quantum computing unit, quantum computing device

Abstract

To realize a quantum arithmetic unit having both functions of quantum operation, and an imparting function of entanglement with a quantum system to a photon having a wavelength different from that of a photon used for the quantum operation.SOLUTION: A quantum arithmetic unit includes: a nano optical fiber (20); and a quantum system (22) disposed on the nano optical fiber. The quantum system includes a ground level, a first excitation level, a second excitation level, and a third excitation level having energy differing from that of the second excitation level, and a first resonance wavelength (λ1) equivalent to a level difference between the ground level and the first excitation level and a second resonance level (λ2) equivalent to a level difference between the second excitation level and the third excitation level differ mutually. The quantum system also mutually operates with a first photon having a first resonance wavelength, and further generates a second photon that has an entanglement with the quantum system and has a second resonance wavelength in the nano optical fiber.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a quantum processing unit and a quantum processor including the quantum processing unit. [Background technology]

[0002] In recent years, quantum computing devices have been devised that include quantum computing units that contain atoms, ions, diamond NV centers, semiconductor quantum dots, etc. as quantum systems. For example, by connecting multiple quantum computing units of the quantum computing device via a quantum channel, the quantum computing units can be used as quantum repeaters for long-distance quantum communication or as quantum computing units of a distributed quantum computing device.

[0003] Here, quantum operations using a quantum operation unit are performed, for example, by utilizing the retention of quantum information in a quantum system and quantum state manipulation of the quantum system. Here, in order to perform the quantum operations, it is necessary to utilize a level of the quantum system that has a relatively long coherence time. Therefore, quantum operations using a quantum operation unit typically utilize a transition between the ground level and the excited level of the quantum system, and the wavelength of light that resonates with the transition is often less than 1.0 μm.

[0004] On the other hand, optical fibers are often used for quantum channels connecting quantum processing units. In this case, to sufficiently reduce the loss of quantum information in the optical fiber, the wavelength of the photons propagating through the optical fiber must be within the 1.3 μm to 1.6 μm wavelength band, commonly known as the communication wavelength band.

[0005] Therefore, in order to achieve both quantum operations using quantum operation units and low-loss quantum communication between quantum operation units, quantum wavelength conversion is required to convert the wavelength of photons output from the quantum operation units into a communication wavelength band. Non-Patent Documents 1 to 3 disclose quantum wavelength converters that convert the wavelength of photons. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] R. Ikuta et al., "Wide-band quantum interface for visible-to-telecommunication wavelength conversion", Nature Communications 2, 537 (2011) [Non-patent document 2] S. Zaske et al., “Visible-to-Telecom Quantum Frequency Conversion of Light from a Single Quantum Emitter”, Phys. Rev. Lett. 109, 147404 (2012) [Non-patent document 3] K. De Greve et al., “Quantum-dot spin-photon entanglement via frequency downconversion to telecom wavelength”, Nature 491, 421 (2012) Summary of the Invention [Problem to be solved by the invention]

[0007] When the quantum wavelength converters described in Non-Patent Documents 1 to 3 are introduced into a quantum computing device, the structure of the quantum computing device becomes complicated. Furthermore, when quantum wavelength conversion is performed using the quantum wavelength converters described in Non-Patent Documents 1 to 3, noise may be introduced, or photon loss may occur. [Means for solving the problem]

[0008] In order to solve the above problem, a quantum operation unit according to one embodiment of the present disclosure includes a nano-optical fiber connected to an optical fiber that propagates photons via a tapered portion, and a quantum system arranged on the nano-optical fiber, wherein the quantum system has a ground level, a first excitation level, a second excitation level, and a third excitation level having energy different from that of the second excitation level, and a first resonant wavelength corresponding to a level difference between the ground level and the first excitation level and a second resonant wavelength corresponding to a level difference between the second excitation level and the third excitation level are different from each other, and the quantum system interacts with a first photon having the first resonant wavelength and is entangled with the quantum system, and generates a second photon having the second resonant wavelength in the nano-optical fiber. [Effects of the Invention]

[0009] A quantum operation unit having both the functions of quantum operation and the function of imparting entanglement with a quantum system to a second photon having a wavelength different from that of the first photon used in the quantum operation can be realized without using a quantum wavelength converter.By using this quantum operation unit, a quantum operator can be realized that can impart entanglement between photons having different wavelengths without requiring a quantum wavelength converter. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an enlarged schematic diagram of the vicinity of a resonator QED system of a quantum operation unit according to an embodiment of the present disclosure, and an energy diagram showing each level of the quantum system. [Figure 2] FIG. 1 is a schematic diagram of a quantum computing device according to an embodiment of the present disclosure. [Figure 3] FIG. 1 is a close-up schematic diagram of a single photon source according to an embodiment of the present disclosure. [Figure 4] 1 is a schematic diagram illustrating a first operation mode of a quantum computing device according to an embodiment of the present disclosure. [Figure 5] 1 is an energy diagram showing a transition process in a quantum system of a quantum processing unit in a first operation mode of a quantum processing device according to an embodiment of the present disclosure. [Figure 6]FIG. 10 is a schematic diagram illustrating a second operation mode of the quantum computing device according to the embodiment of the present disclosure. [Figure 7] 10 is an energy diagram showing a transition process in a quantum system of a quantum processing unit in a second operation mode of the quantum processing device according to the embodiment of the present disclosure. [Figure 8] 10 is an energy diagram showing another example of a transition process in a quantum system of a quantum processing unit in a second operation mode of the quantum processing device according to the embodiment of the present disclosure. [Figure 9] 10 is an energy diagram showing another example of a transition process in a quantum system of a quantum processing unit in a second operation mode of the quantum processing device according to the embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Embodiment] <Quantum calculator> A quantum processor 2 according to this embodiment will be described with reference to Fig. 2. Fig. 2 is a schematic diagram showing the quantum processor 2 according to this embodiment. As shown in Fig. 2, the quantum processor 2 includes a plurality of quantum processing units 4, a single-photon source 6, a polarizing beam splitter 8, a first single-photon detector 10, at least one half beam splitter 12, and a second single-photon detector 14. The quantum processor 2 also includes a single-mode optical fiber F that propagates single photons between the above-mentioned components.

[0012] The quantum operation unit 4 according to this embodiment includes a resonator QED (quantum electrodynamics) system 16 and a laser light source 18. The resonator QED system 16 includes a quantum system and is a unit for realizing quantum operations using the quantum system. The laser light source 18 is a light source for irradiating the quantum system of the resonator QED system 16 with laser light for quantum operations using the resonator QED system 16. Details of the laser light source 18 will be described later.

[0013] <Resonator QED system> The cavity QED system 16 will be described in more detail with reference to Fig. 1. Fig. 1 shows an enlarged schematic diagram of the cavity QED system 16 and its vicinity, and an energy diagram D1 showing the levels of the quantum system included in the cavity QED system 16.

[0014] 1, the cavity QED system 16 includes a nano-optical fiber 20 and at least one quantum system 22. The nano-optical fiber 20 is connected to a single-mode optical fiber F via tapered portions 24 located at both ends. Therefore, photons propagating through the single-mode optical fiber F propagate through the nano-optical fiber 20 via the tapered portions 24.

[0015] 1, the optical fiber F may include a core portion FA and a cladding portion FB surrounding the core portion FA, and photons may be propagated in the core portion FA. Alternatively, the nano-optical fiber 20 may be formed in a heated portion of the optical fiber F by heating the portion using various heating methods including a ceramic heater or an oxygen-water flame, and then pulling the heated portion from both ends. In this case, the nano-optical fiber 20 may also include a core portion that propagates photons and a cladding portion surrounding the core portion.

[0016] The quantum system 22 is disposed on the nano-optical fiber 20. The quantum system 22 has, for example, a plurality of levels. For example, as shown in the energy diagram D1 in FIG. 1, the quantum system 22 has a first ground level g and a second ground level u as ground levels, and a first excitation level e1, a second excitation level e2, and a third excitation level e3 as excitation levels. The first excitation level e1, the second excitation level e2, and the third excitation level e3 have higher energies than the first ground level g and the second ground level u.

[0017] Here, at least the second excitation level e2 and the third excitation level e3 have different energies. In particular, as shown in the energy diagram D1 of FIG. 1, the second excitation level e2 may have higher energy than the first excitation level e1 and the third excitation level e3. However, the first excitation level e1 may be degenerate with either the second excitation level e2 or the third excitation level e3, or may be the same level as either the second excitation level e2 or the third excitation level e3. Furthermore, each level included in the quantum system 22 may have a magnetic sublevel.

[0018] In this specification, the states of quantum system 22 corresponding to the first ground level g and the second ground level u are referred to as state |g> and state |u>, respectively. Also, the states of quantum system 22 corresponding to the first excitation level e1, the second excitation level e2, and the third excitation level e3 are referred to as state |e1>, state |e2>, and state |e3>, respectively.

[0019] Generally, the states |g> and |u> are more stable and have a longer coherence time than the states |e1>, |e2>, and |e3>. Therefore, the first basis level g and the second basis level u can function as a quantum memory that retains the quantum state for a relatively long time.

[0020] As shown in the energy diagram D1 in Fig. 1, the wavelength resonating with the transition from state |g> to state |e1> is defined as the first resonant wavelength λ1, and the wavelength resonating with the transition between state |e2> and state |e3> is defined as the second resonant wavelength λ2. In this specification, the wavelength resonating with the transition between certain levels refers to the wavelength of a photon having energy corresponding to the energy difference between the levels, in other words, the wavelength corresponding to the level difference between the levels. In other words, the wavelength corresponding to the energy difference between the first ground level g and the first excited level e1 is defined as the first resonant wavelength λ1, and the wavelength corresponding to the energy difference between the second excited level e2 and the third excited level e3 is defined as the second resonant wavelength λ2.

[0021] Here, the energy difference between the first ground level g and the first excitation level e1 is different from the energy difference between the second excitation level e2 and the third excitation level e3. Therefore, the first resonance wavelength λ1 and the second resonance wavelength λ2 are different from each other. In particular, the energy difference between the first ground level g and the first excitation level e1 may be larger than the energy difference between the second excitation level e2 and the third excitation level e3. In this case, the second resonance wavelength λ2 is longer than the first resonance wavelength λ1. For example, the first resonance wavelength λ1 is less than 1.0 μm, and the second resonance wavelength λ2 is 1.3 μm or more and 1.6 μm or less.

[0022] In this embodiment, the wavelength that resonates with the transition from state |e1> to state |e2> is defined as a third resonant wavelength λ3, the wavelength that resonates with the transition from state |g> to state |e3> is defined as a fourth resonant wavelength λ4, and the wavelength that resonates with the transition from state |g> to state |e2> is defined as a fifth resonant wavelength λ5.

[0023] In this embodiment, the quantum system 22 may be, for example, a cesium atom. In this case, in this embodiment, the state |g> has 6 2 S 1 / 2 ,F=4 states, state |u> has 6 2 S 1 / 2 , F=3 states can be assigned. In this embodiment, the state |e1> can be assigned to 6 states. 2 P 3 / 2 ,F=3 states, state |e2> has 7 2 S 1 / 2 ,F=4 states, state |e3> has 6 2 P 1 / 2 ,F=3 states can be filled.

[0024] 1, the first resonant wavelength λ1 corresponding to the energy difference between the first ground level g and the first excited level e1 is 852 nm, the second resonant wavelength λ2 corresponding to the energy difference between the second excited level e2 and the third excited level e3 is 1470 nm, and the fourth resonant wavelength λ4 corresponding to the energy difference between the first ground level g and the third excited level e3 is 894 nm.

[0025] The quantum system 22 interacts with a first photon having a first resonant wavelength λ1 propagating through the nano-optical fiber 20. More specifically, a state transition occurs in the quantum system 22 due to the interaction with the first photon propagating through the nano-optical fiber 20. The interaction between the quantum system 22 and the first photon propagating through the nano-optical fiber 20 may occur due to an interaction between the quantum system 22 and an evanescent wave seeping out from the nano-optical fiber 20 as the first photon propagates through the nano-optical fiber 20.

[0026] Here, in this specification, "the quantum system 22 is disposed in the nano-optical fiber 20" does not only mean that the quantum system 22 is in contact with the nano-optical fiber 20. For example, in this embodiment, the quantum system 22 may be disposed at a distance from the nano-optical fiber 20 to such an extent that interaction between the quantum system 22 and the first photon propagating through the nano-optical fiber 20 is possible.

[0027] Furthermore, the quantum system 22 generates a second photon having a second resonant wavelength λ2 in the nano-fiber 20. In particular, the second photon generated from the quantum system 22 has a correlation with the state of the quantum system 22, in other words, has entanglement with the quantum system 22. A detailed method for generating the second photon from the quantum system 22 will be described later.

[0028] In this embodiment, the quantum system 22 is an example of a cesium atom having the above-mentioned levels, but is not limited thereto. For example, the quantum system 22 may include various quantum systems used in quantum computation, as long as it has a ground level, a first excitation level, a second excitation level, and a third excitation level having an energy different from that of the second excitation level. For example, the quantum system 22 may include, in addition to atoms, ions, diamond particles having nitrogen defects, quantum dots, etc.

[0029] <Resonator> Returning to reference to cavity QED system 16 of FIG. 1 , cavity QED system 16 further includes a first resonator 26 and a second resonator 28 located within nanooptical fiber 20 or within optical fiber F. First resonator 26 and second resonator 28 are configured such that at least a portion of their respective resonant optical paths include nanooptical fiber 20. First resonator 26 has a first resonant wavelength λ1, and second resonator 28 has a second resonant wavelength λ2. Thus, quantum system 22 interacts with a first photon propagating through first resonator 26, and quantum system 22 generates a second photon in second resonator 28.

[0030] Each of the first resonator 26 and the second resonator 28 includes, for example, a pair of a first fiber Bragg grating 30 and a second fiber Bragg grating 32 formed in each of two optical fibers F connected to the nano-optical fiber 20. The first fiber Bragg grating 30 and the second fiber Bragg grating 32 include a first resonance wavelength λ1 and a second resonance wavelength λ2 in their reflection bands, respectively. The pair of first fiber Bragg grating 30 and second fiber Bragg grating 32 are arranged so that the optical path lengths of the first and second propagating photons, respectively, over one round trip are an integer multiple of the wavelength of each photon.

[0031] At least one of the first fiber Bragg grating 30 and the second fiber Bragg grating 32 may be formed in the nano-optical fiber 20 instead of the optical fiber F. The first fiber Bragg grating 30 or the second fiber Bragg grating 32 formed in the nano-optical fiber 20 may have a photonic crystal formed by a method such as forming periodic defects in a part of the nano-optical fiber 20.

[0032] <Quantum Bit> In this embodiment, the first resonator 26 has a first resonant wavelength λ1 that resonates with the transition from state |g> to state |e1> as a resonant wavelength, and therefore is coupled to the transition from state |g> to state |e1>. On the other hand, the first resonator 26 is not coupled to the transition from state |u> to state |e1>. In addition, in this embodiment, the second resonator 28 has a second resonant wavelength λ2 that resonates with the transition from state |e2> to state |e3> as a resonant wavelength, and therefore is coupled to the transition from state |e2> to state |e3>.

[0033] However, for example, a two-photon stimulated Raman process can be generated in the quantum system 22 by irradiating the quantum system 22 with control light that resonates with the transitions from the state |g> and the state |u> to the state |e1>. Alternatively, the quantum system 22 may be irradiated with an electromagnetic wave that resonates with the transition from the state |g> to the state |u>. In this way, the transition from the state |u> to the state |e1> can also be controlled by irradiating the quantum system 22 with control light from the laser light source 18 with a certain amplitude. Here, the control light irradiated from the laser light source 18 to the quantum system 22 may include laser light having a wavelength corresponding to the transition from the state |g> to the state |e1> and laser light having a wavelength corresponding to the transition from the state |u> to the state |e1>.

[0034] Therefore, by controlling the interaction between the quantum system 22 and the first photon and the irradiation of the control light from the laser light source 18 onto the quantum system 22, it is possible to control the state of the entire system of the quantum system 22. Therefore, the quantum system 22 can be in an arbitrary superposition state ρ g |g>+ρ u |u>. Note that ρ g and ρ u is |ρ g 2 |+|ρ u 2 are complex coefficients that satisfy |=1.

[0035] As a result, in the cavity QED system 16, the quantum system 22 has a qubit |Ψ>=ρg |g>+ρ u In this case, a one-qubit gate using quantum system 22 is realized by irradiating quantum system 22 with control light that resonates with the transitions from state |g> and state |u> to state |e1>.

[0036] <Single photon source> Next, the single-photon source 6 will be described in more detail with reference to FIG. 3 . FIG. 3 is an enlarged schematic diagram showing the vicinity of the single-photon source 6. Each of the single-photon sources 6 according to this embodiment is a single-photon source that generates a single first photon having a first resonant wavelength λ1 and inputs the photon to each quantum operation unit 4. For example, as shown in FIG. 3 , the single-photon source 6 may have the same configuration as the cavity QED system 16 except that it has a single quantum system 22 and does not have a second cavity 28. In other words, the single-photon source 6 includes a nano-optical fiber 20 connected at both ends to an optical fiber F via tapered portions 24, a quantum system 22 disposed on the nano-optical fiber 20, and a first cavity 26 that resonates with the first photon.

[0037] The single-photon source 6 may generate a single first photon in the first resonator 26, for example, based on the Purcell effect, in which spontaneous emission of a single first photon accompanying a transition from state |g> to state |e1> is emphasized by coupling between the quantum system 22 and the first resonator 26. In other words, the single-photon source 6 may generate a single first photon by utilizing the phenomenon in which a state of the quantum system 22 is excited by control light from a laser light source (not shown), and a single photon is generated when the state returns to the ground state.

[0038] Alternatively, the single-photon source 6 may generate a single first photon in the first resonator 26 by irradiating the first resonator 26 with control light whose amplitude gradually increases from 0 using the quantum system 22. In this case, the waveform of the first photon can be controlled by controlling the change in the amplitude of the control light over time. Alternatively, the single-photon source 6 may be a conventionally known single-photon source that generates a single first photon, such as a heralded single-photon source.

[0039] In the present embodiment, an example has been described in which the single-photon source 6 has a single quantum system 22, but this is not limiting. For example, the single-photon source 6 may have a plurality of quantum systems 22, just like the quantum systems 22 included in the resonator QED system 16. In this case, for example, none of the plurality of quantum systems 22 included in the single-photon source 6 may be coupled to the first resonator 26, and the quantum system 22 may be coupled to the first resonator 26 by irradiating one of the quantum systems 22 with an optical shift beam or the like. With the above configuration, the single-photon source 6 can generate a first photon only by the quantum system 22 coupled to the first resonator 26.

[0040] <Single photon detector> Returning to FIG. 2 , the polarizing beam splitter 8 is a beam splitter that switches between reflecting and transmitting an incident photon depending on the polarization of the photon. In this embodiment, the polarizing beam splitter 8 reflects photons having V polarization and transmits photons having H polarization. For example, photons generated in each resonator QED system 16 by a method described later are propagated from each resonator QED system 16 to each polarizing beam splitter 8. Note that, as shown in FIG. 2 , the quantum processor 2 may be provided with a half-wave plate 34, and photons incident on each polarizing beam splitter 8 may be transmitted through the half-wave plate 34.

[0041] The first single-photon detector 10 includes a first photon detecting element 36 that detects photons having V polarization that are reflected by the polarizing beam splitter 8, and a second photon detecting element 38 that detects photons having H polarization that are transmitted through the polarizing beam splitter 8. Thus, the first single-photon detector 10 detects single photons that are reflected by or transmitted through the polarizing beam splitter 8.

[0042] Half beam splitter 12 reflects incident photons with a probability of about 50% and transmits them with a probability of about 50%. Single photons output from two resonator QED systems 16 are incident on half beam splitter 12 by a method described below. Half beam splitter 12 is positioned so that two simultaneously incident single photons interfere with each other if they have the same wavelength.

[0043] The second single-photon detector 14 includes a third photon detecting element 40 and a fourth photon detecting element 42. The third photon detecting element 40 detects a single photon generated in one resonator-QED system 16 and transmitted through the half beam splitter 12, and a single photon generated in the other resonator-QED system 16 and reflected by the half beam splitter 12. On the other hand, the fourth photon detecting element 42 detects a single photon generated in one resonator-QED system 16 and reflected by the half beam splitter 12, and a single photon generated in the other resonator-QED system 16 and transmitted through the half beam splitter 12. Therefore, the half beam splitter 12 causes interference between the mode (wave packet) of the second photon generated in one resonator-QED system 16 and the mode (wave packet) of the second photon generated in the other resonator-QED system 16.

[0044] The optical fiber F propagates photons between the above-mentioned components of the quantum processor 2. The quantum processor 2 further includes an optical circulator 44. The optical circulator 44 propagates photons incident from each single-photon source 6 to each resonator-QED system 16, and propagates photons incident from each resonator-QED system 16 to the optical fiber F on the side of each polarization beam splitter 8. The optical circulator 44 may include, for example, a half-wave plate 46 in the optical path from each single-photon source 6 to each resonator-QED system 16, and photons incident from each single-photon source 6 to the nano-optical fiber 20 of each resonator-QED system 16 may pass through the half-wave plate 46. The quantum processor 2 further includes an optical switch 48. The optical switch 48 switches between propagating photons generated from each resonator-QED system 16 to either the polarization beam splitter 8 or the half beam splitter 12.

[0045] The quantum computing device 2 may have, instead of the optical switch 48, an element that, depending on the wavelength of an incident photon, determines whether the photon is to be propagated to one of the polarization beam splitters 8 or the half beam splitter 12. For example, the quantum computing device 2 may have, instead of the optical switch 48, a WDM filter or a dichroic mirror.

[0046] 2 , the quantum processor 2 may include more quantum operation units 4, single-photon sources 6, polarizing beam splitters 8, first single-photon detectors 10, half beam splitters 12, and second single-photon detectors 14. In this case, the quantum processor 2 may include an optical switch 50 that switches to which half beam splitter 12 a photon generated from each resonator QED system 16 is propagated. Furthermore, the quantum processor 2 may include multiple quantum operation units 4 for each single-photon source 6.

[0047] The quantum computing device 2 may include a control unit (not shown) for operating each of the included units. The control unit may control each of the units of the quantum computing device 2 based on a program recorded in a memory (not shown) or the like.

[0048] <First Operational Form> A first operation mode of the quantum processor 2 according to this embodiment will be described with reference to Fig. 4 and Fig. 5. Fig. 4 is a schematic diagram of the quantum processor 2 for explaining the operation of each part of the quantum processor 2 in the first operation mode of the quantum processor 2 according to this embodiment. Fig. 5 is a diagram showing an energy diagram D2 representing the state transition of the quantum system 22 in the first operation mode of the quantum processor 2 according to this embodiment.

[0049] In the first operation mode of the quantum processor 2 according to this embodiment, first, a quantum gate operation between quantum systems 22 in a single quantum operation unit 4 will be described. In the first operation mode of the quantum processor 2 according to this embodiment, first, the quantum processor 2 generates a single first photon SF1 having a first resonant wavelength λ1 from each single-photon source 6 by the above-mentioned method or the like. Here, the first photon SF1 has a specific state, for example, transverse polarization |h>.

[0050] Next, the quantum processor 2 transmits the first photon SF1 through the optical circulator 44 and makes it incident on the resonator QED system 16 of the quantum operation unit 4. As a result, the first photon SF1 incident on the optical circulator 44 from the single-photon source 6 propagates to the resonator QED system 16. Note that in quantum gate operations between quantum systems 22 within a single quantum operation unit 4, the polarization of the first photon SF1 may be prevented from changing by aligning the optical axis of the half-wave plate 46 parallel to the horizontal polarization |h>. The first photon SF1 incident on the resonator QED system 16 resonates in the first resonator 26, which has a first resonant wavelength λ1 as its resonant wavelength. Here, the quantum operation unit 4 controls the irradiation of control light L1 from the laser light source 18 to a specific quantum system 22.

[0051] When the control light L1 is not irradiated from the laser light source 18 to a specific quantum system 22, the transition of the quantum system 22 from the state |g> to the state |e1> is coupled to the first resonator 26. On the other hand, in the above case, the transition of the quantum system 22 from the state |u> to the state |e1> is not coupled to the first resonator 26. Therefore, when the quantum system 22 is in the state |g> and when it is in the state |u>, the phase shift that the resonator QED system 16 imparts to the first photon SF1 when the first photon SF1 is reflected in the resonator QED system 16 differs.

[0052] The quantum operation unit 4 changes the wavelength that resonates with the transition of a specific quantum system 22 from state |g> to state |e1> by irradiating the specific quantum system 22 with control light L1 from the laser light source 18. This allows the quantum operation unit 4 to effectively decouple the transition from the first resonator 26. In this case, the phase shift that the resonator QED system 16 imparts to the first photon SF1 is approximately the same when the quantum system 22 is in state |g> and when it is in state |u>. As described above, by irradiating the specific quantum system 22 with control light L1, the quantum operation unit 4 can control whether or not there is a difference in the phase shift that the resonator QED system 16 imparts to the first photon SF1 when the quantum system 22 is in state |g> and when it is in state |u>.

[0053] In the present embodiment, an example has been described in which the coupling between the transition of the quantum system 22 from the state |g> to the state |e1> and the first resonator 26 is effectively released by irradiating the quantum system 22 with the control light L1, but the present invention is not limited to this. For example, in the present embodiment, in a state in which the quantum system 22 is not irradiated with the control light L1, the transition of the quantum system 22 from the state |g> to the state |e1> may not be coupled to the first resonator 26. In this case, the transition of the quantum system 22 from the state |g> to the state |e1> may be coupled to the first resonator 26 by irradiating the quantum system 22 with the control light L1.

[0054] Following the incidence of the first photon SF1 into the resonator QED system 16, a first photon SF1', which is the first photon SF1 reflected by the resonator QED system 16, is emitted from the resonator QED system 16. Here, as described above, whether or not there is a difference in the phase shift that the resonator QED system 16 imparts to the first photon SF1 when the first photon SF1 is reflected by the resonator QED system 16, depends on whether or not there is irradiation of the control light L1, depending on the state of the specific quantum system 22. For this reason, by irradiating the specific quantum system 22 with the control light L1, the quantum operation unit 4 can control whether or not there is a difference in the phase shift between the first photon SF1 and the first photon SF1' when the quantum system 22 is in the state |g> and the state |u>.

[0055] The quantum operation unit 4 may control the irradiation of the control light L1 onto the specific quantum system 22 to cause, for example, a transition T1', which is a state transition from the state |e1> to the state |g> in the specific quantum system 22. This state transition causes the superposition state ρ g |g>+ρ u A first photon SF1′ entangled with |u> may be generated in the first resonator 26. Note that, since the transition from the state |e1> to the state |g> is coupled to the first resonator 26, the generation of the first photon SF1 by the transition T1′ is promoted by the first resonator 26.

[0056] 4, in the first operation mode of the quantum processor 2 according to this embodiment, following emission of the first photon SF1′ from the resonator QED system 16, the quantum processor 2 propagates the first photon SF1′ to the polarization beam splitter 8. For example, the quantum processor 2 controls the optical switch 48 so that the first photon SF1′ that has passed through the optical circulator 44 and the optical switch 48 propagates to the polarization beam splitter 8. Here, in quantum gate operations between quantum systems 22 within a single quantum operation unit 4, the polarization of the first photon SF1′ may not change even at the half-wave plate 34.

[0057] As a result, the quantum processor 2 causes the first photon SF1' from the quantum processing unit 4 to be incident on the polarizing beam splitter 8. However, the polarization of the first photon SF1' does not change from horizontal polarization |h>. Therefore, by taking the sum of the detection results of the two photon detecting elements of the first single-photon detector 10, it can be considered that the first photon SF1' has effectively been measured by a single photon detector.

[0058] Here, for example, if the states of the two quantum systems 22 included in the resonator QED system 16 are both |u>, the optical response in the quantum systems 22 is the same as when the quantum systems 22 do not exist, and therefore the first photon SF1 is reflected with almost no loss and its phase is shifted by π. On the other hand, if the state of either of the two quantum systems 22 included in the resonator QED system 16 is not |u>, the optical response in the quantum systems 22 corresponds to a transition between |g> and |e>, and therefore the incident first photon SF1 is reflected without a phase shift.

[0059] Therefore, the states of the two quantum systems 22 that have interacted with the first photon SF1 correspond to the state before the interaction with the first photon SF1 after a CPF gate has been applied. Furthermore, if the number of quantum systems that interact with the first photon SF1 is N, an integer greater than or equal to 3, the states of the N quantum systems 22 that have interacted with the first photon SF1 correspond to the state before the interaction with the first photon SF1 after an N-bit controlled phase gate has been applied. As described above, in the first operation method, a gate between multiple quantum systems 22 is realized.

[0060] As described above, the quantum operation unit 4 can perform a quantum gate operation using the interaction between the first photon SF1 and any quantum system 22. Therefore, in the first operation mode of the quantum operation device 2, the quantum operation device 2 can perform a quantum operation in each quantum operation unit 4 using the interaction between the first photon SF1 and any quantum system 22.

[0061] As described above, when the quantum gate operation between quantum systems 22 in a single quantum operation unit 4 is performed by the quantum operator 2, the quantum operator 2 is not limited to the configuration described above. For example, when performing the quantum gate operation, the quantum operator 2 does not need to include the polarizing beam splitter 8, the half-wave plate 34, and the half-wave plate 46, and the first single-photon detector 10 may have only a single photon detection element.

[0062] Next, a description will be given of quantum gate operations between quantum systems 22 between multiple quantum operation units 4 in a first operation mode of the quantum operator 2 according to this embodiment. In this case, the quantum operator 2 includes multiple quantum operation units 4 for each single-photon source 6.

[0063] In quantum gate operations between quantum systems 22 among multiple quantum operation units 4, first, a first photon SF1 having a first resonant wavelength λ1 and a specific state such as transverse polarization |h> is generated from each single-photon source 6 by the same method as described above. Next, the quantum operator 2 causes the first photon SF1 to be incident on the resonator QED system 16 of any of the quantum operation units 4.

[0064] Here, the quantum processor 2 may include, instead of the optical circulator 44, an optical switch for each quantum processor unit 4 that determines into which resonator QED system 16 of which quantum processor unit 4 the first photon SF1 is to be incident. Also, the quantum processor 2 may change the polarization state of the first photon SF1 from each single-photon source 6 by a half-wave plate 46.

[0065] As a result, the first photon SF1 incident on the optical circulator 44 from the single-photon source 6 propagates to the resonator-QED system 16. As a result, the above-mentioned first photon SF1 is reflected in the resonator-QED system 16, and a first photon SF1′ is emitted from the resonator-QED system 16. Here, the quantum processor 2 propagates the first photon SF1′ emitted from the resonator-QED system 16 so that it is incident on another different resonator-QED system 16. As a result, the quantum processor 2 causes the first photon SF1 to be incident on a plurality of resonator-QED systems 16, and also reflects the first photon SF1′ from each resonator-QED system 16. Note that the phase shift of the first photon SF1′ in each resonator-QED system 16 is controlled by controlling the state of the quantum system 22 of each resonator-QED system 16, as described above.

[0066] Next, the quantum processor 2 propagates the first photon SF1′ to the polarizing beam splitter 8. The quantum processor 2 may change the polarization state of the first photon SF1′ from each resonator QED system 16 by using a half-wave plate .

[0067] Polarizing beam splitter 8 reflects or transmits the first photon SF1' depending on the polarization state of the incident first photon SF1'. Thereafter, first single-photon detector 10 detects the first photon SF1' emitted from polarizing beam splitter 8, and quantum processor 2 measures the polarization state of the first photon SF1'.

[0068] Here, the first photon SF1′ is entangled with each quantum system 22 of the multiple resonator QED systems 16 from which it is reflected. Therefore, there is a correlation between the state of each quantum system 22 of the multiple resonator QED systems 16 and the polarization state of the first photon SF1′.

[0069] Next, the quantum operation unit 4 performs an operation on the quantum system 22 of the other quantum operation unit 4 according to the polarization state of the measured first photon SF1'. When the quantum operator 2 has two resonator QED systems 16, for example, when the polarization state of the first photon SF1' is transversely polarized |h>, which is the state of the first photon SF1 when the first photon SF1 was generated, a Z-gate operation is performed on the quantum system 22 of the other quantum operation unit 4. The Z-gate operation on a specific quantum system 22 is realized, for example, by the quantum operation unit 4 controlling the irradiation of the control light L1 onto the quantum system 22. On the other hand, when the polarization state of the first photon SF1' is not transversely polarized |h>, such as transversely polarized |V>, no operation is performed on the specific quantum system 22. In other words, a rotation operation is performed on the particular quantum system 22 according to the polarization rotation of the photon by the half-wave plate 46 and the half-wave plate 34 and the measurement result of the polarization of the first photon SF1′ at the first single-photon detector 10.

[0070] By the above operation, the quantum processor 2 can separate the first photon SF1' from the entanglement between the first photon SF1' and the quantum systems 22 of each resonator QED system 16. In other words, by the above operation, the quantum processor 2 can leave only the entanglement between the quantum systems 22 of the multiple resonator QED systems 16. As a result, the quantum processor 2 can project the states of the quantum systems 22 of the multiple resonator QED systems 16 into a specific entangled state. Here, an example has been described in which the quantum processor 2 has two resonator QED systems 16, but the operation of separating the first photon SF1' is also possible when the quantum processor 2 has three or more resonator QED systems 16.

[0071] As described above, the quantum processor 2 can perform quantum gate operations using the interaction between the first photon SF1 and any quantum system 22 in each of the multiple quantum operation units 4. Therefore, in the first operation mode of the quantum processor 2, the quantum processor 2 can perform quantum operations using the interaction between the first photon SF1 and any quantum system 22 between the multiple quantum operation units 4.

[0072] The quantum gate operation in the quantum operation unit 4 described above is an example, and in this embodiment, the quantum operator 2 may cause the quantum operation unit 4 to execute quantum operations using various conventionally known gate operations using the quantum system 22. For example, in a first operation mode of the quantum operator 2, each quantum operation unit 4 may irradiate a plurality of quantum systems 22 with control light L1 and perform quantum operations using the plurality of quantum systems 22. Furthermore, the quantum operator 2 may cause the first photon SF1 to be incident on a plurality of resonator QED systems 16, detect the reflected first photon SF1′, and then cause the first photon SF1′ to be incident on only a specific resonator QED system 16 and detect the reflected first photon SF1′.

[0073] <Second operational form> A second operation mode of the quantum processor 2 according to this embodiment will be described with reference to Fig. 6 and Fig. 7. Fig. 6 is a schematic diagram of the quantum processor 2 for explaining the operation of each part of the quantum processor 2 in the second operation mode of the quantum processor 2 according to this embodiment. Fig. 7 is a diagram showing an energy diagram D3 representing the state transition of the quantum system 22 in the second operation mode of the quantum processor 2 according to this embodiment.

[0074] In a second operating mode of the quantum computing device 2 according to this embodiment, first, in each quantum computing unit 4, pump light L2 is irradiated from the laser light source 18 onto an arbitrary quantum system 22. Here, the pump light L2 includes, for example, a first laser light having a first resonance wavelength λ1, a third laser light having a third resonance wavelength λ3, and a fourth laser light having a fourth resonance wavelength λ4.

[0075] Here, the frequencies of the first laser light, the third laser light, and the fourth laser light are defined as a first resonant frequency ω1, a third resonant frequency ω3, and a fourth resonant frequency ω4, respectively. The frequency of photons having the second resonant wavelength λ2 is defined as a second resonant frequency ω2. Here, the first laser light having the first resonant wavelength λ1 causes a transition T1 from state |g> to state |e1>, as shown in energy diagram D3 in FIG. 7 . The third laser light having the third resonant wavelength λ3 causes a transition T2 from state |e1> to state |e2>, and the fourth laser light having the fourth resonant wavelength λ4 causes a transition T3 from state |e3> to state |g>.

[0076] As a result, when the quantum system 22 is irradiated with the pump light L2, a non-degenerate four-wave mixing process occurs in the quantum system 22. For example, in this embodiment, a single second photon SF2 having a second resonant wavelength λ2 that resonates with the transition from the state |e2> to the state |e3> is generated from the quantum system 22 via transitions T1, T2, and T3. The quantum system 22 generates the second photon SF2 in the second resonator 28. Because the transition from the state |e2> to the state |e3> is coupled to the second resonator 28, the generation of the second photon SF2 by the non-degenerate four-wave mixing process is promoted by the second resonator 28.

[0077] In the present embodiment, the case where the non-degenerate four-wave mixing process in the quantum system 22 is used to generate the second photon SF2, in other words, the case where the first excitation level e1 and the third excitation level e3 have different energies, has been described. However, this is not limiting. For example, the first excitation level e1 and the third excitation level e3 may have the same energy.

[0078] Here, when the second photon SF2 is first generated from the quantum system 22, the quantum system 22 is in an arbitrary superposition state ρ g |g>+ρ u In this case, in the second operation mode of the quantum processor 2 according to this embodiment, the quantum operation unit 4 performs an inversion operation on the quantum system 22 by, for example, the two-photon stimulated Raman process or the like described above, following the generation of the second photon SF2. As a result, the state of the quantum system 22 is changed to ρ g |g>+ρ u |u> to ρ u |g>+ρ g After the inversion operation on the quantum system 22, the quantum operation unit 4 controls the irradiation of the quantum system 22 with the pump light L2 again, thereby causing a non-degenerate four-wave mixing process to occur again in the quantum system 22.

[0079] The second photon SF2 generated from the quantum system 22 by the non-degenerate four-wave mixing process has entanglement with the quantum system 22. Therefore, the second photon SF2 generated before and after the above-mentioned inversion operation is in an arbitrary superposition state ρ g |u>|pE>+ρ u Here, states |pE> and |pL> are the states of the second photon SF2 generated before and after the inversion operation, respectively. Therefore, the second photon SF2 is entangled with quantum system 22, and forms a time bin qubit based on states |pE> and |pL>.

[0080] 6 , in the second operation mode of the quantum processor 2 according to this embodiment, after generating the second photon SF2 in the second resonator 28, the quantum processor 2 propagates the second photon SF2 to the half beam splitter 12. For example, the quantum processor 2 emits the second photon SF2 in the second resonator 28 from the resonator QED system 16 of the quantum operation unit 4 to the outside of the resonator QED system 16. Here, the quantum processor 2 controls the optical switch 48 so that the second photon SF2 that has passed through the optical circulator 44 and the optical switch 48 propagates to the half beam splitter 12.

[0081] As a result, the quantum processor 2 causes the second photon SF2 from the quantum operation unit 4 to be incident on the half beam splitter 12. Here, the quantum processor 2 causes the second photons SF2 from each of the two quantum operation units 4 to be incident on the same half beam splitter 12. In particular, the quantum processor 2 causes the two quantum operation units 4 to emit the second photons SF2 so that the multiple second photons SF2 are incident on the same half beam splitter 12 at approximately the same time.

[0082] Therefore, in this embodiment, in response to two second photons SF2 that are incident on the same half beam splitter 12 at approximately the same time, interference occurs in the half beam splitter 12 between the modes (wave packets) of the two second photons SF2.

[0083] Therefore, by the above operation, the quantum processor 2 imparts entanglement to the specific quantum systems 22 included in each of the resonator QED systems 16 of the two mutually different quantum operation units 4. In other words, by the above operation, the quantum processor 2 can transfer the state of the quantum system 22 included in the specific quantum operation unit 4 to the state of the quantum system 22 included in the other quantum operation unit 4. Therefore, by combining the first operation form and the second operation form, the quantum processor 2 can use the result of a quantum operation in a specific quantum operation unit 4 for the quantum operation of the other quantum operation unit 4. Note that the quantum processor 2 is not limited to the above, and may use the entanglement generated in the quantum operation unit 4 as it is for subsequent quantum computation without transferring the state of the quantum system 22 included in the specific quantum operation unit 4 to the state of the quantum system 22 included in the other quantum operation unit 4.

[0084] <Summary> The quantum operation unit 4 according to this embodiment executes quantum operation using the quantum system 22 through an interaction between the first photon SF1 incident on the resonator QED system 16 and the quantum system 22 included in the resonator QED system 16. Furthermore, the quantum operation unit 4 according to this embodiment generates a second photon SF2 entangled with the quantum system 22 through irradiation of the quantum system 22 with pump light L2.

[0085] Here, the first photon SF1 has a first resonant wavelength λ1, and the second photon SF2 has a second resonant wavelength λ2 that is different from the first resonant wavelength λ1. Therefore, quantum operation unit 4 achieves both functions of performing quantum operation using quantum system 22 and imparting entanglement between quantum system 22 and second photon SF2, which has a wavelength different from that of the first photon SF1 used in the quantum operation, without using a quantum wavelength converter.

[0086] Generally, quantum operations using a quantum system require that quantum information be retained in the quantum system for a certain period of time, so a transition between the ground state, which has a relatively long coherence time, and the excited state just above it is used. For example, quantum operations using cesium atoms, as mentioned above, are performed in the ground state 6 2 S1 / 2 ,F=3 state and its immediately higher excited state 6 2 P 3 / 2 ,F=3 states are used. In this case, the wavelength corresponding to the energy difference between the ground state of the quantum system and its immediately higher excited state is generally less than 1.0 μm. Therefore, by making the wavelength of the photon less than 1.0 μm, quantum operations can be efficiently performed using the interaction between the quantum system and the photon.

[0087] Furthermore, the loss of photons propagating through a single-mode optical fiber generally increases with the propagation distance. Here, the loss of photons is large when the wavelength of the photons is less than 1.0 μm, but is relatively small when the wavelength of the photons is 1.0 μm or more and 1.8 μm or less, which includes the wavelength band generally referred to as the communication wavelength band. More preferably, the loss of photons propagating through an optical fiber is smaller when the wavelength of the photons is 1.3 μm or more and 1.6 μm or less. In other words, when the wavelength of the photons is 1.0 μm or more and 1.8 μm or less, more preferably 1.3 μm or more and 1.6 μm or less, it is possible to propagate photons over long distances while reducing the loss of the photons.

[0088] In this embodiment, the first resonant wavelength λ1, which is the wavelength of the first photon SF1, is shorter than the second resonant wavelength λ2, which is the wavelength of the second photon SF2. In particular, the first resonant wavelength λ1 is less than 1.0 μm, and the second resonant wavelength λ2 is 1.3 μm or more and 1.6 μm or less. Therefore, the quantum operation unit 4 according to this embodiment can efficiently perform quantum operations using the interaction between the first photon SF1 and the quantum system 22. Furthermore, the quantum operation unit 4 can generate the second photon SF2 that is entangled with the quantum system 22 and has little loss even when propagated over a relatively long distance.

[0089] The length of the optical fiber F between each single-photon source 6 and each quantum operation unit 4, and between each quantum operation unit 4 and each polarizing beam splitter 8 may be short enough to sufficiently reduce the loss of the first photon SF1. This allows the quantum operator 2 to efficiently perform quantum operations using the first photon SF1 in each quantum operation unit 4 while reducing the loss of the first photon SF1.

[0090] Furthermore, the length of the optical fiber F between each quantum operation unit 4 and each half beam splitter 12 may be longer than the length of the optical fiber F between each quantum operation unit 4 and each polarization beam splitter 8. This allows the quantum operation device 2 to propagate the operation result of a specific quantum operation unit 4 over a relatively long distance using the second photon SF2, which has a relatively small loss even when propagated over a long distance through the optical fiber F. Therefore, the quantum operation device 2 can make the distance between two quantum operation units 4 longer.

[0091] Therefore, the quantum processor 2 can efficiently perform quantum operations using the first photon SF1 in each quantum operation unit 4, while efficiently propagating the results of the quantum operation in a specific quantum operation unit 4 to other quantum operation units 4 that are located far away.

[0092] <Specific operation methods of quantum computing devices> As a more specific operation method of the quantum processor 2, for example, the quantum processor 2 performs a quantum operation using a first photon SF1 and a quantum system 22 in a first quantum operation unit 4 according to the first operation mode described above. Next, the quantum processor 2 propagates the result of the quantum operation to a quantum system 22 of a second quantum operation unit 4 different from the first quantum operation unit 4 according to the second operation mode described above, using a second photon SF2 that is entangled with the quantum system 22.

[0093] Here, in the propagation of the result of the quantum operation using the second photon SF2 in the second operating mode of the quantum operator 2, although the loss of the second photon SF2 in the optical fiber F is reduced, some of the second photons SF2 may be lost.

[0094] In this case, the quantum processor 2 can execute a quantum operation using the second quantum operation unit 4 having the quantum system 22 to which the result of the quantum operation has been propagated in the above-described first operation mode. As a result, the quantum processor 2 may correct a quantum error that occurs due to the loss of the second photon SF2, using the second quantum operation unit 4. Furthermore, the quantum processor 2 may propagate the corrected quantum information to the quantum system 22 of yet another quantum operation unit 4, using the second photon SF2, in the above-described second operation mode.

[0095] Note that it is also possible to first generate entanglement between distant quantum operation units 4 and then perform quantum operation by repeatedly attempting to generate and propagate the second photon SF2 until the generation of entanglement is successful. In this way, even if the second photon SF2 is lost in the first attempt, entanglement between the quantum operation units 4 can be generated more reliably by performing multiple attempts.

[0096] This allows the quantum processor 2 to efficiently propagate the result obtained by a quantum operation in a specific quantum operation unit 4 to a distant quantum operation unit 4 while restoring the quantum information. Therefore, the quantum processor 2 achieves a quantum communication device that propagates the result obtained by a quantum operation in a specific quantum operation unit 4 over a long distance with low loss. In this case, each quantum operation unit 4 functions as a quantum repeater of the quantum communication device.

[0097] As another specific operation method, for example, the quantum processor 2 propagates the result of the quantum operation in the first quantum operation unit 4 to the quantum system 22 of a second quantum operation unit 4 different from the first quantum operation unit 4, using the first operation mode described above. Next, the quantum processor 2 executes a quantum operation using the quantum system 22, using the second quantum operation unit 4 having the quantum system 22 to which the result of the quantum operation has been propagated.

[0098] In this case, the quantum processor 2 can use the result obtained from a quantum operation in a specific quantum operation unit 4 to perform a quantum operation in another quantum operation unit 4. Therefore, the quantum processor 2 achieves a distributed quantum processor that can increase the number of quantum systems 22 available for quantum operations overall by performing a quantum operation in each of a plurality of quantum operation units 4 that are separated from one another. In this case, each quantum operation unit 4 functions as a quantum operation unit of the distributed quantum processor.

[0099] In this operation method, quantum operations in the multiple quantum operation units 4 in the first operation mode may be executed in parallel. Also, in this operation method, propagation of the quantum state or sharing of entanglement between the multiple quantum operation units 4 in the second operation mode may be executed multiple times during the quantum operations in the first operation mode described above.

[0100] <Additional Notes> In this embodiment, each resonator QED system 16 includes a first resonator 26 having a first resonant wavelength λ1 as a resonant wavelength, and a second resonator 28 having a second resonant wavelength λ2 as a resonant wavelength. In each quantum operation unit 4, a first photon SF1 in the first resonator 26 interacts with the quantum system 22, and a second photon SF2 entangled with the quantum system 22 is generated in the second resonator 28.

[0101] The quantum operation unit 4 causes the first photon SF1 in the first resonator 26 coupled to the transition used for quantum operation of the quantum system 22 to interact with the quantum system 22. As a result, the quantum operation unit 4 allows the interaction between the quantum system 22 and the first photon to occur more efficiently, and executes quantum operation using the quantum system 22 more efficiently.

[0102] Furthermore, the quantum operation unit 4 generates the second photon SF2 in the second resonator 28 coupled to a transition corresponding to a second resonant wavelength λ2, which is the wavelength of the second photon SF2 generated using a non-degenerate four-wave mixing process in the quantum system 22. Therefore, in the quantum operation unit 4, the second photon SF2 is generated more efficiently using a non-degenerate four-wave mixing process in the quantum system 22.

[0103] The first resonator 26 and the second resonator 28 respectively include a first fiber Bragg grating 30 and a second fiber Bragg grating 32 formed in the optical fiber F or the nano-optical fiber 20. Therefore, the first resonator 26 and the second resonator 28 can be formed by a relatively simple method, such as forming the first fiber Bragg grating 30 and the second fiber Bragg grating 32 by irradiating the optical fiber F or the nano-optical fiber 20 with laser light. Furthermore, the quantum operation unit 4 can configure the first resonator 26 and the second resonator 28 with a simpler structure than when a ring resonator or the like is included.

[0104] In this embodiment, the quantum computation using the quantum system 22 and the generation of the second photon SF2 entangled with the quantum system 22 both use the transition T1 from the state |g> to the state |e1>, but this is not limiting. For example, the quantum system 22 may further have a fourth excitation level e4 having an energy different from that of the second excitation level e2, and a state |e4> corresponding to the fourth excitation level e4 may be defined. The fourth excitation level e4 may be, for example, a magnetic sublevel of the first excitation level e1.

[0105] In this case, the above-mentioned transition T1 is used in quantum computation using the quantum system 22, and transition T4 from the state |g> to the state |e4> may be used instead of transition T1 to generate the second photon SF2. In this case, the above-mentioned transition T2 is interpreted as a transition from the state |e4> to the state |e2>. In other words, the quantum operation unit 4 may generate the second photon SF2 by a four-wave mixing process using transition T4 from the state |g> to the state |e4>, transition T2 from the state |e4> to the state |e2>, and transition T3 from the state |e3> to the state |g> in the quantum system 22.

[0106] In the four-wave mixing process, the pump light L2 from the laser light source 18 irradiated to the quantum system 22 may contain, instead of the first laser light, a second laser light that resonates with the transition between the first ground level g and the fourth excited level e4. Furthermore, the third laser light contained in the pump light L2 may be a laser light that resonates with the transition between the fourth excited level e4 and the second excited level e2.

[0107] [Variation 1] <Another example 1 of a method for generating second photons> Another example of the method for generating a second photon SF2 entangled with the quantum system 22 in the second operation mode according to the present embodiment will be described as Modification 1 with reference to Fig. 8. Fig. 8 is a diagram showing an energy diagram D4 representing another example of the state transition of the quantum system 22 in the second operation mode of the quantum computing device 2 according to this modification.

[0108] The quantum processor 2 according to this modification is identical to the quantum processor 2 according to the present embodiment, and differs only in the pump light L2 irradiated to the quantum system 22 when the second photon SF2 is generated in the second operation mode, and the state transition in the quantum system 22 irradiated with the pump light L2. In the second operation mode according to this modification, for example, the pump light L2 irradiated to the quantum system 22 when the second photon SF2 is generated may include the above-mentioned fourth laser light and a fifth laser light whose second harmonic wave resonates with the transition between the first ground level g and the second excited level e2.

[0109] For example, the fifth laser light is a laser light having a frequency ω', where the frequency ω' satisfies ω'=(ω1+ω3) / 2. Therefore, the fifth laser light has energy that is approximately half the energy difference between the first ground level g and the second excited level e2. Therefore, the second harmonic of the fifth laser light resonates with the transition from the state |g> to the state |e2>.

[0110] When the quantum system 22 is irradiated with the fourth and fifth laser beams described above, as shown in the energy diagram D4 of FIG. 8, a transition T3 from the state |e3> to the state |g> and a transition from the state |g> to the state |e2> via transitions T4 and T5 occur. Here, the transitions T4 and T5 occur consecutively due to the fifth laser beam. Specifically, the fifth laser beam causes a transition T4 from the state |g> to a state excited to an energy level approximately midway between the first ground level g and the second excited level e2. Furthermore, the fifth laser beam causes a transition T5 from the state excited to an energy level approximately midway between the first ground level g and the second excited level e2 to the state |e2> consecutively with the transition T4. As a result, in this modification, a three-level four-wave mixing process occurs in the quantum system 22 irradiated with the pump light L2, and the second photon SF2 described above is generated.

[0111] In this modification as well, the second photon SF2 has the second resonant wavelength λ2 and is entangled with the quantum system 22. Therefore, in this modification as well, the quantum operation unit 4 achieves both the quantum operation using the interaction between the first photon and the quantum system 22 and the function of imparting entanglement between the quantum system 22 and the second photon SF2 without using a quantum wavelength converter.

[0112] [Variation 2] <Another example 2 of the method for generating second photons> Another example of the method for generating a second photon SF2 entangled with the quantum system 22 in the second operation mode according to the present embodiment will be described as Modification 2 with reference to Fig. 9. Fig. 9 is a diagram showing an energy diagram D5 illustrating another example of the state transition of the quantum system 22 in the second operation mode of the quantum computing device 2 according to this modification.

[0113] The quantum processor 2 according to this modification is identical to the quantum processor 2 according to the present embodiment, and differs only in the pump light L2 irradiated to the quantum system 22 when the second photon SF2 is generated in the second operation mode, and the state transition in the quantum system 22 irradiated with the pump light L2. In the second operation mode according to this modification, for example, the pump light L2 irradiated to the quantum system 22 when the second photon SF2 is generated may include the above-mentioned fourth laser light and a sixth laser light resonating with the transition between the first ground level g and the second excited level e2.

[0114] For example, the sixth laser light is a laser light having a frequency ω5, where the frequency ω5 satisfies ω5=ω1+ω3. Therefore, the sixth laser light has energy equivalent to the energy difference between the first ground level g and the second excited level e2. Therefore, the sixth laser light resonates with the transition from the state |g> to the state |e2>.

[0115] When quantum system 22 is irradiated with the fourth and sixth laser beams described above, transition T3 from state |e3> to state |g> and transition from state |g> to state |e2> via transition T6 occur, as shown in energy diagram D5 in Fig. 9. Here, transition T6 is caused by the sixth laser beam. Specifically, transition T6 from state |g> to state |e2> occurs by the sixth laser beam without passing through any other excited level. As a result, in this modification, three-level parametric down-conversion occurs in the quantum system 22 irradiated with the pump light L2, and the above-mentioned second photon SF2 is generated. In this modification as well, the second photon SF2 has the second resonant wavelength λ2 and is entangled with the quantum system 22. Therefore, in this modification as well, the quantum operation unit 4 achieves both the functions of quantum operation using the interaction between the first photon and the quantum system 22 and the function of imparting entanglement between the quantum system 22 and the second photon SF2 without using a quantum wavelength converter.

[0116] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure. [Explanation of symbols]

[0117] 2 Quantum computing unit 4 Quantum Computation Unit 6 Single photon source 8 Polarizing Beam Splitter 10 First single photon detector 12 Half beam splitter 14 Second single photon detector 16 Resonator QED system 18 Laser light source 20 Nano-optical fiber 22 Quantum system 24 Tapered section 26 1st resonator 28 Second resonator 30 First Fiber Bragg Grating 32 Second Fiber Bragg Grating SF1 1st photon SF2 second photon

Claims

1. a nano-optical fiber connected to an optical fiber that propagates photons via a tapered portion; a quantum system disposed on the nano-optical fiber; the quantum system has a ground level, a first excited level, a second excited level, and a third excited level having an energy different from that of the second excited level; a first resonant wavelength corresponding to a level difference between the ground level and the first excitation level and a second resonant wavelength corresponding to a level difference between the second excitation level and the third excitation level are different from each other, The quantum system interacts with a first photon having the first resonant wavelength and is entangled with the quantum system to generate a second photon having the second resonant wavelength in the nano-optical fiber.

2. The quantum operation unit according to claim 1 , wherein the second resonant wavelength is longer than the first resonant wavelength.

3. 3. The quantum operation unit according to claim 2, wherein the first resonant wavelength is less than 1.0 μm, and the second resonant wavelength is 1.0 μm or more and 1.8 μm or less.

4. 4. The quantum operation unit according to claim 3, wherein the second resonant wavelength is 1.3 μm or more and 1.6 μm or less.

5. The optical fiber further includes a first resonator having the first resonant wavelength and a second resonator having the second resonant wavelength, the first resonant wavelength being formed in at least one of the nano-optical fiber and the optical fiber; The quantum operation unit according to claim 1 , wherein the quantum system interacts with the first photon in the first resonator to generate the second photon in the second resonator.

6. The quantum operation unit according to claim 5, wherein the first resonator has a pair of first fiber Bragg gratings whose reflection band includes the first resonance wavelength, and the second resonator has a pair of second fiber Bragg gratings whose reflection band includes the second resonance wavelength.

7. The quantum operation unit according to claim 1 , further comprising a laser light source that irradiates the quantum system with a first laser light having the first resonant wavelength.

8. the quantum system further has a fourth excited level having an energy different from that of the second excited level; The quantum operation unit according to claim 7, wherein the laser light source further irradiates the quantum system with a second laser light that resonates with a transition between the ground level and the fourth excited level, a third laser light that resonates with a transition between the fourth excited level and the second excited level, and a fourth laser light that resonates with a transition between the third excited level and the ground level.

9. The quantum operation unit according to claim 7, wherein the laser light source further irradiates the quantum system with a fourth laser light that resonates with the transition between the third excited level and the ground level, and a fifth laser light whose second harmonic resonates with the transition between the ground level and the second excited level.

10. A plurality of quantum operation units according to any one of claims 1 to 4; a plurality of single-photon sources that generate single photons having the first resonant wavelength and input the single photons to each of the quantum operation units; a plurality of polarizing beam splitters onto which single photons having the first resonant wavelength output from each of the quantum operation units are incident; a plurality of first single-photon detectors for detecting single photons exiting each of the polarizing beam splitters; At least one half beam splitter that causes single photons having the second resonant wavelength output from each of the two quantum operation units to interfere with each other; at least one second single-photon detector for detecting single photons exiting the half beam splitter; A quantum computing device equipped with

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