Quantum computing device

Abstract

This quantum computing device comprises a single photon generator, four resonators, and an optical polarization measurement device. Photonic qubits emitted from the single photon generator interact with each of four atomic qubits trapped by the four resonators, and the optical polarization measurement device is configured to measure the photonic qubits that have undergone interaction.

Description

quantum computing device

[0001] The present invention relates to a technique for performing quantum computing using resonator QED.

[0002] In recent years, it has been shown that the application of quantum mechanics to calculations has made it possible to perform calculations at speeds not previously possible. In quantum computing, a cavity quantum electrodynamics (QED) system, in which photons confined in a cavity interact quantum mechanically with atoms, has attracted attention as an important physical system that can deterministically manipulate quantum bits of light or atoms.

[0003] In quantum computing using a cavity QED system, photons are injected into a cavity in which atoms are placed, causing the photons and atoms to interact quantum mechanically. These photons are then emitted from the cavity. If the photon state and the atom state are appropriately selected as quantum bits, a quantum gate between the photon quantum bit and the atomic quantum bit can be realized. Quantum gates are applied as a core technology in quantum computing.

[0004] However, because qubits are vulnerable to external noise, in actual quantum computing, information must be stored using a logical qubit, which is newly defined using the state of multiple qubits. This method of redundancy of information, in which multiple qubits are used to create a new logical qubit, is called encoding. Hereinafter, qubits that are not logical qubits will be referred to as physical qubits to clearly distinguish them from logical qubits. Photon qubits and atomic qubits are both examples of physical qubits.

[0005] For example, if multiple physical qubits are used to construct a logical qubit that holds the information of one original physical qubit, even if noise occurs in some of the physical qubits, it is possible to prevent the information in the redundant logical qubit from being destroyed. A quantum computing mechanism that can arbitrarily reduce the error rate of calculations by encoding is called error-tolerant quantum computing.

[0006] Andreas Reiserer, "Cavity-based quantum networks with single atoms and optical photons," REVIEWS OF MODERN PHYSICS, VOLUME 87, OCTOBER-DECEMBER 2015.

[0007] In the prior art, there are no examples that propose a specific device configuration for error-tolerant quantum computing based on a cavity QED system. Therefore, in order to construct error-tolerant quantum computing in a cavity QED system, it is necessary to clarify a specific device configuration.

[0008] The present invention has been made in view of the above points, and has as its object to provide a technique for realizing error-tolerant quantum computing using a resonator QED system.

[0009] According to the disclosed technology, there is provided a quantum computing device including: a single-photon generator; four resonators; and an optical polarization measurement device, wherein a photon quantum bit emitted from the single-photon generator interacts with each of four atomic quantum bits trapped in the four resonators; and the optical polarization measurement device is configured to measure the photon quantum bit that has been subjected to the interaction.

[0010] According to the disclosed technology, it becomes possible to realize error-tolerant quantum computing using a resonator QED system.

[0011] FIG. 1 is a diagram showing a configuration having a resonator 10 and a circulator 20 (quantum gate configuration). FIG. 2 is a diagram showing a basic configuration for performing syndrome measurements of four atomic quantum bits. FIG. 3 is a diagram showing the basic structure of a logical quantum bit using a surface code. FIG. 4 is a diagram for explaining Example 1. FIG. 5 is a diagram for explaining Example 1. FIG. 6 is a diagram for explaining Example 2. FIG. 7 is a diagram showing the allocation of atomic quantum bits to each resonator in a four-resonator logical quantum bit of Example 2. FIG. 8 is a diagram showing the configuration of a quantum computing system 300. FIG. 9 is a diagram showing an example of the hardware configuration of a computer.

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The embodiment described below is merely an example, and the embodiment to which the present invention is applied is not limited to the following embodiment.

[0013] (Overview of the embodiment) In this embodiment, a technique for realizing error-tolerant quantum computing using a resonator QED system is described. Specifically, in this embodiment, an error-tolerant surface-coded logical quantum bit is realized using a resonator QED system. A surface code is a code that is known to be easy to implement experimentally and capable of performing error-tolerant quantum computing even with a high noise generation rate.

[0014] Surface codes require constant measurement, called syndrome measurement, to detect noise-induced errors. Whether or not this error detection can consistently detect errors without amplifying the noise determines whether or not the logical qubit information can be accurately retained.

[0015] In actual syndrome measurements, the multiple physical qubits that make up a logical qubit are divided into groups of four, and the quantum state of the entire group is measured for each group. This makes it possible to determine whether an error has occurred in a physical qubit without accessing each individual physical qubit, whose state would be destroyed if measured individually.

[0016] In this embodiment, we consider constructing a logical quantum bit using an atomic quantum bit trapped in a resonator, and describe a technology for realizing syndrome measurement of a group consisting of the above-mentioned four physical quantum bits. In this technology, the basic structure is a structure in which four resonators containing atomic quantum bits are arranged, a photon quantum bit interacts with each of the four atomic quantum bits once, and finally the photon quantum bit is measured.

[0017] In the above basic structure, it is possible to indirectly measure the syndromes of the four atoms by measuring the photon qubit, which contains the information of the four atomic qubits through successive interactions with each atomic qubit. The photon qubit corresponds to an auxiliary physical qubit.

[0018] The configuration and operation of the device according to this embodiment will be described in detail below.

[0019] (Regarding the resonator) Fig. 1 shows a configuration including a resonator 10 and a circulator 20, which are components of a quantum computing device 100 according to this embodiment. As shown in Fig. 1, the resonator 10 according to this embodiment has a configuration in which two highly reflective mirrors 11 and 12 face each other. A single atom is trapped inside the resonator 10 shown in Fig. 1 as an atomic quantum bit.

[0020] Of the mirrors 11 and 12 that make up the resonator 10, the mirror 11 has a slight transparency, allowing photons from a waveguide (or optical fiber) to be incident on the resonator 10 via the mirror 11 and reflected (output) from the resonator 10.

[0021] In this embodiment, the resonator uses two mirrors as shown in Fig. 1, but this is just an example. A resonator using, for example, a nano-optical fiber may also be used.

[0022] As shown in Fig. 1, the atomic qubit used in this embodiment is defined by the lower two levels (|0>, |1>) of a lambda-type three-level system. In addition, a photon qubit defined by two orthogonal polarization states (|H>, |V>) is used as an auxiliary physical qubit for measuring the quantum states of the four atomic qubits. A quantum gate is realized between the photon qubit and each atomic qubit based on the interaction between them. As a technology for this part, the technology for realizing a quantum gate between a three-level atomic qubit and a polarized photon qubit, disclosed in Non-Patent Document 1, can be used.

[0023] (Device Configuration for Performing Syndrome Measurement) Fig. 2 shows an example of a basic configuration for performing syndrome measurement of four atomic quantum bits. The device having the configuration shown in Fig. 2 is an example of the quantum computing device 100 according to this embodiment.

[0024] As shown in Fig. 2, a quantum computing device 100 having a basic configuration includes a single-photon generator 50, an optical polarization detection device 60, resonators 1 to 4, half-wave plates 31 and 32, and circulators 21 to 24. In the example of Fig. 2, one atomic quantum bit is trapped in each resonator. The configuration consisting of one resonator and one circulator connected to the resonator is the same as the configuration shown in Fig. 1.

[0025] In the configuration of FIG. 2, a single-photon generator 50 and an optical polarization detector 60 are connected to resonators 1 to 4 trapping atomic quantum bits 1 to 4, respectively, via circulators 1 to 4.

[0026] The resonator and the circulator are connected by a waveguide or optical fiber. The single-photon generator 50 and the circulator 21 are connected by a waveguide or optical fiber via a half-wave plate 31, the two circulators are connected by a waveguide or optical fiber, and the circulator 24 and the optical polarization measurement device 60 are connected by a waveguide or optical fiber via a half-wave plate 32.

[0027] The single-photon generator 50 emits a photon quantum bit as a pulse. The photon quantum bit passes through the half-wave plate 31 and reaches the circulator 21. The circulators 21 to 24 each have the function of directing the photon quantum bit arriving from the single-photon generator 50 side toward the connected resonator, and directing the photon quantum bit returning from the resonator toward the optical polarization detection device 60. The photon quantum bit output from the circulator 24 passes through the half-wave plate 32 and reaches the optical polarization detection device 60. The operation of the quantum computing device 100 with a basic configuration is as follows.

[0028] A photon quantum bit is emitted as a pulse from single-photon generator 50. Then, four consecutive quantum gates are applied between the photon quantum bit that has passed through half-wave plate 31 and the atomic quantum bit in each resonator. After passing through the four quantum gates, the photon quantum bit passes through half-wave plate 32, and its polarization is measured by optical polarization detector 60.

[0029] The quantum gate is, for example, a CZ gate that uses an atomic quantum bit as a control quantum bit.

[0030] A surface-encoded logical quantum bit can be realized if it is possible to perform syndrome measurements on four adjacent atomic quantum bits as shown in FIG. 3 when quantum bits are conveniently arranged on a two-dimensional lattice.

[0031] With the basic configuration shown in Figure 2, by using photon qubits as auxiliary atomic qubits, it is possible to perform syndrome measurements on four atomic qubits without directly measuring the atomic qubits (without destroying the state of the atomic qubits).

[0032] Two examples (Example 1 and Example 2) will be described below as more specific examples for realizing a logical quantum bit.

[0033] First Embodiment First, a description will be given of a first embodiment. Fig. 4(a) shows an example of the configuration of a quantum computing device 100 that realizes a two-dimensionally arranged logical quantum bit in the first embodiment.

[0034] In this structure, resonators are actually arranged in a two-dimensional lattice, and each resonator is connected to a single-photon generator 50 and an optical polarization detector 60 via a switch 70. Also, as shown in the figure, a half-wave plate 30 is provided. Therefore, the photon quantum bit emitted from the single-photon generator 50 passes through the half-wave plate 30. Furthermore, the photon quantum bit that has passed through the half-wave plate 30 is incident on the optical polarization detector 60.

[0035] Using this structure, the direction of travel of the photon quantum bit is appropriately set by the switch 70, so that the photon quantum bit emitted from the single-photon generator 50 circulates through four adjacent resonators and is incident on the optical polarization detection device 60. Fig. 4(b) shows an image of the photon quantum bit circulating through four adjacent resonators.

[0036] FIG. 5 shows the arrangement of the resonators (=atomic quantum bits) when the configuration of FIG. 4(b) is viewed from directly above, and an image of a photon quantum bit circulating around the four resonators.

[0037] Second Embodiment Next, a description will be given of a second embodiment. Fig. 6 shows a configuration example of a quantum computing device 100 that realizes a four-resonator logical quantum bit in the second embodiment.

[0038] As shown in Fig. 6, the configuration of the quantum computing device 100 of the second embodiment is the same as the basic configuration shown in Fig. 2. The second embodiment differs from the basic configuration in that multiple atomic quantum bits are trapped in each resonator.

[0039] That is, in the quantum computing device 100 of the second embodiment, four resonators 1 to 4 are connected in the same manner as in the basic configuration, and a plurality of atomic quantum bits are trapped in each resonator.

[0040] In Example 2, an external laser is applied to the atomic qubit, shifting the energy level of the atomic qubit from the resonant frequency of the resonator, thereby determining whether or not to allow the atomic qubit to interact with the photon qubit. In other words, by controlling the external laser, it is possible to select the atomic qubit that will interact with the photon qubit in each resonator.

[0041] Therefore, in the structure of Example 2 shown in FIG. 6, one atomic quantum bit is selected from each resonator by a laser to interact with the photon quantum bit, and syndrome measurements are sequentially performed on a total of four atomic quantum bits.

[0042] As shown in Fig. 7, atoms are arranged in each resonator so that four adjacent atomic quantum bits on a two-dimensional lattice considered in terms of surface codes are allocated without overlap to resonators 1 to 4. In Fig. 7, the numbers in the atomic quantum bits indicated by circles indicate the numbers (codes) of the resonators.

[0043] In the example of the logical qubit in Fig. 7, for example, eight atomic qubits are trapped in resonator 1. For example, when performing syndrome measurements on four atomic qubits indicated by A, B, C, and D in Fig. 7, atomic qubit A is selected in resonator 1 as the atomic qubit that interacts with the photon qubit, atomic qubit B is selected in resonator 2 as the atomic qubit that interacts with the photon qubit, atomic qubit C is selected in resonator 3 as the atomic qubit that interacts with the photon qubit, and atomic qubit D is selected in resonator 4 as the atomic qubit that interacts with the photon qubit. Thereafter, the photon qubit is emitted from single-photon generator 50, and the photon qubit is measured in optical polarization measurement device 60.

[0044] Although the method in Example 2 has a lower degree of parallelism in the syndrome measurement operation than the two-dimensionally arranged logical quantum bit described in Example 1, it has the advantage that it requires fewer resonators, single-photon generators, optical polarization detectors, and switches.

[0045] It is also possible to combine Example 1 with Example 2. This makes it possible to increase the number of logical quantum bits while keeping the device configuration of Example 1 unchanged.

[0046] (Configuration Example of a System Using the Quantum Computing Device 100) A configuration example of a quantum computing system 300 including the quantum computing device 100 is shown in Fig. 8. The "quantum computing system" may also be called a "quantum computer."

[0047] 8, the quantum computing system 300 includes a quantum computing device 100 and a control device 200. The quantum computing device 100 has the configuration described in the basic configuration, the first embodiment, the second embodiment, or the like.

[0048] The control device 200 performs error-tolerant quantum computing by transmitting a control signal instructing measurement or operation to the quantum computing device 100 and acquiring measurement results from the quantum computing device 100. The control device 200 can be realized by a classical computer.

[0049] The quantum computing device 100 is not limited to a device that includes an actual resonator. For example, the quantum computing device 100 may be a simulator that simulates, on a classical computer, a single-photon generator, a half-wave plate, a resonator, an atomic qubit trapped in the resonator, a photon qubit, a circulator, and an optical polarimetry device.

[0050] In this case, the quantum computing device 100 may be a classical computer such as a PC, or may be a virtual machine on the cloud. When the quantum computing device 100 is a simulator, the quantum computing device 100 may be provided inside the control device 200. That is, in this case, the quantum computing system 300 may be configured with a classical computer.

[0051] (Configuration Example of Classical Computer) As described above, when a classical computer is used as the quantum computing device 100, the control device 200, or the quantum computing system 300, the device (quantum computing device 100, the control device 200, or the quantum computing system 300) can be realized by having the classical computer execute a program. This classical computer may be a physical computer or a virtual machine on the cloud. Hereinafter, the classical computer will be referred to as a "computer."

[0052] That is, the device can be realized by executing a program corresponding to the processing performed by the device using hardware resources such as a CPU and memory built into a computer. The program can be recorded on a computer-readable recording medium (such as a portable memory) and stored or distributed. The program can also be provided via a network such as the Internet or email.

[0053] Fig. 9 is a diagram showing an example of the hardware configuration of the computer. The computer in Fig. 9 includes a drive device 1000, an auxiliary storage device 1002, a memory device 1003, a CPU 1004, an interface device 1005, a display device 1006, an input device 1007, an output device 1008, and the like, all of which are interconnected by a bus BS. The computer may further include a GPU.

[0054] The program that realizes the processing on the computer is provided by a recording medium 1001, such as a CD-ROM or a memory card. When the recording medium 1001 storing the program is set in the drive device 1000, the program is installed from the recording medium 1001 to the auxiliary storage device 1002 via the drive device 1000. However, the program does not necessarily have to be installed from the recording medium 1001, but may be downloaded from another computer via a network. The auxiliary storage device 1002 stores the installed program as well as necessary files, data, etc.

[0055] The memory device 1003 reads and stores a program from the auxiliary storage device 1002 when an instruction to start the program is received. The CPU 1004 realizes functions related to the device in accordance with the program stored in the memory device 1003. The interface device 1005 is used as an interface for connecting to a network, etc. The display device 1006 displays a GUI (Graphical User Interface) or the like according to the program. The input device 1007 is composed of a keyboard, mouse, buttons, a touch panel, etc., and is used to input various operation instructions. The output device 1008 outputs the results of calculations.

[0056] (Summary, Effects, etc. of the Embodiments) As described above, the technology described in the present embodiment makes it possible to realize logical quantum bits required for error-tolerant quantum computing using a resonator QED system.

[0057] The following additional notes are provided regarding the above-described embodiments.

[0058] <Appendix> (Appendix 1) A quantum computing device including a single-photon generator, four resonators, and an optical polarization measurement device, configured such that a photon quantum bit emitted from the single-photon generator interacts with each of four atomic quantum bits trapped in the four resonators, and the optical polarization measurement device measures the photon quantum bit that has been interacted with. (Appendix 2) The quantum computing device of appendix 1, in which the four atomic quantum bits form a logical quantum bit, and in which syndrome measurement is performed as the measurement. (Appendix 3) The quantum computing device of appendix 1 or 2, in which a plurality of resonators are arranged on a two-dimensional lattice, and the single-photon generator and the optical polarization measurement device are connected to each resonator via a switch, and in which four atomic quantum bits in four adjacent resonators are grouped together, and in which syndrome measurement is performed for each of the plurality of groups. (Supplementary Item 4) The quantum computing device according to any one of Supplementary Items 1 to 3, wherein a plurality of atomic quantum bits are trapped in each of the four resonators, and one atomic quantum bit is selected in each resonator to interact with a photon quantum bit, and syndrome measurements are performed on the selected four atomic quantum bits.

[0059] Although the present embodiment has been described above, the present invention is not limited to such a specific embodiment, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

[0060] 1 to 4, 10 resonator 11, 12 mirror 20 to 24 circulator 30 to 32 half-wave plate 50 single-photon generator 60 optical polarization measurement device 70 switch 100 quantum computing device 200 control device 300 quantum computing system 1000 drive device 1001 recording medium 1002 auxiliary storage device 1003 memory device 1004 CPU 1005 interface device 1006 display device 1007 input device 1008 output device

Claims

1. A quantum computing device comprising a single photon generator, four resonators, and an optical polarization measurement device, wherein a photon qubit emitted from the single photon generator interacts with each of four atomic qubits trapped in the four resonators, and the optical polarization measurement device is configured to measure the photon qubit that has undergone the interaction.

2. The quantum computing device according to claim 1, wherein a logical qubit is constituted by the four atomic qubits, and a syndrome measurement is performed as the measurement.

3. The quantum computing device according to claim 1 or 2, wherein a plurality of resonators are arranged on a two-dimensional lattice, the single photon generator and the optical polarization measurement device are connected to each resonator via a switch, four atomic qubits in four adjacent resonators are taken as one group, and a syndrome measurement is performed for each of the plurality of groups.

4. The quantum computing device according to claim 1 or 2, wherein a plurality of atomic qubits are trapped in each of the four resonators, one atomic qubit that interacts with a photon qubit is selected in each resonator, and a syndrome measurement is performed on the selected four atomic qubits.

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