2 is made incident on half-wave plate 15-(M+1), and the state of the photon is rotated by π / 4 before being emitted (step S36).

[0087] (7) In step S36, the photon emitted from the half-wave plate 15-(M+1) is made incident on the polarization state measuring device 16, and the polarization state of the photon is measured. As a result, the state of the photon is contracted to a certain polarization state, and the selected atoms sa1, sa2, and sa that are entangled with the state of this photon are M The state of the selected atom sa1 is also contracted according to the measurement result of the polarization state of the photon. Then, as described in the first embodiment, the selected atom sa1 trapped in the quantum computing unit 13-1 is rotated according to the measurement result of the polarization state of the photon (step S37).

[0088] Selected atoms sa1, sa2, and sa3 after the above-mentioned operations S11 to S13, S24 to S29, S12', S13', and S34 to S37 are MThe state of the selected atoms sa1,sa2,sa before the operation is performed. M For the state of x1+x2+x M This is the result of applying a Toffoli gate to the 1-bit gate.

[0089] [Fourth embodiment] Next, a fourth embodiment of the present invention will be described.

[0090] As illustrated in FIG. 12, the quantum computing device 4 of this embodiment includes N single photon sources 41-1 to 41-N (single photon sources S(1), ..., S(N)), M quantum computing units 13-1 to 13-M (quantum computing units U(1), ..., U(M)), an optical switch 44, half-wave plates 45-1 to 45-Y, polarization state measuring devices 46-1 to 46-N, and an optical fiber 12. Here, N is an integer of 2 or more, M is an integer of 2N or more, and Y is an integer of 1 or more. The configuration of the quantum computing units 43-m (m=1, ..., M) is the same as that of the quantum computing unit 13-m of the first embodiment. The configuration of the single photon source 41-j (j=1, ..., N) is the same as that of the single photon source 11 of the first embodiment. The configuration of the polarization state measuring device 46-j (j=1, ..., N) is also the same as that of the polarization state measuring device 16 of the first embodiment. The configuration of the half-wave plate 45-y (y=1, ..., Y) is also the same as that of the half-wave plate 15-i' (i'∈{1, ..., M+1}) of any one of the first to third embodiments. The optical switch 44 is a device that outputs an incident photon to a selected optical path. The photon is propagated to the optical switch 44 and from the optical switch 44 to the quantum computing unit 13-m by the optical fiber 12.

[0091] <Operation of quantum computing device 4> In this embodiment, the optical switch 44 optically connects each single photon source 41-j (j=1,...,N) and a pair of two selected quantum computation units 13-α1(j) and 13-α2(j). However, α1(j),α2(j)∈{1,...,M}, and α1(j)≠α2(j). Each single photon source 41-j emits a photon 411-j (single photon). For j=1,...,N, the photon emitted from each single photon source 41-j is input to an optical circuit C(j) including a pair of two quantum computation units 13-α1(j) and 13-α2(j). This allows distributed quantum computation to be performed in parallel by the optical circuits C(1),...,C(N). Each optical circuit C(j) may include one more quantum computing unit 13-α3(j) (α3(j)∈{1, ..., M}, α1(j) ≠ α2(j) ≠ α3(j)) in addition to the set of two quantum computing units 13-α1(j) and 13-α2(j), may include one or more half-wave plates 45-y, and may include a polarization state measuring device 46-j. There is no limitation on the optical circuit C(j), but examples of the optical circuits (FIGS. 6, 8, 9, and 11) exemplified in the first to third embodiments can be given. Specific examples of distributed quantum computing by each optical circuit C(j) are as exemplified in the first to third embodiments.

[0092] [Other variations] The present invention is not limited to the above-mentioned embodiment. For example, in the above-mentioned embodiment, the fiber Bragg gratings 123a-m and 123b-m are provided in the cores of the end 121a-m and end 121b-m of the quantum computing unit 13-m, respectively. However, instead of providing the fiber Bragg grating 123a-m in the core of the end 121a-m, the fiber Bragg grating may be provided in the tapered portion 122a-m or the nano-optical fiber 131-m. Similarly, instead of providing the fiber Bragg grating 123b-m in the core of the end 121b-m, the fiber Bragg grating may be provided in the tapered portion 122b-m or the nano-optical fiber 131-m. However, it is preferable that all atoms 132-m are arranged along the nano-optical fiber region between the two fiber Bragg gratings provided in the quantum computing unit 13-m. That is, it is desirable that the strength of the evanescent field at the positions of all atoms 132-m for photons propagating within nano-optical fiber 131-m satisfies the above-mentioned condition.

[0093] In the above embodiment, an example has been shown in which the fiber Bragg gratings 123a-0 and 123b-0 are provided in the cores of the end 121a-0 and end 121b-0 of the single-photon source 11, respectively. However, instead of providing the fiber Bragg grating 123a-0 in the core of the end 121a-0, a fiber Bragg grating may be provided in the tapered section 122a-0 or the nano-optical fiber 111. Similarly, instead of providing the fiber Bragg grating 123b-0 in the core of the end 121b-0, a fiber Bragg grating may be provided in the tapered section 122b-0 or the nano-optical fiber 111. However, it is preferable that the atom 112 is disposed along the nano-optical fiber region between the two fiber Bragg gratings provided in the single-photon source 11. In other words, it is preferable that the strength of the evanescent field at the position of the atom 112 for the photon propagating in the nano-optical fiber 111 satisfies the above-mentioned condition.

[0094] In the above embodiment, the nano-optical fibers 111, 131-m and the tapered portions 122a-0, 122b-0, 122am, and 122b-m are surrounded by a vacuum. However, the nano-optical fibers 111, 131-m and the tapered portions 122a-0, 122b-0, 122am, and 122b-m do not need to be surrounded by a vacuum, and any space having a smaller refractive index than the cladding of the nano-optical fibers 111, 131-m and the tapered portions 122a-0, 122b-0, 122am, and 122b-m may be used. For example, this space may be filled with a gas such as air or nitrogen, or a liquid such as water or alcohol. In the above embodiment, the atom 132-m is used as the "quantum system functioning as a quantum bit that interacts with the state of photons" in the quantum computing unit 13-m. However, instead of the atom 132-m, a quantum system in a solid, such as an artificial atom (e.g., a quantum dot) or a color center in a crystal (e.g., an NV center in diamond), which shows an optical response similar to that of the atom 132-m, may be used (see, for example, References 4, 5, etc.). Similarly, in the above-mentioned embodiment, the atom 112 is used as the "quantum system having multiple levels" in the single-photon source 11. However, instead of the atom 112, a quantum system in a solid, such as an artificial atom (e.g., a quantum dot) or a color center in a crystal (e.g., an NV center in diamond), which shows an optical response similar to that of the atom 112, may be used. In such a case, the solid having the quantum system may be fixed to the nano-optical fibers 111 and 131-m, respectively.

[0095] Reference 4: Ramachandrarao Yalla, Fam Le Kien, M Morinaga, K Hakuta, "Efficient Channeling of Fluorescence Photons From Single Quantum Dots Into Guided Modes of Optical Nanofiber," Phys Rev Lett. 2012 Aug 10;109(6):063602. doi: 10.1103 / PhysRevLett.109.063602. Epub 2012 Aug 8. Reference 5: Masazumi Fujiwara1, Kazuma Yoshida1, Tetsuya Noda, Hideaki Takashima, Andreas W Schell, Norikazu Mizuochi and Shigeki Takeuchi, "Manipulation of single nanodiamonds to ultrathin fiber-taper nanofibers and control of NV-spin states toward fiber-integrated λ-systems," Published 7 October 2016, Nanotechnology, Volume 27, Number 45. Such a solid can be fixed to the nano-optical fiber 111, 131-m by, for example, van der Waals force. In the above embodiment, a Λ-type three-level atom is exemplified, but other three-level atoms or atoms having four or more levels may be used.

[0096] The number of half-wave plates, single-photon sources, and optical state measuring devices is not limited, and may be appropriately set according to the distributed quantum computing to be realized. Needless to say, other appropriate changes are possible without departing from the spirit of the present invention.

[0097] [summary] The features of each embodiment and its modified examples will be summarized below.

[0098] As described above, the quantum computing unit 13-m for distributed quantum computing in each embodiment includes a nano-optical fiber 131-m optically connected to the optical fiber 12 that propagates incident photons via a tapered portion 121a-m, and a plurality of atoms a1-m, ..., ... K(m) In this way, the nano-optical fiber 131-m is used to configure the resonator, and in principle, atoms a1-m, ..., a1-m can be coupled to the nano-optical fiber 131-m while maintaining a small mode cross-sectional area. K(m)The length of the region in which the atoms a1-m, ..., a1-m are trapped can be made as long as desired. In addition, the nano-optical fiber 131-m is optically connected to the end 121a-m of the optical fiber 12 that propagates the incident photons via the tapered portion 122a-m, so the total loss of the resonator can be made small. Therefore, the resonator and the individual atoms a1-m, ..., ... K(m) While keeping the coupling rate g with -m sufficiently large relative to the amplitude relaxation rate κ and the relaxation rate γ (i.e., keeping the cooperation coefficient C larger than TH1 (first threshold)), many atoms a1-m, ..., a K(m) In addition, such quantum computing units 13-m can be optically connected with low loss by optical fiber 12, so that a deterministic quantum circuit for distributed quantum computing can be configured by coupling multiple quantum computing units 13-m.

[0099] As mentioned above, the photons propagating in the nano-optical fiber 131-m are reflected by multiple atoms a1-m, ..., a K(m) The strength of the evanescent field at the position a1-m is equal to or greater than a threshold th2 (second threshold), and multiple atoms a1-m, ..., a K(m) It is desirable that the van der Waals force between each of the atoms a1-m, a2-m, and the nano-optical fiber 131-m is equal to or less than a threshold th3 (third threshold). In this embodiment, by using the nano-optical fiber 131-m, the coupling rate g is made sufficiently large relative to the amplitude relaxation rate κ and the relaxation rate γ, and under this condition, the coupling rate g is made sufficiently large relative to the amplitude relaxation rate κ and the relaxation rate γ. K(m) Atoms a1-m, ..., a-m can be aligned in a line along the longitudinal direction of the nano-optical fiber 131-m. K(m) The -m are aligned in a line, making it easier to shine a control light on individual atoms.

[0100] The above-mentioned features make it possible to perform distributed quantum computing by coupling multiple quantum computing units each trapping multiple atoms. For example, a first quantum computing unit and a second quantum computing unit selected from the multiple quantum computing units are optically connected, and photons are input to an optical circuit including the first quantum computing unit and the second quantum computing unit, thereby performing distributed quantum computing.

[0101] For example, as illustrated in Figures 9 and 11, distributed quantum computing is also possible using three quantum computing units, namely, quantum computing unit 13-1 (first quantum computing unit), quantum computing unit 13-2 (second quantum computing unit), and quantum computing unit 13-3 or 13-M (third quantum computing unit), each of which traps multiple atoms.

[0102] For example, as illustrated in FIG. 9 , a photon is incident on half-wave plate 15-1 (first half-wave plate), a photon emitted from half-wave plate 15-1 is incident on quantum computing unit 13-1 (first quantum computing unit), a photon emitted from quantum computing unit 13-1 is incident on half-wave plate 15-2 (second half-wave plate), a photon emitted from half-wave plate 15-2 (second half-wave plate) is incident on quantum computing unit 13-2 (second quantum computing unit), a photon emitted from quantum computing unit 13-2 is incident on half-wave plate 15-1 (third half-wave plate), a photon emitted from half-wave plate 15-1 (third half-wave plate) is incident on quantum computing unit 13-1, a photon emitted from quantum computing unit 13-1 is incident on half-wave plate 15-3 (fourth half-wave plate), and a photon emitted from half-wave plate 15-3 (third half-wave plate) is incident on quantum computing unit 13-2. Distributed quantum computing can also be performed by making a photon emitted from quantum computing unit 13-2 incident on half-wave plate 15-1 (fourth half-wave plate) into quantum computing unit 13-3 (third quantum computing unit), making a photon emitted from quantum computing unit 13-3 incident on half-wave plate 15-1 (fifth half-wave plate), making a photon emitted from half-wave plate 15-1 (fifth half-wave plate) into quantum computing unit 13-1, making a photon emitted from quantum computing unit 13-1 incident on half-wave plate 15-2 (sixth half-wave plate), making a photon emitted from half-wave plate 15-2 (sixth half-wave plate) into quantum computing unit 13-2, making a photon emitted from quantum computing unit 13-2 incident on half-wave plate 15-1 (seventh half-wave plate), and making a photon emitted from half-wave plate 15-1 (seventh half-wave plate) into quantum computing unit 13-1. Incidentally, the half-wave plates 15-1, 15-2, and 15-3 (first to seventh half-wave plates) rotate the state of the incident photons by π / 4 and emit the photons. In the example of FIG. 9, the half-wave plate 15-1 is used as the first, third, fifth, and seventh half-wave plates, the half-wave plate 15-2 is used as the second and sixth half-wave plates, and the half-wave plate 15-3 is used as the fourth half-wave plate. However, this does not limit the present invention. For example, one half-wave plate may be used as the first to seventh half-wave plates, seven half-wave plates may be used as the first to seventh half-wave plates, or any other number of half-wave plates may be used as the first to seventh half-wave plates. That is, the first to seventh half-wave plates may be the same as each other, or may not be the same as each other, or only some of the first to seventh half-wave plates may be the same as each other.

[0103] For example, as illustrated in FIG. 11, a photon is made incident on half-wave plate 15-1 (first half-wave plate), the photon emitted from half-wave plate 15-1 (first half-wave plate) is made incident on quantum computing unit 13-1 (first quantum computing unit), the photon emitted from quantum computing unit 13-1 is made incident on half-wave plate 15-2 (second half-wave plate), and the photon emitted from half-wave plate 15-2 (second half-wave plate) is made incident on quantum computing unit 13-2 (second quantum computing unit). The photon emitted from the quantum computing unit 13-2 is made incident on the half-wave plate 15-1 (third half-wave plate), the photon emitted from the half-wave plate 15-1 (third half-wave plate) is made incident on the quantum computing unit 13-1, the photon emitted from the quantum computing unit 13-1 is made incident on the half-wave plate 15-3 (fourth half-wave plate), and the photon emitted from the half-wave plate 15-3 (fourth half-wave plate) is made incident on the half-wave plate 15-3 (fourth half-wave plate). Distributed quantum computing can also be performed by making the photon emitted from the quantum computing unit 13-M (third quantum computing unit), making the photon emitted from the quantum computing unit 13-M incident on half-wave plate 15-1 (fifth half-wave plate), making the photon emitted from the half-wave plate 15-1 (fifth half-wave plate) incident on the quantum computing unit 13-1, making the photon emitted from the quantum computing unit 13-1 incident on half-wave plate 15-M (sixth half-wave plate), making the photon emitted from the half-wave plate 15-M (sixth half-wave plate) incident on the quantum computing unit 13-2, making the photon emitted from the quantum computing unit 13-2 incident on half-wave plate 15-(M+1) (seventh half-wave plate), measuring the quantum state of the photon emitted from the half-wave plate 15-(M+1) (seventh half-wave plate), and performing a rotation operation on the atoms of the quantum computing unit 13-1 according to the measurement result. Incidentally, the half-wave plates 15-1, 15-2, 15-3, and 15-(M+1) (first to seventh half-wave plates) rotate the state of the incident photons by π / 4 and emit them. In the example of FIG. 11, the half-wave plate 15-1 is used as the first, third, and fifth half-wave plates, the half-wave plate 15-2 is used as the second and sixth half-wave plates, the half-wave plate 15-3 is used as the fourth half-wave plate, and the half-wave plate 15-(M+1) is used as the seventh half-wave plate. However, this does not limit the present invention. For example, one half-wave plate may be used as the first to seventh half-wave plates, seven half-wave plates may be used as the first to seventh half-wave plates, or any other number of half-wave plates may be used as the first to seventh half-wave plates.That is, the first to seventh half-wave plates may or may not be identical to one another, or only parts of the first to seventh half-wave plates may be identical to one another.

[0104] Furthermore, as described in the fourth embodiment, the quantum computing device 4 has N single photon sources 41-1 to 41-N (single photon sources S(1), ..., S(N)), M quantum computing units 13-1 to 13-M (quantum computing units U(1), ..., U(M)), and an optical switch 44 that optically connects the single photon source 41-j and a set of two quantum computing units 13-α1(j) and 13-α2(j) (U(α1(j)) and U(α2(j))). By inputting a photon emitted from the single photon source 41-j to an optical circuit C(j) including a set of two quantum computing units 13-α1(j) and 13-α2(j) for j=1, ..., N, it is also possible to perform distributed quantum computing in parallel by the optical circuits C(1), ..., C(N).

[0105] In each embodiment and its modified example, a fiber Bragg grating having a reflection band including the wavelength of a photon is provided in an optical fiber or a nano-optical fiber, and the fiber Bragg grating functions as a mirror that reflects photons. This makes it possible to provide the mirror only by processing the optical fiber or the nano-optical fiber.

[0106] Furthermore, the single photon sources 11, 41-1 to 41-N in each embodiment include a nano optical fiber 111 optically connected via a tapered portion 122a-0 to an optical fiber that propagates photons, and atoms 112 disposed outside the nano optical fiber 111. Since the nano optical fiber 111 is optically connected to the optical fiber via the tapered portion 122a-0, the generated single photons can be efficiently supplied to the optical fiber.

[0107] As described above, the atom may be replaced with a quantum system such as an artificial atom (e.g., a quantum dot) or a color center in a crystal (e.g., an NV center). That is, the quantum computing unit may have a nano-optical fiber optically connected to an optical fiber propagating incident photons via a tapered portion, and a plurality of quantum systems arranged outside the nano-optical fiber and spaced apart along the longitudinal direction of the nano-optical fiber. However, at least one of the quantum systems functions as a quantum bit that interacts with the state of the photon. It is preferable that all of the plurality of quantum systems interact with the state of the photon. Also, the single photon source may have a nano-optical fiber optically connected to an optical fiber propagating photons via a tapered portion, and a quantum system having a plurality of levels arranged outside the nano-optical fiber. As described above, the quantum system may include, for example, at least one of an atom, an artificial atom, a quantum dot, and a color center in a crystal (e.g., an NV center). [Explanation of symbols]

[0108] 1~4 Quantum computing device 13-m Quantum Computing Unit 11,41-j Single photon source

Claims

1. A quantum computing unit for quantum computing, comprising: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having Among the plurality of quantum systems, a quantum system selected by the optically shifted light functions as a quantum bit that interacts with the state of the photon; A quantum computing unit having a cooperation factor greater than a first threshold.

2. A quantum computing unit for quantum computing, comprising: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having Among the plurality of quantum systems, a quantum system selected by the optically shifted light functions as a quantum bit that interacts with the state of the photon; A quantum computing unit, wherein the strength of an evanescent field at the position of the plurality of quantum systems for the photons propagating within the nano-optical fiber is greater than or equal to a predetermined threshold.

3. 3. The quantum computing unit of claim 1 or 2, a fiber Bragg grating having a reflection band including the wavelength of the photon is provided in the optical fiber or the nano-optical fiber; A quantum computing unit, wherein the fiber Bragg grating acts as a mirror to reflect the photons.

4. A quantum computing unit for quantum computing, comprising: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having Among the plurality of quantum systems, a quantum system selected by the optically shifted light functions as a quantum bit that interacts with the state of the photon; A quantum computing unit, wherein the nano-optical fiber is provided with a fiber Bragg grating having a reflection band that includes the wavelength of the photon.

5. 5. The quantum computing unit of claim 1 , A quantum computing unit, wherein a plurality of said quantum systems act as qubits that interact with the state of said photons.

6. A quantum computing unit for quantum computing, comprising: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having At least one of the quantum systems functions as a quantum bit that interacts with the state of the photon; a plurality of said quantum systems function as quantum bits that interact with the state of said photons; A quantum computing unit in which a quantum system selected from the plurality of quantum systems by the optically shifted light functions as a quantum bit that interacts with the state of the photon.

7. 7. The quantum computing unit of claim 1, A quantum computing unit, wherein the multiple quantum systems are aligned in a line along the longitudinal direction of the nano-optical fiber.

8. 8. The quantum computing unit of claim 1 , The quantum computing unit includes at least one of an atom, an artificial atom, a quantum dot, and an NV center.

9. one or more quantum computing units that are optically connected and perform distributed quantum computing; Each of the quantum computing units comprises: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having Among the plurality of quantum systems, a quantum system selected by the optically shifted light functions as a quantum bit that interacts with the state of the photon; A quantum computing device, the cooperation factor being greater than a first threshold.

10. one or more quantum computing units that are optically connected and perform distributed quantum computing; Each of the quantum computing units comprises: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having Among the plurality of quantum systems, a quantum system selected by the optically shifted light functions as a quantum bit that interacts with the state of the photon; A quantum computing device, wherein the strength of an evanescent field at the position of the plurality of quantum systems for the photons propagating within the nano-optical fiber is equal to or greater than a predetermined threshold.

11. The quantum computing device of claim 9 or 10, A quantum computing device that optically connects a first quantum computing unit and a second quantum computing unit selected from the plurality of quantum computing units, and performs distributed quantum computing by injecting photons into an optical circuit including the first quantum computing unit and the second quantum computing unit.

12. one or more quantum computing units that are optically connected and perform distributed quantum computing; Each of the quantum computing units comprises: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having At least one of the quantum systems functions as a quantum bit that interacts with the state of the photon; the plurality of quantum computing units includes a first quantum computing unit, a second quantum computing unit, and a third quantum computing unit; a photon is incident on a first half-wave plate, a photon emitted from the first half-wave plate is incident on the first quantum computing unit, a photon emitted from the first quantum computing unit is incident on a second half-wave plate, a photon emitted from the second half-wave plate is incident on the second quantum computing unit, a photon emitted from the second quantum computing unit is incident on a third half-wave plate, a photon emitted from the third half-wave plate is incident on the first quantum computing unit, a photon emitted from the first quantum computing unit is incident on a fourth half-wave plate, and a photon emitted from the fourth half-wave plate is incident on the a photon emitted from the third quantum computing unit is incident on a fifth half-wave plate, the photon emitted from the fifth half-wave plate is incident on the first quantum computing unit, the photon emitted from the first quantum computing unit is incident on a sixth half-wave plate, the photon emitted from the sixth half-wave plate is incident on the second quantum computing unit, the photon emitted from the second quantum computing unit is incident on a seventh half-wave plate, and the photon emitted from the seventh half-wave plate is incident on the first quantum computing unit, thereby performing distributed quantum computing.

13. one or more quantum computing units that are optically connected and perform distributed quantum computing; Each of the quantum computing units comprises: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having At least one of the quantum systems functions as a quantum bit that interacts with the state of the photon; the plurality of quantum computing units includes a first quantum computing unit, a second quantum computing unit, and a third quantum computing unit; A photon is incident on a first half-wave plate, a photon emitted from the first half-wave plate is incident on the first quantum computing unit, a photon emitted from the first quantum computing unit is incident on a second half-wave plate, a photon emitted from the second half-wave plate is incident on the second quantum computing unit, a photon emitted from the second quantum computing unit is incident on a third half-wave plate, a photon emitted from the third half-wave plate is incident on the first quantum computing unit, a photon emitted from the first quantum computing unit is incident on a fourth half-wave plate, and a photon emitted from the fourth half-wave plate is incident on the third quantum computing unit. a photon emitted from the third quantum computing unit is incident on a fifth half-wave plate, a photon emitted from the fifth half-wave plate is incident on the first quantum computing unit, a photon emitted from the first quantum computing unit is incident on a sixth half-wave plate, a photon emitted from the sixth half-wave plate is incident on the second quantum computing unit, a photon emitted from the second quantum computing unit is incident on a seventh half-wave plate, a quantum state of the photon emitted from the seventh half-wave plate is measured, and a rotation operation is performed on the quantum system of the first quantum computing unit in accordance with a measurement result, thereby performing distributed quantum computing.

14. 14. The quantum computing device of claim 12 or 13, The first to seventh half-wave plates rotate the state of the incident photon by π / 4 and emit the photon.

15. one or more quantum computing units that are optically connected and perform distributed quantum computing; Each of the quantum computing units comprises: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates incident photons; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having At least one of the quantum systems functions as a quantum bit that interacts with the state of the photon; N single-photon sources S(1), ..., S(N); M quantum computing units U(1), ..., U(M); A single photon source S(j) and two quantum computing units U(α １ (j)) and U(α ２ (j) an optical switch for optically connecting the sets of j=1,...,N, N is an integer equal to or greater than 2, M is an integer equal to or greater than 2N, and α １ (j), α ２ (j) ∈ {1, ..., M}, and α １ (j) ≠ α ２ (j), where for j = 1, ..., N, the photons emitted from the single-photon source S(j) are １ (j)), U(α ２ , C(N), and performing distributed quantum computation in parallel using optical circuits C(1), ..., C(N).

16. 1. A single photon source for quantum computing, comprising: a nano-optical fiber optically connected via a tapered portion to an optical fiber that propagates photons; a quantum system having multiple levels disposed outside the nano-optical fiber; having The quantum system having the multiple levels emits a single photon when excited by the control light. Single photon source.

17. A quantum computing method using a quantum computing unit, comprising: The quantum computing unit includes: a nano-optical fiber optically connected to the optical fiber via a tapered portion; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; The quantum computing method includes: Irradiating at least one of the plurality of quantum systems with optically shifted light; injecting photons into the optical fiber; Including, Among the plurality of quantum systems, a quantum system selected by the optically shifted light functions as a quantum bit that interacts with the state of the photon; A quantum computing method, wherein the cooperation factor is greater than a first threshold.

18. A quantum computing method using a quantum computing unit, comprising: The quantum computing unit includes: a nano-optical fiber optically connected to the optical fiber via a tapered portion; a plurality of quantum systems disposed outside the nano-optical fiber and spaced apart along a length of the nano-optical fiber; having The quantum computing method includes: Irradiating at least one of the plurality of quantum systems with optically shifted light; injecting photons into the optical fiber; Including, Among the plurality of quantum systems, a quantum system selected by the optically shifted light functions as a quantum bit that interacts with the state of the photon; A quantum computing method, wherein the strength of an evanescent field at the position of the plurality of quantum systems for the photons propagating in the nano-optical fiber is greater than or equal to a predetermined threshold.

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