Quantum entanglement generating device, quantum entanglement generating method, and quantum computer

Abstract

A quantum entanglement generation device 10 comprises, when n is an integer of 2 or more, two superconducting quantum bit elements 20a and 20b each having three electrodes, a coupled resonator 30 disposed between the adjacent superconducting quantum bit elements 20a and 20b, and waveguides 50a and 50b capacitive-coupled to the superconducting quantum bit elements 20a and 20b, respectively. The coupled resonator 30 generates quantum entanglement between the adjacent superconducting quantum bit elements 20a and 20b by applying a two-qubit gate between the adjacent superconducting quantum bit elements 20a and 20b. The superconducting quantum bit elements 20a and 20b generate a two-dimensional cluster state by emitting the quantum entanglement as propagating microwave photons to the waveguides 50a and 50b.

Description

[Technical field]

[0001] The present invention relates to a quantum entanglement generating device, a quantum entanglement generating method, and a quantum computer. [Background technology]

[0002] Measurement-based quantum computing has been proposed as one of the promising methods for realizing a quantum computer (eg, Non-Patent Documents 1 and 2). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] R. Raussendorf and HJ Briegel "A One-Way Quantum Computer", Phys. Rev. Lett. 86, 5188,(2001) [Non-Patent Document 2] R. Raussendorf, DE Browne, HJ Briegel "Measurement-based quantum computation with cluster states", Phys. Rev. A 68, 022312 (2003) [Non-Patent Document 3] SE Economou, N. Lindner, and T. Rudolph "Optically Generated 2-Dimensional Photonic Cluster State from Coupled Quantum Dots", Phys. Rev. Lett. 105, 093601 (2010) [Non-Patent Document 4] J. Ilves, S. Kono, Y. Sunada, S. Yamazaki, M. Kim, K. Koshino, Y. Nakamura "On-demand generation and characterization of a microwave time-bin qubit", npj Quantum Information volume 6, Article number: 34 (2020) .

Direct Environment 5

Outdoor Configuration 6

Direct Environment 7

[0004] In measurement-based quantum computing, it is necessary to prepare a large-scale quantum entangled state called a cluster state. Non-Patent Document 3 discloses an idea of ​​generating a cluster state of a two-dimensional photon train from two coupled quantum dot pairs. Non-Patent Document 4 discloses a method of generating microwave time-bin qubits on demand using a superconducting circuit quantum electrodynamics (circuit-QED) architecture. Non-Patent Document 5 discloses a superconducting qubit device having three electrodes. These technologies are elemental technologies for generating a two-dimensional cluster state of a microwave photon train using a superconducting qubit device. However, a specific device for generating a two-dimensional cluster state of a microwave photon train has not yet been proposed.

[0005] The present invention has been made in view of the above problems, and has an object to provide an apparatus for generating a two-dimensional cluster state of a microwave photon train. [Means for solving the problem]

[0006] In order to solve the above problems, a quantum entanglement generating device according to an embodiment of the present invention includes n quantum bit elements, where n is an integer equal to or greater than 2, a coupled resonator disposed between adjacent quantum bit elements, and a waveguide (e.g., a coaxial line or a coplanar waveguide) capacitively coupled to each of the quantum bit elements. A two-qubit gate is applied between adjacent quantum bit elements using the coupled resonator to generate quantum entanglement between the adjacent quantum bit elements. Furthermore, a propagating microwave photon train having quantum entanglement with the quantum bit is generated from each quantum bit, and is sequentially emitted into the waveguide to generate a two-dimensional cluster state.

[0007] Alternatively, adjacent qubit elements may be directly coupled without using a coupled resonator. In other words, even when the qubits are directly coupled, the two-qubit gate required for this method can still be used.

[0008] Each of the n qubit elements may have three electrodes.

[0009] The quantum entanglement generating device of the embodiment may include a photon emitting resonator or photon emitting quantum bit that conditionally generates an excitation depending on the state of the quantum bit element and emits the excitation as a propagating microwave photon into the waveguide, independent of the quantum bit element.

[0010] The quantum entanglement generating device according to the embodiment may include a readout resonator that reads out the state of the quantum bit element.

[0011] Two of the three electrodes may have the shape of half a ring having a concentric contour when viewed in the direction of the waveguide.

[0012] The quantum entanglement generating device according to the embodiment may include a conductor cavity having a cavity therein. The quantum bit element and the coupled resonator may be fixed within the cavity of the conductor cavity.

[0013] The quantum bit element may initialize the quantum bit to a ground state, half-excite the ground state to a first excited state, excite the first excited state to a second excited state, excite the ground state to the first excited state, convert the second excited state to an excitation of the photon emitting resonator or the photon emitting quantum bit, and then emit a propagating microwave photon from the photon emitting resonator or the photon emitting quantum bit into the waveguide.

[0014] The qubit device may be a superconducting qubit device.

[0015] Another aspect of the present invention is a method for generating quantum entanglement using the above-mentioned quantum entanglement generating apparatus, comprising the steps of initializing a quantum bit to a ground state, semi-exciting the ground state to a first excited state, exciting the first excited state to a second excited state, exciting the ground state to the first excited state, converting the second excited state to an excitation of a photon-emitting resonator or a photon-emitting quantum bit, and then emitting a propagating microwave photon from the photon-emitting resonator or the photon-emitting quantum bit into a waveguide.

[0016] Yet another aspect of the present invention is a quantum computer including the above-mentioned quantum entanglement generating device.

[0017] The quantum computer of the embodiment may perform measurement-based quantum computation in which measurements are repeatedly performed on the quantum entangled state generated by the quantum entanglement generation device.

[0018] The quantum computer of the embodiment may perform measurement-type quantum computation by temporarily storing the cluster state generated by the quantum entanglement generation device as propagating photons from the waveguide to a superconducting delay line, absorbing the propagating photons with another quantum bit element, applying a two-qubit gate between the quantum bit element for entanglement generation and the photons, performing a basis conversion, and then measuring the photons. The measurement is then repeated while selecting the next measurement basis based on the result of the previous measurement.

[0019] Any combination of the above components, and any transformation of the present invention into an apparatus, method, system, recording medium, computer program, etc., are also effective as aspects of the present invention. Effect of the Invention

[0020] According to the present invention, it is possible to provide an apparatus for generating a two-dimensional cluster state of quantum bits. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram showing a one-dimensional cluster state. [Diagram 2] FIG. 1 is a schematic diagram showing a two-dimensional cluster state. [Diagram 3] FIG. 1 is a schematic diagram showing the generation and emission of propagating microwave photons by a device that couples a quantum bit with a resonator. [Figure 4] FIG. 4 is a state transition diagram when propagating microwave photons are generated and emitted using the device in FIG. [Diagram 5] FIG. 2 is a schematic diagram showing how a cluster state is generated using the quantum entanglement generation device according to the first embodiment. [Figure 6] 1 is a perspective view of a quantum entanglement generating device according to a first embodiment. [Figure 7] 7 is a plan view of a superconducting quantum bit device in the quantum entanglement generation device of FIG. 6. [Figure 8] FIG. 7 is a diagram showing step 1 of a procedure for generating a cluster state using the quantum entanglement generation device of FIG. 6. [Figure 9] FIG. 7 is a diagram showing step 2-1 of a procedure for generating a cluster state using the quantum entanglement generation device of FIG. 6. [Figure 10] FIG. 7 is a diagram showing step 2-3 of the procedure for generating a cluster state using the quantum entanglement generation device of FIG. 6. [Figure 11] FIG. 7 is a diagram showing steps 2-4 of a procedure for generating a cluster state using the quantum entanglement generation device of FIG. 6. [Figure 12] FIG. 7 is a diagram showing steps 2-5 of a procedure for generating a cluster state using the quantum entanglement generation device of FIG. 6. [Figure 13] FIG. 7 is a diagram showing that steps (2-1) to (2-5) are repeated “desired photon chain length−1 times” in the procedure for generating a cluster state using the quantum entanglement generating device of FIG. [Figure 14] 7 is a flowchart showing a procedure for generating a cluster state using the quantum entanglement generation device of FIG. 6. [Figure 15] FIG. 11 is a perspective view of a quantum entanglement generating device according to a second embodiment. [Figure 16] FIG. 13 is a perspective view of a quantum computer according to a fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The present invention will be described below with reference to the drawings based on preferred embodiments. The embodiments are illustrative and do not limit the invention. All features and combinations described in the embodiments are not necessarily essential to the invention. The same or equivalent components, members, and processes shown in each drawing are given the same reference numerals, and duplicated descriptions are omitted as appropriate. In addition, the scale and shape of each part shown in each drawing are set for convenience to facilitate explanation, and are not to be interpreted as being limiting unless otherwise specified. In addition, when terms such as "first" and "second" are used in this specification or claims, unless otherwise specified, these terms do not represent any order or importance, but are only intended to distinguish one configuration from another. In addition, some of the members that are not important in explaining the embodiment in each drawing are omitted.

[0023] Before describing specific embodiments, the basic knowledge will be described first. A quantum computer is a computer that realizes high-speed calculations by utilizing quantum mechanical phenomena, and can efficiently solve some problems that are difficult to solve in a realistic calculation time with a classical computer. The gate method, which is the mainstream method of realizing a quantum computer, is to create a large number of quantum bits one by one, combine them to form wiring between the quantum bits for calculation, and perform calculations while performing quantum operations in sequence. While the gate method has been actively researched as a standard quantum calculation method, there is a problem that it is difficult to scale up because wiring and control become complicated as the number of quantum bits increases.

[0024] In contrast, another realization method, "measurement-based quantum computing" (also called "one-way quantum computing"), first prepares a large number of quantum bits in a specific quantum entangled state (cluster state) and performs calculations by measuring these quantum bits individually. In this respect, measurement-based quantum computing differs from gate methods that require controlling the interactions between quantum bits (quantum gates) depending on the content of the calculation. The cluster state is a state in which any quantum calculation pattern is superimposed, and the principle of measurement-based quantum computing is that any calculation can be performed by adaptively repeating measurements on the cluster state. The advantage of measurement-based quantum computing is that as long as a cluster state with a sufficient number of quantum bits and an appropriate quantum entanglement structure is prepared at the beginning, any quantum calculation can be realized by relatively simple measurements of each quantum bit. Here, "cluster state with an appropriate quantum entanglement structure" refers to a general-purpose quantum entanglement that can realize any quantum calculation using multiple inputs, and the simplest example known is called a "two-dimensional cluster state". Measurement-based quantum computing makes it possible to perform large-scale quantum calculations on relatively small hardware.

[0025] A "quantum bit" (also called a "qubit" or "qubit") is the smallest unit of quantum information in a quantum computer. In a classical computer, a bit can only have the value 0 or 1. In other words, in this case, the state (classical state) is two states. In contrast, a quantum bit can be a quantum mechanical superposition of these two states.

[0026] "Quantum entanglement" refers to the correlation between two or more quantum bits in a quantum many-body system that can only be explained by quantum mechanics. Quantum entanglement is used in various information processing technologies that apply quantum mechanics (quantum measurement, quantum communication, quantum computing, etc.). The cluster state described below is also a type of quantum entanglement.

[0027] A "cluster state" is a quantum entanglement used in measurement-based quantum computing. When illustrating a cluster state, quantum bits are often represented as circles and quantum entanglement between quantum bits is often represented as lines. The type of quantum computing that can be performed using a cluster state is determined by the structure of the cluster state. For example, a single chain-like cluster state (one-dimensional cluster state) allows only one-input, one-output calculations. In contrast, to be able to perform any quantum computation with multiple inputs and multiple outputs, a two-dimensional cluster state with a mesh-like structure is required. Figure 1 shows a schematic of a one-dimensional cluster state. Figure 2 shows a schematic of a two-dimensional cluster state.

[0028] Quantum mechanics is usually applied to microscopic physical systems such as atoms and electrons. However, electronic devices such as superconducting circuits including Josephson devices exhibit quantum mechanical behavior even though they are macroscopic physical systems. A "superconducting qubit device" uses such a superconducting circuit as an element that functions as a qubit. In other words, a superconducting qubit device is an artificially created quantum mechanical physical system on a superconducting electrical circuit. Since it is relatively easy to integrate and control the device characteristics, superconducting qubit devices are expected to be a key device for realizing quantum computers. In the following, not only superconducting qubit devices but also artificial elements that function as qubits may be called "qubit devices".

[0029] Photons with energy in the microwave range are called "microwave photons". Microwave frequencies are on the order of 10 GHz, making them electrically controllable. In addition, microwave wavelengths are on the order of 1 cm, making it possible to design a variety of devices using conductor cavities and thin film patterns. However, the energy of microwave photons is extremely small, specifically equivalent to a temperature of 500 millikelvin (mK). For these reasons, microwave photons must be generated and detected at cryogenic temperatures.

[0030] By integrating and mounting superconducting quantum bit elements on a chip, a macroscopic quantum circuit can be formed. However, there is a limit to the number of superconducting quantum bit elements that can be mounted on one chip. Therefore, a method has been proposed to form a quantum network by quantum connecting chips using the propagation of microwave photons, thereby increasing the total number of superconducting quantum bit elements (for example, Non-Patent Documents 6 and 7). Microwave photons that carry quantum information between quantum bits are sometimes called "propagating microwave photons."

[0031] The process of generating and emitting a propagating microwave photon will be described with reference to Fig. 3. Fig. 3 shows a system in which a device 1 is coupled to a waveguide 4. The device 1 is configured by capacitively coupling a quantum bit 2 and a resonator 3. Below, the process of generating a propagating microwave photon 5 using the device 1 and emitting the generated propagating microwave photon 5 to the waveguide 4 (such as a coaxial line) capacitively coupled to the resonator 3 will be described.

[0032] First, the quantum bit 2 is set to the desired quantum state. Next, the quantum bit 2 is irradiated with microwaves, so that the quantum state of the quantum bit 2 is transferred to the resonator 3. As a result, the resonator 3 has a photon state that corresponds to the quantum state of the quantum bit 2. Finally, the photon state of the resonator 3 is spontaneously emitted into the waveguide 4, generating a pulse of propagating microwave photons 5.

[0033] FIG. 4 is a state transition diagram when a propagating microwave photon is generated and emitted using the device in FIG. 3. Below, the procedure for generating one pulse of a propagating microwave photon will be described with reference to FIG. 4. In this example, the quantum bit is a three-level system consisting of the ground state |g>, the first excited state |e>, and the second excited state |f>. Also, it is assumed that there are two quantum states in the resonator: a vacuum state |0> with zero photons, and a one-photon state |1> with one photon. In the following, the character on the left side of the ket |> indicates the state of the quantum bit, and the character on the right side indicates the number of photons in the resonator. For example, |e0> indicates that the quantum bit is in the first excited state, and the number of photons in the resonator is zero.

[0034] In this example, the frequencies corresponding to the energy of each state of the system when |g0> is used as the reference are as follows: |g0>:0GHz |e0>:8.5GHz |g1>:10.6GHz |f0>:16.6GHz

[0035] Propagating microwave photons are generated in five steps: (Step i) Initialize the quantum bit to the ground state |g〉. (Step ii) Set the quantum bit to the desired state α|g>+β|e>. (Step iii) The quantum bit is excited from the first excited state |e> to the second excited state |f> by irradiating it with microwaves having a frequency (16.6 GHz-8.5 GHz=8.1 GHz) equivalent to the energy difference between the second excited state |f> and the first excited state |e>. (Step iv) By irradiating the drive microwaves with a frequency (16.6 GHz-10.6 GHz = 6.0 GHz) equivalent to the energy difference between states |f0> and |g1>, the transition from state |f0> to |g1> is driven. This conditionally excites the resonator when the quantum bit is in the second excited state |f>, and the quantum bit state α|g>+β|e> is transferred to the quantum state α|0>+β|1> of the resonator. (Step v) A pulse of propagating microwave photons, α|0>+β|1>, is generated by spontaneous emission from the resonator into the waveguide. The system state returns to |g0>. As mentioned above, in this specification, when expressing the quantum state of the entire device, notations such as |f0>, |g1>, etc. are used, and when focusing only on quantum bits, notations such as |f>, |g>, etc. are used (same below).

[0036] The above procedure can also be performed in a device that has a photon-emitting qubit instead of a resonator, where the vacuum state and one-photon state of the resonator correspond to the ground state and first excited state of the photon-emitting qubit, respectively.

[0037] As will be described later, a similar procedure can be used to generate a state in which a series of propagating microwave photons are entangled in a chain shape. In the embodiment described below, the propagating microwave photons play an important role in generating the cluster state.

[0038] [First embodiment] FIG. 5 shows a schematic diagram of a cluster state generated by using a quantum entanglement generating device according to the first embodiment. In this device, a photon emission quantum bit is used instead of a photon emission resonator. This quantum entanglement generating device includes two entanglement generating quantum bits 6a and 6b, two photon emission quantum bits 7a and 7b coupled to the entanglement generating quantum bits 6a and 6b, respectively, and microwave waveguides 8a and 8b coupled to the photon emission quantum bits 7a and 7b. The system consisting of the entanglement generating quantum bit 6a, the photon emission quantum bit 7a, and the microwave waveguide 8a is called the first column, and the system consisting of the entanglement generating quantum bit 6b, the photon emission quantum bit 7b, and the microwave waveguide 8b is called the second column. A two-qubit gate can be applied between two adjacent entanglement generating quantum bits 6a and 6b.

[0039] A two-qubit gate between the two entanglement generation qubits 6a and 6b generates quantum entanglement between the first and second columns. As described above, the photon emission qubits 7a and 7b can be conditionally excited depending on the state of the entanglement generation qubits. The excitation of the photon emission qubits is then spontaneously emitted into the microwave waveguides 8a and 8b, thereby successively generating propagating microwave photons that are quantum entangled with the entanglement generation qubits. A two-dimensional cluster state is generated by applying a two-qubit gate between the entanglement generation qubits every time a propagating microwave photon is generated.

[0040] FIG. 6 shows a schematic diagram of a quantum entanglement generating device 10 according to a first embodiment. The quantum entanglement generating device 10 includes superconducting quantum bit devices 20a and 20b, a coupled resonator 30, readout resonators 40a and 40b, waveguides 50a and 50b, readout lines 60a and 60b, and a conductor cavity 80. The superconducting quantum bit devices 20a and 20b include an entanglement generating quantum bit and a photon emitting quantum bit, respectively. That is, the quantum entanglement generating device 10 includes the entanglement generating quantum bit 6a and the photon emitting quantum bit 7a of FIG. 5, which are integrated into the superconducting quantum bit device 20a. Similarly, the entanglement generating quantum bit 6b and the photon emitting quantum bit 7b are integrated into the superconducting quantum bit device 20b.

[0041] 6, the readout resonator 40a, the superconducting quantum bit device 20a, the coupled resonator 30, the superconducting quantum bit device 20b, and the readout resonator 40b are arranged in a chain shape on a silicon substrate 70. The readout resonator 40a, the superconducting quantum bit device 20a, the coupled resonator 30, the superconducting quantum bit device 20b, and the readout resonator 40b are fabricated, for example, by dry etching a niobium thin film.

[0042] The superconducting quantum bit device 20a and the coupled resonator 30 are capacitively coupled to each other. Similarly, the superconducting quantum bit device 20b and the coupled resonator 30 are capacitively coupled to each other.

[0043] The superconducting qubit device 20a and the readout resonator 40a are capacitively coupled to each other. Similarly, the superconducting qubit device 20b and the readout resonator 40b are capacitively coupled to each other.

[0044] The superconducting quantum bit device 20a and the waveguide 50a are capacitively coupled to each other. Similarly, the superconducting quantum bit device 20b and the waveguide 50b are capacitively coupled to each other.

[0045] The readout resonator 40a and the readout line 60a are capacitively coupled to each other, and similarly, the readout resonator 40b and the readout line 60b are capacitively coupled to each other.

[0046] The conductor cavity 80 is an aluminum block having a cylindrical hollow inside. The silicon substrate 70 is fixed inside the hollow of the conductor cavity 80. The conductor cavity 80 has through holes at positions corresponding to directly above the readout resonator 40a, directly above the superconducting quantum bit device 20a, directly above the superconducting quantum bit device 20b, and directly above the readout resonator 40b. The readout line 60a, the waveguide 50a, the waveguide 50b, and the waveguide to be the readout line 60b are inserted through these through holes. As a result, the conductor cavity 80 forms an outer conductor, and the coupled resonator 30 and the readout resonators 40a and 40b form inner conductors. As a result, the quantum entanglement generating device 10 has a structure of a coaxial line resonator. The coaxial line resonator has a larger mode volume than a two-dimensional resonator created using a coplanar line or the like, and therefore has the advantage of having a small internal loss. Furthermore, since the coaxial line resonator has a simple structure, it can be easily produced at low cost.

[0047] The readout resonators 40a and 40b are formed of elongated superconducting thin film wires, for example, made by dry etching a niobium thin film. The readout resonators 40a and 40b are used to calibrate the quantum entanglement generation device 10 and to read out the state of the entanglement generation quantum bit, and are not involved in the generation of the actual microwave photon train. Therefore, it should be noted that they are not essential components in this embodiment.

[0048] FIG. 7 illustrates a plan view of a superconducting quantum bit device 20a configured using three electrodes as an example of a superconducting quantum bit device in the quantum entanglement generation device 1 of FIG. 6. The superconducting quantum bit device 20a includes a first electrode 101, a second electrode 102, and a third electrode 103. The first electrode 101 and the second electrode 102 each have a shape of a ring cut in half with a concentric outline. The third electrode 103 has a circular shape. The first electrode 101 and the second electrode 102 are disposed facing each other with the third electrode 103 sandwiched therebetween. The first electrode 101 and the third electrode 103 are joined by a Josephson junction J1. Similarly, the second electrode 102 and the third electrode 103 are joined by a Josephson junction J2.

[0049] The superconducting quantum bit device 20a can function as both an entanglement generation quantum bit and a photon emission quantum bit depending on the vibration mode of the electromagnetic field generated by the first electrode 101, the second electrode 102, and the third electrode 103. For example, when a positive potential is applied to the first electrode 101, a zero potential is applied to the second electrode 102, and a negative potential is applied to the third electrode 103, the mode of the generated electromagnetic field is strongly coupled to the adjacent superconducting quantum bit. Therefore, in this case, the superconducting quantum bit device 20a functions as an entanglement generation quantum bit. On the other hand, when a positive potential is applied to the first electrode 101, a negative potential is applied to the second electrode 102, and a positive potential is applied to the third electrode 103, the mode of the generated electromagnetic field is strongly coupled to the coaxial line used as a waveguide in this embodiment. Therefore, in this case, the superconducting quantum bit device 20a functions as a photon emission quantum bit.

[0050] A conventional superconducting quantum bit, for example a transmon quantum bit, is composed of a circuit in which one Josephson junction and one capacitor are connected in parallel. In this case, there are two electrodes (i.e., one capacitor). Unlike this embodiment, this superconducting quantum bit cannot realize both the functions of an entanglement generation quantum bit and a photon emission quantum bit. In this respect, this embodiment is significantly different from conventional superconducting quantum bits.

[0051] The configuration and operation of the superconducting quantum bit device 20b are the same as those of the above-mentioned superconducting quantum bit device 20a, and therefore a detailed description thereof will be omitted.

[0052] Hereinafter, a procedure for generating a two-dimensional cluster state using the quantum entanglement generation device 1 will be described with reference to Fig. 8 to Fig. 13. The symbols in Fig. 4 apply. The two-dimensional cluster state is generated by the following steps. (Step 1) Initialize the quantum bit to |g> (Figure 8). (Step 2) The following steps (2-1) to (2-5) are repeated “desired photon chain length-1 times” (FIG. 13). (Step 2-1) |g> is semi-excited to |e> (Figure 9). (Step 2-2) A controlled-Z gate is applied between two adjacent quantum bits. (Step 2-3) Excite |e> to |f> (Figure 10). (Step 2-4) |g> is excited to |e> (Figure 11). (Step 2-5) Drive the |f0> → |g1> transition and emit a propagating microwave photon (Figure 12). (Step 3) |g> is semi-excited to |e>. (Step 4) A controlled-Z gate is applied between two adjacent quantum bits. (Step 5) Excite |e> to |f>. (Step 6) Drive the |f0> → |g1> transition and emit a propagating microwave photon.

[0053] FIG. 14 is a flowchart showing the procedure for generating the above cluster state.

[0054] By the above procedure, a two-dimensional cluster state consisting of two microwave photon trains of any length can be generated.

[0055] In the above-described embodiment, the quantum entanglement generating device includes a conductor cavity. However, the present invention is not limited to this, and any suitable housing may be used as long as it can electromagnetically isolate the superconducting qubit and the microwave resonator from the outside world. For example, if a coplanar resonator is used instead of the coaxial line resonator, and a coplanar waveguide is used instead of the coaxial line, implementation without using a conductor cavity is possible.

[0056] In the embodiments described above, the quantum entanglement generation quantum bit and the photon emission quantum bit are integrated into one superconducting quantum bit element. However, this is not limited to this, and a photon emission resonator or a photon emission quantum bit may be provided independently of the quantum entanglement generation quantum bit.

[0057] According to this embodiment, it is possible to realize a device that generates a two-dimensional cluster state of quantum bits.

[0058] [Second embodiment] 15 is a schematic diagram of a quantum entanglement generating device 11 according to a second embodiment. The quantum entanglement generating device 11 includes superconducting quantum bit devices 21a, 21b, and 21c, coupled resonators 31a and 31b, readout resonators 41a, 41b, and 41c, waveguides 51a, 51b, and 51c, readout lines 61a, 61b, and 61c, and a conductor cavity 81 (the readout lines 61b and 61c are omitted to avoid cluttering the drawing). That is, the quantum entanglement generating device 11 includes a superconducting quantum bit device 21c, a coupled resonator 31b, a readout resonator 41c, a waveguide 51c, and a readout line 61c in addition to the configuration of the quantum entanglement generating device 10 in FIG. 6.

[0059] The superconducting quantum bit device 21a and the coupled resonator 31a are capacitively coupled to each other. The superconducting quantum bit device 21b is capacitively coupled to the coupled resonators 31a and 31b. The superconducting quantum bit device 21c and the coupled resonator 31b are capacitively coupled to each other.

[0060] The superconducting quantum bit device 21a and the readout resonator 41a are capacitively coupled to each other. Similarly, the superconducting quantum bit device 21b and the readout resonator 41b are capacitively coupled to each other. Similarly, the superconducting quantum bit device 21c and the readout resonator 41c are capacitively coupled to each other.

[0061] Superconducting quantum bit device 21a and waveguide 51a are capacitively coupled to each other. Similarly, superconducting quantum bit device 21b and waveguide 51b are capacitively coupled to each other. Similarly, superconducting quantum bit device 21c and waveguide 51c are capacitively coupled to each other.

[0062] The readout resonator 41a and the readout line 61a are capacitively coupled to each other. Similarly, the readout resonator 41b and the readout line 61b are capacitively coupled to each other. Similarly, the readout resonator 41c and the readout line 61c are capacitively coupled to each other.

[0063] A two-qubit gate can be applied between adjacent superconducting quantum bit device 21a and superconducting quantum bit device 21b. Similarly, a two-qubit gate can be applied between adjacent superconducting quantum bit device 21b and superconducting quantum bit device 21c.

[0064] The detailed procedure for generating the cluster state is the same as in the first embodiment, so a description thereof will be omitted.

[0065] 6 generates a two-dimensional cluster state consisting of two rows of propagating microwave photons, whereas the quantum entanglement generating device 11 generates a two-dimensional cluster state consisting of three rows of propagating microwave photons. In other words, according to this embodiment, a larger scale two-dimensional cluster state can be generated.

[0066] [Third embodiment] The third embodiment is a quantum entanglement generation method. This method generates a quantum entangled state using the above-mentioned quantum entanglement generation device. This method includes a first step of initializing a quantum bit to a ground state, a second step of semi-exciting the ground state to a first excited state, a third step of exciting the first excited state to a second excited state, a fourth step of exciting the ground state to the first excited state, and a fifth step of driving a transition from the second excited state and then emitting a propagating microwave photon from the resonator to a waveguide. According to this embodiment, a two-dimensional cluster state of propagating microwave photons can be generated using the quantum entanglement generation device.

[0067] [Fourth embodiment] The fourth embodiment is a quantum computer. This quantum computer includes the above-mentioned quantum entanglement generation device. In particular, this quantum computer may perform measurement-based quantum computation, which repeats measurements on the quantum entangled state (cluster state) generated by the above-mentioned quantum entanglement generation device. According to this embodiment, it is possible to realize a quantum computer capable of performing large-scale quantum computation with relatively small-scale hardware.

[0068] FIG. 16 shows a schematic diagram of a quantum computer 12 according to a fourth embodiment. The quantum computer 12 includes a superconducting quantum bit device 22a, a superconducting quantum bit device 22b, a coupled resonator 32, a readout resonator 42a, a readout resonator 42b, and a superconducting delay line 52. The superconducting quantum bit device 22a functions as a quantum bit device for photon absorption and basis conversion. The superconducting quantum bit device 22b functions as a quantum bit device for entanglement generation and photon transmission. The coupled resonator 32 functions as a coupled resonator that mediates a two-qubit gate for generating quantum entanglement between propagating photons emitted at separate times. The readout resonator 42a functions as a readout resonator for the quantum bit for basis conversion. The readout resonator 42b functions as a readout resonator for the quantum bit for photon transmission.

[0069] Quantum computer 12 may execute measurement-type quantum computation, for example, through the following process: In other words, quantum computer 12 may temporarily store the quantum entangled state (cluster state) generated by superconducting quantum bit device 22b as a propagating photon in superconducting delay line 52, absorb the propagating photon in superconducting quantum bit device 22a, perform basis conversion, and then measure the photon, and repeat the measurement while selecting the next measurement basis based on the result of the previous measurement.

[0070] As a modification of quantum computer 12, for example, the following quantum computer may be realized. That is, in this modification, a controlled-Z gate may be applied between superconducting quantum bit device 22a, which absorbs a propagating photon extracted from superconducting delay line 52, and superconducting quantum bit device 22b, to generate quantum entanglement between propagating photons emitted at different times, thereby generating a three-dimensional cluster state using multiplexing in the time direction.

[0071] As a modified example of the quantum computer 12, a quantum computer having an error correction function using a three-dimensional cluster state may be realized.

[0072] The present invention has been described above based on the embodiments. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of each component and each treatment process, and that such modifications are also within the scope of the present invention.

[0073] Any combination of the above-described embodiments and modifications is also useful as an embodiment of the present invention. A new embodiment resulting from the combination has the combined effects of each of the combined embodiments and modifications. [Industrial Applicability]

[0074] INDUSTRIAL APPLICABILITY The present invention can be used in a quantum entanglement generating device, a quantum entanglement generating method, and a quantum computer. [Explanation of symbols]

[0075] 10. Quantum entanglement generation device, 11. Quantum entanglement generation device, 12. Quantum computers, 20a··Superconducting quantum bit device, 20b Superconducting qubit device, 21a··Superconducting qubit device, 21b Superconducting qubit device, 21c··Superconducting qubit device, 22a··Superconducting qubit device, 22b Superconducting qubit device, 30...coupled resonator, 31a...coupled resonator, 31b...coupled resonator, 32...coupled resonator, 40a Readout resonator; 40b Readout resonator; 41a Readout resonator; 41b Readout resonator; 41c Readout resonator; 42a Readout resonator; 42b Readout resonator; 50a...coaxial line, 50b...Coaxial line, 51a...Coaxial line, 51b...Coaxial line, 51c...Coaxial line, 52. Superconducting delay line, 60a Readout line, 60b Readout line, 61a Readout line, 61b Readout line, 61c Readout line, 70··Silicon substrate, 80··conductor cavity, 81 Conductor cavity, 101... first electrode; 102... second electrode; 103... third electrode; J1: Josephson junction J2: Josephson junction.

Claims

1. n quantum bit elements, where n is an integer equal to or greater than 2; a coupled resonator disposed between adjacent quantum bit elements; a waveguide capacitively coupled to each of the quantum bit elements; generating quantum entanglement between adjacent quantum bit elements by operating a two-qubit gate between the adjacent quantum bit elements using the coupled resonator; The quantum bit element generates a two-dimensional cluster state by emitting the quantum entanglement as a propagating microwave photon into the waveguide.

2. 2. The quantum entanglement generating device according to claim 1, wherein each of the n quantum bit elements has three electrodes.

3. 3. The quantum entanglement generating device according to claim 1, wherein the quantum bit element includes a photon emitting quantum bit that transfers the quantum entanglement to a propagating microwave photon and emits the propagating microwave photon into the waveguide.

4. 3. The quantum entanglement generating device according to claim 1, further comprising a photon emitting resonator or a photon emitting quantum bit, which transfers the quantum entanglement to a propagating microwave photon and emits the propagating microwave photon into the waveguide, independent of the quantum bit element.

5. 5. The quantum entanglement generating device according to claim 1, further comprising a readout resonator for reading out the state of the quantum bit element.

6. 3. The quantum entanglement generating device according to claim 2, wherein two of the three electrodes have a shape resembling half of a ring having a concentric outline when viewed from the direction of the waveguide.

7. A conductor cavity is provided with a hollow space extending therethrough; 7. The quantum entanglement generating device according to claim 1, wherein the quantum bit element and the coupled resonator are fixed within the hollow of the conductor cavity.

8. The quantum bit device includes: Initialize the qubit to its ground state, The ground state is half-excited to the first excited state, 8. A quantum entanglement generating device as described in any one of claims 1 to 7, characterized in that after exciting the first excited state to a second excited state, exciting the ground state to the first excited state, and driving a transition from the second excited state, a propagating microwave photon is emitted from the resonator to the waveguide, and the first excited state is semi-excited to the second excited state.

9. 9. The quantum entanglement generating device according to claim 1, wherein the quantum bit element is a superconducting quantum bit element.

10. A quantum entanglement generating method using the quantum entanglement generating device according to claim 1, A quantum entanglement generating method, characterized in that the quantum entanglement generating device executes the steps of initializing a quantum bit to a ground state, semi-exciting the ground state to a first excited state, exciting the first excited state to a second excited state, exciting the ground state to the first excited state, driving a transition from the second excited state, and then emitting a propagating microwave photon from the resonator to a waveguide, and semi-exciting the first excited state to the second excited state.

11. A quantum computer comprising the quantum entanglement generating device according to any one of claims 1 to 9.

12. 12. The quantum computer according to claim 11, which executes measurement-based quantum computation by repeatedly measuring the quantum entangled state generated by the quantum entanglement generation device.

13. The quantum computer of claim 11, characterized in that the quantum entanglement state generated by the quantum entanglement generation device is temporarily stored as propagating photons from the waveguide to a superconducting delay line, and then is again caused to interact with the photon generation device, and a measurement device having a basis is used to repeat measurements while selecting a next measurement basis based on the previous measurement result, thereby performing measurement-type quantum computation.

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