Control method for quantum computing systems and quantum devices

Abstract

A quantum computation system (1) has a quantum device (21), and a control unit (10) that controls the quantum device. The quantum device has a first qubit (100), a second qubit (200), a magnetic flux qubit (300) for coupling that can couple with the first qubit and the second qubit, and a first magnetic flux application unit (340) that applies a magnetic flux to the magnetic flux qubit for coupling. The control unit causes the first magnetic flux application unit to apply a first magnetic flux, which is provided with a first time modulation, during a first period, and apply a second magnetic flux, which is provided with a second time modulation differing from the first time modulation, during a second time period differing from the first time period. This quantum computation system can be used, for example, in quantum computing.

Description

Technical Field

[0001] The present disclosure relates to a quantum computing system and a method for controlling a quantum device.

Background Art

[0002] Conventionally, a quantum computing system capable of coupling two flux qubits using flux qubits has been proposed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a conventional quantum computing system, the coupling between qubits cannot be cut, and there is a risk of interaction due to unnecessary coupling.

[0005] An object of the present disclosure is to provide a quantum computing system and a method for controlling a quantum device capable of turning off the coupling between two qubits.

Means for Solving the Problems

[0006] According to one embodiment of the present disclosure, a quantum computing system is provided, comprising a quantum device and a control unit for controlling the quantum device, wherein the quantum device comprises a first qubit, a second qubit, a coupling flux qubit capable of coupling with the first qubit and the second qubit, and a first flux application unit for applying a magnetic flux to the coupling flux qubit, and the control unit causes the first flux application unit to apply a first magnetic flux with first time modulation during a first period, and to apply a second magnetic flux with second time modulation different from the first time modulation during a second period different from the first period. [Effects of the Invention]

[0007] According to this disclosure, the coupling of two qubits can be turned off. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a block diagram showing a quantum computing system according to the first embodiment. [Figure 2] Figure 2 is a circuit diagram showing a quantum device in the first embodiment. [Figure 3] Figure 3 is a timing chart showing the time variation of the magnetic flux applied to the SQUID during the first period. [Figure 4] Figure 4 is a timing chart showing the time variation of the magnetic flux applied to the SQUID during the second period. [Figure 5] Figure 5 is a diagram (part 1) showing the energy states of a magnetic flux qubit. [Figure 6] Figure 6 is a diagram (part 2) showing the energy states of a magnetic flux qubit. [Figure 7] Figure 7 shows the results of the first simulation (part 1). [Figure 8] Figure 8 shows the results of the first simulation (part 2). [Figure 9] Figure 9 is a timing chart (part 1) showing the time variation of the magnetic flux applied to the SQUID during the second period in the second simulation. [Figure 10] Figure 10 is a timing chart (part 2) showing the time variation of the magnetic flux applied to the SQUID in the second period in the second simulation. [Figure 11] Figure 11 is a timing chart (part 3) showing the time variation of the magnetic flux applied to the SQUID in the second period in the second simulation. [Figure 12] Figure 12 is a timing chart (part 4) showing the time variation of the magnetic flux applied to the SQUID in the second period in the second simulation. [Figure 13] Figure 13 is a diagram (part 1) showing the results of the second simulation. [Figure 14] Figure 14 is a diagram (part 2) showing the results of the second simulation. [Figure 15] Figure 15 is a diagram (part 1) showing the results of the third simulation. [Figure 16] Figure 16 is a diagram (part 2) showing the results of the third simulation. [Figure 17] Figure 17 is a block diagram showing a quantum computing system according to the second embodiment. [Figure 18] Figure 18 is a circuit diagram showing a quantum device in the second embodiment. [Figure 19] Figure 19 is a timing chart showing the time variation of the current generated by the current source and the magnetic flux applied to the SQUID in the first period. [Figure 20] Figure 20 is a timing chart showing the time variation of the current generated by the current source and the magnetic flux applied to the SQUID in the second period. [Figure 21] Figure 21 is a diagram showing the relationship between the magnetic flux of the coupled flux qubit and the internal current of the charge qubit coupled thereto. [Figure 22] Figure 22 is a plan view showing a first example of the configuration of a quantum device in the second embodiment. [Figure 23] Figure 23 is a cross-sectional view showing a first example of the configuration of a quantum device in the second embodiment. [Figure 24] Figure 24 is a cross-sectional view showing a portion of the qubit substrate in the second embodiment. [Figure 25] Figure 25 is a cross-sectional view showing a portion of another qubit substrate in the second embodiment. [Figure 26] Figure 26 is a plan view showing a second example of the configuration of a quantum device in the second embodiment. [Figure 27] Figure 27 is a cross-sectional view showing a second example of the configuration of a quantum device in the second embodiment. [Figure 28] Figure 28 is a plan view showing a third example of the configuration of the quantum device in the second embodiment. [Modes for carrying out the invention]

[0009] Embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configuration will be denoted by the same reference numerals to avoid redundant descriptions.

[0010] (First Embodiment) First, the first embodiment will be described. The first embodiment relates to a quantum computing system. Figure 1 is a block diagram showing the quantum computing system according to the first embodiment.

[0011] The quantum computing system 1 according to the first embodiment includes a control unit 10 and a quantum device 21. The control unit 10 controls the quantum device 21.

[0012] The control unit 10 is a computer and includes a bus 11, an input device 12, an output device 13, a storage device 14, a memory device 15, an arithmetic processing unit 16, and an interface device 17. The input device 12, output device 13, storage device 14, memory device 15, arithmetic processing unit 16, and interface device 17 are connected to the bus 11. The input device 12, output device 13, storage device 14, memory device 15, arithmetic processing unit 16, and interface device 17 are interconnected via the bus 11.

[0013] The input device 12 is a device for inputting various types of information and can be implemented as, for example, a keyboard or a pointing device. The output device 13 is for outputting various types of information and can be implemented as, for example, a display. The interface device 17 includes a LAN card or the like and is used to connect to a network.

[0014] The memory device 14 stores a control program for controlling the quantum device 21. The memory device 15 reads the control program from the memory device 14 and stores it when the quantum computing system 1 is started. The arithmetic processing unit 16 then performs various processes as described later, according to the control program stored in the memory device 15.

[0015] Figure 2 is a circuit diagram showing the quantum device 21. The quantum device 21 includes a flux qubit 100, a flux qubit 200, and a coupling flux qubit 300. The coupling flux qubit 300 can be inductively coupled with the flux qubits 100 and 200. Here, an example of inductive coupling between the flux qubit 100 and the flux qubit 200 is disclosed, but the coupling can also be capacitive coupling, or a combination of inductive and capacitive coupling. The quantum device 21 further includes flux application units 130, 140, 230, 240, 330, and 340.

[0016] The magnetic flux qubit 100 has a main loop 110 and a superconducting quantum interference device (SQUID) 120. 0 has a Josephson junction element 121, a Josephson junction element 122, an inductor 123, and an inductor 124 connected in a ring and series in this order. The main loop 110 has inductors 112 and 113 connected in series with each other. The end of inductor 112 opposite to inductor 113 is connected between Josephson junction elements 121 and 122. The end of inductor 113 opposite to inductor 112 is connected between inductor 123 and inductor 124. For example, the flux qubit 100 contains niobium as a superconducting material. The flux qubit 100 is an example of a first qubit and a first flux qubit.

[0017] The flux qubit 200 has a main loop 210 and a SQUID 220. The SQUID 220 has a Josephson junction element 221, a Josephson junction element 222, an inductor 223, and an inductor 224 connected in a ring and series in this order. The main loop 210 has inductors 212 and 213 connected in series with each other. The end of inductor 213 opposite to inductor 212 is connected between Josephson junction elements 221 and 222. The end of inductor 212 opposite to inductor 213 is connected between inductors 223 and 224. For example, the flux qubit 200 contains niobium as the superconducting material. The flux qubit 200 is an example of a second qubit and a second flux qubit.

[0018] The coupling flux qubit 300 has a main loop 310 and a SQUID 320. The SQUID 320 has a Josephson junction element 321, a Josephson junction element 322, an inductor 323, and an inductor 324 connected in a ring and in series in this order. The main loop 310 has inductors 311, 313, and 312 connected in series in this order. The end of inductor 312 opposite to inductor 313 is connected between Josephson junction elements 321 and 322. The end of inductor 311 opposite to inductor 313 is connected between inductor 323 and 324. Inductor 311 and inductor 112 are inductively coupled to each other, and inductor 312 and inductor 212 are inductively coupled to each other.

[0019] The magnetic flux application unit 130 includes inductors 131 and 132 and a pulse signal generator 133. Inductors 131 and 132 are connected in series with each other. The end of inductor 132 opposite to inductor 131 is grounded. The pulse signal generator 133 is connected between the end of inductor 131 opposite to inductor 132 and ground. Inductor 131 and inductor 123 are inductively coupled with each other. Inductor 132 and inductor 124 are inductively coupled with each other. The magnetic flux application unit 130 is an example of a second magnetic flux application unit.

[0020] The magnetic flux application unit 230 includes inductors 231 and 232 and a pulse signal generator 233. Inductors 231 and 232 are connected in series with each other. The end of inductor 231 opposite to inductor 232 is grounded. The pulse signal generator 233 is connected between the end of inductor 232 opposite to inductor 231 and ground. Inductor 231 and inductor 223 are inductively coupled with each other. Inductor 232 and inductor 224 are inductively coupled with each other. The magnetic flux application unit 230 is an example of a third magnetic flux application unit.

[0021] The magnetic flux application unit 330 includes inductors 331 and 332 and a pulse signal generator 333. Inductors 331 and 332 are connected in series with each other. The end of inductor 332 opposite to inductor 331 is grounded. The pulse signal generator 333 is connected between the end of inductor 331 opposite to inductor 332 and ground. Inductor 331 and inductor 323 are inductively coupled with each other. Inductor 332 and inductor 324 are inductively coupled with each other. The magnetic flux application unit 330 is an example of a first magnetic flux application unit.

[0022] The magnetic flux application unit 140 includes an inductor 141 and a DC power supply 142. The positive terminals of the inductor 141 and the DC power supply 142 are connected to each other. The end of the inductor 141 opposite to the DC power supply 142 is grounded. The negative terminal of the DC power supply 142 is grounded. Inductor 141 and inductor 113 are inductively coupled to each other. The main loop 310 is an example of annular wiring.

[0023] The magnetic flux application unit 240 includes an inductor 241 and a DC power supply 242. The positive terminals of the inductor 241 and the DC power supply 242 are connected to each other. The end of the inductor 241 opposite to the DC power supply 242 is grounded. The negative terminal of the DC power supply 242 is grounded. The inductor 241 and the inductor 213 are inductively coupled to each other.

[0024] The magnetic flux application unit 340 includes an inductor 341 and a DC power supply 342. The positive terminals of the inductor 341 and the DC power supply 342 are connected to each other. The end of the inductor 341 opposite to the DC power supply 342 is grounded. The negative terminal of the DC power supply 342 is grounded. Inductor 341 and inductor 313 are inductively coupled to each other.

[0025] The control unit 10 turns on the coupling between magnetic flux qubit 100 and magnetic flux qubit 200 during the first period, and turns off the coupling between magnetic flux qubit 100 and magnetic flux qubit 200 during the second period. Figure 3 is a timing chart showing the time change of magnetic flux applied to SQUID 120, 220, and 320 during the first period. Figure 4 is a timing chart showing the time change of magnetic flux applied to SQUID 120, 220, and 320 during the second period. Figures 5 and 6 show the energy states of the magnetic flux qubits. In Figures 3 and 4, Φ120 represents the magnetic flux applied to SQUID 120, Φ220 represents the magnetic flux applied to SQUID 220, and Φ320 represents the magnetic flux applied to SQUID 320. Φ0 is the magnetic flux quantum. The horizontal axis in Figures 5 and 6 represents the magnetic flux applied to the main loop.

[0026] In Figure 2, several inductors, such as inductors 123 and 124, are disclosed as individual inductors, but an inductor formed integrally with these may also be used. Furthermore, several DC power supplies, such as DC power supply 142, are examples of power supplies, and other forms of power supplies, such as arbitrary waveform sources, may also be used. Also, the coupling between the flux qubit 200 and the coupling flux qubit 300 is indirect coupling, and may be inductive coupling, capacitive coupling, or other types of coupling.

[0027] As shown in Figure 3, during the first period, under the control of the control unit 10, the magnetic fluxes Φ120, Φ220, and Φ320 are all 0 (Wb) until time t12, rise to Φ0 from time t12 to time t13, and then become Φ0 from time t13 onward. The rise in magnetic flux Φ320 does not necessarily have to be linear with respect to time. Note that the energy potential of the coupling magnetic flux qubit 300 is equivalent in the cases where Φ=0 (Wb) and Φ=Φ0. Therefore, the magnetic flux Φ320 may be maintained at Φ0 until time t12, then lowered to 0 (Wb) between time t12 and time t13, and controlled to be 0 (Wb) from time t13 onward.

[0028] As shown in Figure 4, in the second period, under the control of the control unit 10, both magnetic flux Φ120 and Φ220 are 0 (Wb) until time t22, rise to Φ0 from time t22 to time t23, and become Φ0 from time t23. On the other hand, under the control of the control unit 10, magnetic flux Φ320 is 0 (Wb) until time t21, which is before time t22, rises to 0.5Φ0 from time t21 to time t22, and becomes 0.5Φ0 from time t22. Alternatively, magnetic flux Φ320 may be controlled to become 0.5Φ0 before time t22.

[0029] In all of the flux qubits, including flux qubit 100, flux qubit 200, and coupling flux qubit 300, when the applied magnetic flux is Φ0, there are two ground states, as shown in Figure 6. Furthermore, in all of the flux qubits, including flux qubit 100, flux qubit 200, and coupling flux qubit 300, when the applied magnetic flux is 0.5Φ0, there is one ground state, as shown in Figure 5. Here, as shown in Figure 5, the energy value of the ground state when there is one ground state is low. On the other hand, as shown in Figure 6, the energy value of the ground state when there are two ground states is higher than the state in Figure 5. During the period between time t22 and time t23, if coupling flux qubit 300 is in the state shown in Figure 5, and there is an energy potential difference between it and flux qubit 100 and flux qubit 200, quantum tunneling becomes less likely to occur between flux qubit 100 and flux qubit 200. Therefore, it becomes easier to control the coupling between magnetic flux qubit 100 and magnetic flux qubit 200 to be turned off.

[0030] Therefore, in the first period, the magnetic fluxes Φ120, Φ220, and Φ320 simultaneously become 0.5Φ0, and when the magnetic fluxes Φ120, Φ220, and Φ320 are 0.5Φ0, the magnetic flux qubit 100 and the magnetic flux qubit 200 are in a superposition state. Subsequently, when the magnetic fluxes Φ120, Φ220, and Φ320 rise to Φ0, the magnetic flux qubits 100 and 200 return to a classical state, and the solution converges. For this reason, in the first period, the coupling between the magnetic flux qubit 100 and the magnetic flux qubit 200 is turned ON.

[0031] On the other hand, in the second period, magnetic flux Φ320 becomes 0.5Φ0 before magnetic flux Φ120 and Φ220 become 0.5Φ0, and there is no period in which magnetic flux qubit 100 and magnetic flux qubit 200 are in a superposition state. Therefore, in the second period, the coupling between magnetic flux qubit 100 and magnetic flux qubit 200 is turned off.

[0032] (First Simulation) Next, the first simulation relating to the first embodiment performed by the inventor of the present invention will be described. In the first simulation, the magnetic flux application unit 140 was controlled to make the magnetic flux qubit 100 more likely to be in the "1" state, and the magnetic flux application unit 240 was controlled to apply two types of biases to the magnetic flux qubit 200. With one bias, the probability of the magnetic flux qubit 200 being in the "1" state was set to 50%, and with the other bias, the magnetic flux qubit 200 was made more likely to be in the "0" state. Then, the above-described first and second periods were controlled. For reference, the magnetic flux Φ120 and Φ220 were changed in the same way as in the first period, assuming that there was no coupling magnetic flux qubit 300.

[0033] In the first simulation, the number of trials was set to 1000. The results of the first simulation are shown in Figures 7 and 8. Figure 7 shows the results when the probability of the flux qubit 200 being in the "1" state is 50%, and Figure 8 shows the results when the flux qubit 200 is more likely to be in the "0" state. In Figures 7 and 8, "11" indicates that both flux qubit 100 and flux qubit 200 are in the "1" state, and "10" indicates that flux qubit 100 is in the "1" state and flux qubit 200 is in the "0" state.

[0034] As shown in Figure 7, even when the probability of the flux qubit 200 being in the "1" state was set to 50%, when control was performed during the first period, the flux qubit 200 also entered the "1" state with a high probability. Furthermore, when control was performed during the second period, the same results as when the coupling flux qubit 300 was not activated were obtained.

[0035] As shown in Figure 8, even when the flux qubit 200 was configured to be prone to being in the "0" state, when the control was performed during the first period, the flux qubit 200 also entered the "1" state with a high probability. Furthermore, when the control was performed during the second period, the same results as when the coupling flux qubit 300 was not activated were obtained.

[0036] From the above, it can be said that in the first period, the coupling between magnetic flux qubit 100 and magnetic flux qubit 200 is in the ON state, and in the second period, the coupling between magnetic flux qubit 100 and magnetic flux qubit 200 is in the OFF state.

[0037] (Second Simulation) Next, a second simulation relating to the first embodiment performed by the present inventor will be described. In the second simulation, various controls were performed as control of the second period. Also, in the second simulation, as in the first simulation, the magnetic flux application unit 140 was controlled to make the magnetic flux qubit 100 more likely to be in the "1" state, and two types of biases were applied to the magnetic flux qubit 200 by controlling the magnetic flux application unit 240. With one bias, the probability of the magnetic flux qubit 200 being in the "1" state was set to 50%, and with the other bias, the probability of the magnetic flux qubit 200 being in the "0" state was set to 70%. Figures 9 to 12 are timing charts showing the time change of magnetic flux applied to SQUID 120, 220, and 320 during the second period in the second simulation. In the control patterns shown in Figures 9 to 12, the time change of magnetic flux applied to SQUID 120 and 220 is the same as control pattern C shown in Figure 4.

[0038] In control pattern A shown in Figure 9, the magnetic flux Φ320 was kept at 0 (Wb) at all times.

[0039] In control pattern B shown in Figure 10, the magnetic flux Φ320 was 0 (Wb) until time t22, increased to 0.5Φ0 from time t22 to time t23, and then set to 0.5Φ0 from time t23 onwards.

[0040] In control pattern C shown in Figure 4, the magnetic flux Φ320 was 0 (Wb) until time t21, increased to 0.5Φ0 from time t21 to time t22, and then set to 0.5Φ0 from time t22 onwards.

[0041] In control pattern D shown in Figure 11, the magnetic flux Φ320 was Φ0 until time t22, then decreased to 0.5Φ0 from time t22 to time t23, and then set to 0.5Φ0 from time t23 onwards.

[0042] In control pattern E shown in Figure 12, the magnetic flux Φ320 is Φ0 until time t22, then decreases to 0(Wb) from time t22 to time t23, and then remains 0(Wb) from time t23 onwards.

[0043] In the second simulation, the number of trials was set to 1000. The results of the second simulation are shown in Figures 13 and 14. Figure 13 shows the results when the probability of the flux qubit 200 being in the "1" state is 50%, and Figure 14 shows the results when the probability of the flux qubit 200 being in the "0" state is 70%. In Figures 13 and 14, "11" indicates that both flux qubit 100 and flux qubit 200 are in the "1" state, and "10" indicates that flux qubit 100 is in the "1" state and flux qubit 200 is in the "0" state.

[0044] As shown in Figures 13 and 14, in control patterns B and C, results similar to those obtained when the coupling flux Φ120 and Φ220 were changed in the same way as in the first period were obtained, assuming the absence of the coupling flux qubit 300. In particular, control pattern C yielded results closest to those obtained when the coupling flux qubit 300 was absent. In control patterns B and C, where the magnetic flux is biased so that the energy potential of the coupling flux qubit 300 has only one ground state as shown in Figure 5, the system is more resistant to sudden changes in magnetic flux from external sources and exhibits superior stability compared to control pattern A, where the magnetic flux is unbiased. Furthermore, in control patterns B and C, the quantum tunneling effect is reduced because there is a difference in energy potential between the magnetic flux qubits 100 and 200 and the coupling flux qubit 300 during the period between time t22 and time t23. Therefore, control patterns B and C make it easier to control the coupling between flux qubit 100 and flux qubit 200 to be turned off than in the case of control pattern A.

[0045] (Third simulation) Next, a third simulation relating to the first embodiment performed by the present inventor will be described. In the third simulation, the results were compared between control patterns A and C and the case where there is no coupling flux qubit 300, while varying the bias current I240 flowing through the magnetic flux application unit 240. The number of trials in the third simulation was set to 1000. The results of the third simulation are shown in Figures 15 and 16. The vertical axis in Figure 15 shows the probability that both the flux qubit 100 and the flux qubit 200 are in the "1" state, which is the "11" state. The vertical axis in Figure 16 shows the probability that the flux qubit 100 is in the "1" state and the flux qubit 200 is in the "0" state, which is the "10" state.

[0046] As shown in Figures 15 and 16, control pattern C yielded results closer to those obtained when the coupling flux qubit 300 was absent, compared to control pattern A.

[0047] Based on the above simulation results, control pattern C is particularly preferred. Considering the changes in the energy potential of magnetic flux qubits 100 and 200, it is preferable that magnetic flux Φ320 reaches 0.4Φ0 to 0.6Φ0 before magnetic fluxes Φ120 and Φ220 reach 0.3Φ0.

[0048] The change in magnetic flux may occur over a time period of, for example, 0.1 ns to 100 ms, or 1 μs to 1 ms.

[0049] (Second Embodiment) Next, a second embodiment will be described. The second embodiment relates to a quantum computing system. Figure 17 is a block diagram showing the quantum computing system according to the second embodiment.

[0050] The quantum computing system 2 according to the second embodiment includes a control unit 10 and a quantum device 21. The control unit 10 controls the quantum device 21. The configuration of the control unit 10 is the same as in the first embodiment, except for the contents of the control program.

[0051] Figure 18 is a circuit diagram showing the quantum device 22. The quantum device 22 has a charge qubit 400, a charge qubit 500, and a coupling flux qubit 300. The coupling flux qubit 300 can be inductively coupled with the charge qubits 400 and 500. The quantum device 22 further has charge supply units 430 and 530. The configuration of the coupling flux qubit 300 is the same as in the first embodiment.

[0052] The charge qubit 400 is, for example, a transmon qubit and has a Josephson junction element 401 and a capacitor 402 connected in a ring shape to each other. The charge qubit 400 further has an inductor 403 electrically connected in parallel to the Josephson junction element 401 and the capacitor 402. Inductor 403 and inductor 311 are inductively coupled to each other. One end of the Josephson junction element 401, capacitor 402 and inductor 403 is grounded. For example, the charge qubit 400 contains titanium nitride as a superconducting material. The charge qubit 400 is an example of a first qubit and a first charge qubit. Note that grounding one end of the Josephson junction element 401, capacitor 402 and inductor 403 is not essential, and even if grounded, it may be connected to the ground surface via capacitance.

[0053] A charge supply unit 430 is connected to the other ends of the Josephson junction element 401, capacitor 402, and inductor 403. The charge supply unit 430 has a current source 431 and a capacitor 432. Capacitor 432 is connected between one end of the current source 431 and the Josephson junction element 401, capacitor 402, and inductor 403. The other end of the current source 431 is grounded.

[0054] The charge qubit 500 is, for example, a transmon qubit and has a Josephson junction element 501 and a capacitor 502 connected in a ring shape to each other. The charge qubit 500 further has an inductor 503 electrically connected in parallel to the Josephson junction element 501 and the capacitor 502. Inductor 503 and inductor 312 are inductively coupled to each other. One end of the Josephson junction element 501, capacitor 502 and inductor 503 is grounded. For example, the charge qubit 500 contains titanium nitride as a superconducting material. The charge qubit 500 is an example of a second qubit and a second charge qubit.

[0055] A charge supply unit 530 is connected to the other ends of the Josephson junction element 501, capacitor 502, and inductor 503. The charge supply unit 530 has a current source 531 and a capacitor 532. Capacitor 532 is connected between one end of the current source 531 and the Josephson junction element 501, capacitor 502, and inductor 503. The other end of the current source 531 is grounded.

[0056] The control unit 10 turns on the coupling between the charge qubit 400 and the charge qubit 500 during the first period, and turns off the coupling between the charge qubit 400 and the charge qubit 500 during the second period. Figure 19 is a timing chart showing the time evolution of the current generated by the current source 431, the current generated by the current source 531, and the magnetic flux applied to the SQUID 320 during the first period. Figure 20 is a timing chart showing the time evolution of the current generated by the current source 431, the current generated by the current source 531, and the magnetic flux applied to the SQUID 320 during the second period. In Figures 19 and 20, I431 represents the current generated by the current source 431, I531 represents the current generated by the current source 531, and Φ320 represents the magnetic flux applied to the SQUID 320.

[0057] The case where a gate operation is performed on the charge qubit 400 with the coupling between the charge qubit 400 and the charge qubit 500 in the ON state will be explained using Figure 19. In this example, the period from time t31 to time t32 corresponds to the first period. As shown in Figure 19, in the first period, the control unit 10 controls the current source 431 to generate a current for the gate operation, including the time from time t31 to time t32. While the current source 431 is generating current, the gate operation of the charge qubit 400 is performed. Under the control of the control unit 10, the magnetic flux Φ320 becomes 0.5Φ0 from time t30 to time t31. The current generated by the current source 531 is always 0 (A). Next, during the period between time t31 and time t32, the magnetic flux Φ320 is set to 0 (Wb). During this period, the coupling between the charge qubit 400 and the charge qubit 500 is in the ON state. After that, from time t32 onward, the magnetic flux Φ320 is set to 0.5Φ0 again. It is preferable to return the magnetic flux Φ320 to 0.5Φ0 after time t32, but the magnetic flux Φ320 may be maintained at 0(Wb) even after time t32. Furthermore, since the case where Φ=0(Wb) and the case where Φ=Φ0 are equivalent in terms of the energy potential of the coupling flux qubit 300, the magnetic flux Φ320 may be set to Φ=Φ0 during the period between time t31 and time t32.

[0058] Next, we will explain the case where the coupling between the charge qubit 400 and the charge qubit 500 is turned off and a gate operation is performed on the charge qubit 400, using Figure 20. In this example, the period from time t31 to time t32 corresponds to the second period. As shown in Figure 20, the control unit 10 controls the current source 431 to generate a current for the gate operation so that it includes the period from time t31 to time t32. While the current source 431 is generating current, the gate operation of the charge qubit 400 is performed. The control unit 10 controls the magnetic flux Φ320 to 0.5Φ0 for the period from time t31 to time t32 (the second period). The current generated by the current source 531 is always 0 (A). Note that Figure 20 also shows an example where the magnetic flux Φ320 is controlled to be 0.5Φ0 for the period from time t30 to time t31 and for the period from time t32 onwards.

[0059] As described above, in the coupling flux qubit 300, when the applied magnetic flux is 0 (Wb) or Φ0, there are two ground states, as shown in Figure 6, and when the applied magnetic flux is 0.5Φ0, there is one ground state, as shown in Figure 5. Here, we will explain that when the applied magnetic flux in the coupling flux qubit 300 is 0 (Wb) or Φ0, the coupling can be controlled to the ON state, and when the applied magnetic flux is 0.5Φ0, the coupling can be controlled to the OFF state.

[0060] Figure 21 shows the relationship between the magnetic flux of the coupling flux qubit 300 and the internal current of the coupled charge qubit 400. Figure 21 shows the results obtained by performing a simulation with the resonance frequencies of the charge qubit 400 and charge qubit 500 set to 8.1 GHz. When an 8.1 GHz excitation signal was applied to the charge qubit 500 from a current source 531 as a signal source, the internal state of the charge qubit 400 was calculated and the internal current Ip was observed. At that time, the intensity of the signal applied to the coupling flux qubit 300 (output signal of pulse signal generator 333) was modulated.

[0061] The vertical axis of Figure 21 shows the internal current Ip normalized according to the signal strength (magnetic flux applied by the pulse signal generator 333), with the internal current Ip when the signal magnitude is 0 as the reference. It can be seen that when the magnetic flux is 0 (Wb), the coupling between charge qubit 400 and charge qubit 500 is in the ON state, while when the magnetic flux is 0.5Φ0, the coupling is in the OFF state.

[0062] Simulations confirmed that even when Xmon was used instead of charge qubit 400 and charge qubit 500, the resonance frequency differed, but the trend remained the same as in Figure 21.

[0063] Xmon is a flux qubit composed of single-layer wiring, and for example, includes niobium, titanium nitride, or niobium nitride as a superconducting material. Furthermore, the coupling between the coupling flux qubit 300 and the charge qubits 400 and 500 may be capacitive coupling.

[0064] (First example of the configuration of quantum device 22) Next, we will describe a first example of the configuration of the quantum device 22. Figure 22 is a plan view showing a first example of the configuration of the quantum device 22. Figure 23 is a cross-sectional view showing a first example of the configuration of the quantum device 22. Figure 23 corresponds to a cross-sectional view along the line XXIII-XXIII in Figure 22.

[0065] As shown in Figures 22 and 23, the first example of quantum device 22A includes a qubit substrate 30 and a qubit substrate 40. The qubit substrates 30 and 40 are joined to each other via bumps 25. For example, the qubit substrate 30 is flip-chip bonded to the qubit substrate 40. For example, the bumps 25 are connected to a ground layer provided on the qubit substrate 30 and a ground layer provided on the qubit substrate 40.

[0066] The qubit substrate 30 includes a coupling flux qubit 300, a magnetic flux application unit 330, and a magnetic flux application unit 340. The qubit substrate 30 has a qubit region 31 on which the coupling flux qubit 300 is arranged.

[0067] Although not shown in Figure 23, the qubit substrate 40 may have wiring and electrodes connected to the magnetic flux qubits 100 and 200. The qubits are not limited to magnetic flux qubits, but may be other types of qubits. The qubit substrate 40 may also have a resonator for observing the state of the qubits and electrodes for manipulating the state of the qubits. Furthermore, a path for introducing a signal to manipulate the state of the qubits may be formed from the edge of the qubit substrate 40 by wire bonding, or a through hole may be provided in the qubit substrate 40 to supply the signal from the back surface of the qubit substrate 40. The qubit substrate 30 may have a path for supplying the signal at its edge or back surface. Alternatively, it may be connected to a pattern provided on the qubit substrate 40 side via bumps and supplied from the back surface through the edge or through hole of the qubit substrate 40.

[0068] Figure 24 is a cross-sectional view showing a part of the qubit substrate 30. The qubit substrate 30 includes, for example, a substrate 80, wiring layers 81, 82, 83, and 84, insulating layers 85, 86, and 87, and a Josephson junction element 88. The wiring layer 81 is provided on the substrate 80, and the Josephson junction element 88 is provided on the wiring layer 81. The insulating layer 85 is provided on the substrate 80 so as to cover the Josephson junction element 88 and the wiring layer 81. The wiring layer 82 is provided on the insulating layer 85 so as to be in contact with the Josephson junction element 88 through an opening formed in the insulating layer 85. The insulating layer 86 is provided on the insulating layer 85 so as to cover the wiring layer 82. The wiring layer 83 is provided on the insulating layer 86. The insulating layer 87 is provided on the insulating layer 86 so as to cover the wiring layer 83. The wiring layer 84 is provided on the insulating layer 87 so as to be in contact with the wiring layer 83 through an opening formed in the insulating layer 87. The materials of the wiring layers 81, 82, 83, and 84 are materials that can act as superconductors, such as niobium or niobium nitride. The materials of the insulating layers 85, 86, and 87 are, for example, silicon oxide.

[0069] The Josephson junction element 88 includes, for example, an aluminum film 88A, an aluminum oxide film 88B, and an aluminum film 88C. The aluminum film 88A is connected to the wiring layer 81. The aluminum oxide film 88B is provided on top of the aluminum film 88A. The aluminum film 88C is provided on top of the aluminum oxide film 88B, and the wiring layer 82 is in contact with the aluminum film 88C. The Josephson junction element 88 corresponds to Josephson junction elements 321 and 322.

[0070] The qubit substrate 40 includes a charged qubit 400, a charge supply unit 430, a charged qubit 500, and another charge supply unit 530. The qubit substrate 40 has a qubit region 41 on which the charged qubit 400 is located, and a qubit region 42 on which the charged qubit 500 is located.

[0071] Figure 25 is a cross-sectional view showing a part of the qubit substrate 40. The qubit substrate 40 includes, for example, a substrate 90, wiring layers 91 and 92, and an insulating layer 93. The wiring layer 91 is provided on the substrate 90, and the insulating layer 93 covers the surface of the wiring layer 91. The wiring layer 92 is provided on the insulating layer 93. The wiring layers 91 and 92 and the insulating layer 93 constitute Josephson junction elements corresponding to Josephson junction elements 401 and 501.

[0072] The inductor 311 of the coupling flux qubit 300 and the inductor 403 of the charge qubit 400 face each other and are inductively coupled. The inductor 312 of the coupling flux qubit 300 and the inductor 503 of the charge qubit 500 face each other and are inductively coupled.

[0073] For example, the inductors 311 and 312 of the coupling flux qubit 300 are connected to the wiring layer 84, a portion of the wiring layer 84 and the inductor 403 are inductively coupled to each other, and another portion of the wiring layer 84 and the inductor 503 are inductively coupled to each other.

[0074] (Second example of the configuration of quantum device 22) Next, a second example of the configuration of the quantum device 22 will be described. Figure 26 is a plan view showing the second example of the configuration of the quantum device 22. Figure 27 is a cross-sectional view showing the second example of the configuration of the quantum device 22. Figure 27 corresponds to a cross-sectional view along the line XXVII-XXVII in Figure 26.

[0075] As shown in Figures 26 and 27, the second example of quantum device 22B includes a qubit substrate 30, a qubit substrate 50, and a qubit substrate 60. The qubit substrate 30 and the qubit substrate 50 are joined to each other via bumps 26, and the qubit substrate 30 and the qubit substrate 60 are joined to each other via bumps 27. For example, the qubit substrate 30 is flip-chip bonded to the qubit substrates 50 and 60. For example, bump 26 is connected to a ground layer provided on the qubit substrate 30 and a ground layer provided on the qubit substrate 50, and bump 27 is connected to a ground layer provided on the qubit substrate 30 and a ground layer provided on the qubit substrate 60.

[0076] The qubit substrate 50 includes a charged qubit 400 and a charge supply unit 430. The qubit substrate 50 has a qubit region 51 on which the charged qubit 400 is located. The qubit substrate 60 includes a charged qubit 500 and a charge supply unit 530. The qubit substrate 60 has a qubit region 61 on which the charged qubit 500 is located.

[0077] The inductor 311 of the coupling flux qubit 300 and the inductor 403 of the charge qubit 400 face each other and are inductively coupled. The inductor 312 of the coupling flux qubit 300 and the inductor 503 of the charge qubit 500 face each other and are inductively coupled.

[0078] For example, the inductors 311 and 312 of the coupling flux qubit 300 are connected to the wiring layer 84, a portion of the wiring layer 83 or 84 and the inductor 403 are inductively coupled to each other, and another portion of the wiring layer 83 or 84 and the inductor 503 are inductively coupled to each other.

[0079] Although not shown in Figure 27, the qubit substrate 50 and qubit substrate 60 may have qubits or wiring and electrodes directly or indirectly connected to the qubits. Furthermore, the qubit substrate 50 and qubit substrate 60 may have resonators for observing the state of the qubits or electrodes for manipulating the state of the qubits. Additionally, a path for introducing signals to manipulate the state of the qubits may be formed from the ends of the qubit substrate 50 and qubit substrate 60 by wire bonding. 5 Through holes may be provided in the quantum bit substrate 50 and quantum bit substrate 60 to supply signals from the back surfaces of quantum bit substrate 50 and quantum bit substrate 60. Quantum bit substrate 30 may have signal supply paths at its edges or on its back surface. Alternatively, it may be connected to patterns provided on the quantum bit substrate 50 and quantum bit substrate 60 via bumps, and supplied from the back surfaces through the edges or through holes of quantum bit substrate 50 and quantum bit substrate 60.

[0080] The other components are the same as in the first example.

[0081] (Third example of the configuration of quantum device 22) Next, we will describe a third example of the configuration of the quantum device 22. Figure 28 is a plan view showing the third example of the configuration of the quantum device 22.

[0082] As shown in Figure 28, in the third example of quantum device 22C, the region 52 within the qubit region 51 where the Josephson junction element 401 and capacitor 402 are provided is separated from the qubit substrate 30 in a plan view. Similarly, the region 62 within the qubit region 61 where the Josephson junction element 501 and capacitor 502 are provided is separated from the qubit substrate 30 in a plan view.

[0083] The other configurations are the same as in the second example.

[0084] In the second example, the insulating layers 85, 86, and 87 included in the qubit substrate 30 can become sources of dielectric loss for the Josephson junction element 401 and capacitor 402, and for the Josephson junction element 501 and capacitor 502. In the third example, such dielectric loss can be reduced.

[0085] In the first or second example, a ground layer may be provided on the portion of the qubit substrate 30 that is in contact with the Josephson junction element 401 and capacitor 402, and the portion that is in contact with the Josephson junction element 501 and capacitor 502, in order to reduce dielectric loss.

[0086] The materials for bumps 25, 26, and 27 are preferably materials that can be bonded at low temperatures, such as indium or an indium alloy. Preferably, the materials for bumps 25, 26, and 27 are materials that can be bonded at 200°C or below, and more preferably, materials that can be bonded at 180°C or below. The materials for bumps 25, 26, and 27 may also be materials that can improve adhesion, such as gold. Alternatively, an alloy of indium and gold may be used. In this case, depending on the composition, it may be possible to bond at temperatures below 180°C and also become a superconducting material.

[0087] In the second and third examples, the qubit substrate 30 and either the qubit substrate 50 or 60 may be connected by a coaxial cable containing a superconducting material (hereinafter, a coaxial cable containing a superconducting material may be referred to as a superconducting cable). The superconducting material used in the superconducting cable is, for example, an alloy of niobium and titanium.

[0088] For example, the qubit substrate 30 and the qubit substrate 50 are joined using bumps 26 as in the second example, and the qubit substrate 30 and the qubit substrate 60 are coupled by a superconducting cable. In this case, for example, the inductor 312 of the coupling flux qubit 300 and the superconducting cable are inductively coupled to each other, and the superconducting cable and the inductor 503 of the charge qubit 500 are inductively coupled to each other. For example, the qubit substrate 30 may be flip-chip bonded to the qubit substrate 50. The length of the superconducting cable is often several centimeters to several meters, and is typically 1m to 10m. The coupling state between the charge qubit 400 and the charge qubit 500 can be adjusted by controlling the energy potential of the coupling flux qubit 300.

[0089] Furthermore, the coupling between the superconducting cable and the charge qubit 400 or 500 may be capacitive, and the coupling between the superconducting cable and the coupling flux qubit 300 may also be capacitive. In addition, a superconducting circuit including a Josephson junction element may be further included in the coupling portion and coupled directly, inductively, or capacitively.

[0090] The qubit substrate 30 and the qubit substrate 60 may be joined using bumps 27 as in the second example, and the qubit substrate 30 and the qubit substrate 50 may be coupled by a superconducting cable. Alternatively, the qubit substrate 30 and the qubit substrates 50 and 60 may be coupled by superconducting cables.

[0091] By connecting qubit substrates using superconducting cables, the degree of freedom in qubit substrate placement on the stage within the same dilution refrigerator can be increased. For example, when placing qubit substrates on the lowest temperature stage of a dilution refrigerator, the qubit substrates can be placed at geographically separated locations and protected by different magnetic shields. Furthermore, by using long superconducting cables of 5m to 10m, it is possible to connect qubits from qubit substrates installed in different dilution refrigerators.

[0092] In the first embodiment, when the magnetic flux application unit 330 applies a magnetic flux of Φ0 to the coupling magnetic flux qubit 300, it is preferable that an unstable state does not clearly appear in the shape of the energy potential of the coupling magnetic flux qubit 300 (see Figure 6). For example, 2πLI C The variable β, represented by / Φ0. L The value of is preferably 1.2 to 8.0. Here, L is the total inductance of the coupling flux qubit 300, and I C β is the sum of the critical current values ​​of Josephson junction elements 321 and 322. L When the value is between 1.2 and 6.0, it is more preferable because an inflection point, which would indicate the appearance of an unstable state in the energy potential, does not clearly appear.

[0093] For example, the critical current value I of Josephson junction elements 321 and 322 C The critical current value I may be 1.0 μA to 4.0 μA. C The smaller the value, the easier it is to broaden the range of selection for the overall inductance L of the coupling flux qubit 300. Also, the critical current value I C The larger the value, the narrower the range of selectable inductance L becomes, but the easier it is to stably form Josephson junction elements 321 and 322.

[0094] In order to unify the ground state of the magnetic flux qubit, the applied magnetic flux is 0.5Φ0, but it does not need to be exactly 0.5Φ0; it can be between 0.4Φ0 and 0.6Φ0.

[0095] Furthermore, the coupling flux qubit 300 may have multiple SQUID 320 connected in parallel to each other. In this case, variations in characteristics can be reduced.

[0096] The coupling flux qubit 300 does not necessarily have to include the flux application unit 340.

[0097] The quantum computing system and quantum device relating to this disclosure can be used, for example, in quantum computing.

[0098] Although preferred embodiments have been described in detail above, the invention is not limited to the embodiments described above, and various modifications and substitutions can be made to the embodiments described above without departing from the scope of the claims. [Explanation of Symbols]

[0099] 1, 2: Quantum computing systems 10: Control Unit 21, 22, 22A, 22B, 22C: Quantum devices 30, 40, 50, 60: Quantum bit substrates 31, 41, 42, 51, 61: Qubit regions 100, 200: Magnetic flux qubits 110, 210, 310: Main loop 120, 220, 320: SQUID 130, 140, 230, 240, 330, 340: Magnetic flux application section 300: Coupling flux qubit 400, 500: Charge qubits 430, 530: Charge supply section

Claims

1. Quantum devices and, A control unit for controlling the quantum device, It has, The aforementioned quantum device is The first qubit and The second qubit and A coupling flux qubit that can be coupled with the first qubit and the second qubit, A first magnetic flux application unit that applies magnetic flux to the coupling magnetic flux qubit, It has, The control unit controls the first magnetic flux application unit. During the first period, a first magnetic flux with first time modulation is applied. A quantum computing system that applies a second magnetic flux having a second time modulation different from the first time modulation during a second period different from the first period.

2. The first qubit has a first magnetic flux qubit, The second qubit has a second magnetic flux qubit, The aforementioned quantum device is A second magnetic flux application unit that applies magnetic flux to the first magnetic flux qubit, A third magnetic flux application unit that applies magnetic flux to the second magnetic flux qubit, It has, The control unit controls the second magnetic flux application unit. During the first period, a third magnetic flux having the first time modulation is applied. During the second period, a fourth magnetic flux having a third time modulation different from the second time modulation is applied. The control unit controls the third magnetic flux application unit. During the first period, a fifth magnetic flux having the first time modulation is applied. The quantum computing system according to claim 1, wherein a sixth magnetic flux having the third time modulation is applied during the second period.

3. Upon application of the fourth magnetic flux, the ground state of the first magnetic flux qubit becomes two. Upon application of the sixth magnetic flux, the ground state of the second magnetic flux qubit becomes two. The quantum computing system according to claim 2, wherein the application of the second magnetic flux results in a single ground state for the coupling flux qubit.

4. Magnetic flux quantum Φ ０ In that case, In the third time modulation, the fourth magnetic flux and the sixth magnetic flux change from 0 (Wb) to Φ ０ It rose to that point, In the second time modulation, the fourth magnetic flux and the sixth magnetic flux are Φ ０ During the period in which the second magnetic flux is 0.4Φ ０ ~0.6Φ ０ The quantum computing system according to claim 2 or 3.

5. The fourth magnetic flux and the sixth magnetic flux are Φ ０ Before reaching that point, the second magnetic flux is 0.4Φ ０ ~0.6Φ ０ The quantum computing system according to claim 4, which reaches [a certain level].

6. The fourth magnetic flux and the sixth magnetic flux reach Φ ０ and at the same time, the second magnetic flux reaches 0.4Φ ０ to 0.6Φ ０ The quantum computing system according to claim 4, wherein the quantum computing system reaches

7. The first qubit has Xmon, The second qubit has Xmon, Upon application of the first magnetic flux, the ground state of the coupling flux qubit becomes two. Upon application of the second magnetic flux, the ground state of the coupling flux qubit becomes one, Magnetic flux quantum Φ ０ In that case, During the first period, the first magnetic flux is 0 (Wb) or Φ ０ And so, During the second period, the second magnetic flux is 0.4Φ ０ ~0.6Φ ０ The quantum computing system according to claim 1.

8. Let L be the total inductance of the coupling flux qubit, and I be the critical current value of the Josephson junction element included in the coupling flux qubit. Ｃ , magnetic flux quantum Φ ０ When this is the case, 2πLI Ｃ / Φ ０ The quantum computing system according to claim 1, 2, 3, or 7, wherein the value of is 1.2 to 8.

0.

9. A method for controlling quantum devices, The aforementioned quantum device is The first qubit and The second qubit and A coupling flux qubit that can be coupled with the first qubit and the second qubit, It has, In the first period, the process involves applying a first magnetic flux with first time modulation to the coupling flux qubit, A step of applying a second magnetic flux having a second time modulation different from the first time modulation to the coupling flux qubit in a second period different from the first period, A method for controlling a quantum device, having the following characteristics.

10. The first qubit has a first magnetic flux qubit, The second qubit has a second magnetic flux qubit, The process involves applying a third magnetic flux with the first time modulation to the first magnetic flux qubit during the first period, and applying a fifth magnetic flux with the first time modulation to the second magnetic flux qubit. During the second period, a fourth magnetic flux having a third time modulation different from the second time modulation is applied to the first magnetic flux qubit, and a sixth magnetic flux having the third time modulation is applied to the second magnetic flux qubit; A method for controlling a quantum device according to claim 9, comprising:

11. Upon application of the fourth magnetic flux, the ground state of the first magnetic flux qubit becomes two. Upon application of the sixth magnetic flux, the ground state of the second magnetic flux qubit becomes two. The method for controlling a quantum device according to claim 10, wherein the application of the second magnetic flux results in a single ground state for the coupling flux qubit.

12. The first qubit has a first charge qubit, The second qubit has a second charge qubit, Upon application of the first magnetic flux, the ground state of the coupling flux qubit becomes two. A method for controlling a quantum device according to claim 11, wherein the application of the second magnetic flux results in a single ground state for the coupling flux qubit.

13. Quantum devices and, A control unit for controlling the quantum device, It has, The aforementioned quantum device is The first charge qubit and The second charge qubit, A coupling flux qubit that can be coupled with the first charge qubit and the second charge qubit, A first magnetic flux application unit that applies magnetic flux to the coupling magnetic flux qubit, It has, The control unit controls the first magnetic flux application unit. A quantum computing system that applies a magnetic flux such that, in a first period, the ground state of the energy potential of the coupling magnetic flux qubit becomes one, and in a second period different from the first period, the ground state of the energy potential of the coupling magnetic flux qubit becomes two.

14. A method for controlling quantum devices, The aforementioned quantum device is The first charge qubit and The second charge qubit, A coupling flux qubit that can be coupled with the first charge qubit and the second charge qubit, A first magnetic flux application unit that applies magnetic flux to the coupling magnetic flux qubit, It has, The first magnetic flux application unit is subjected to a magnetic flux such that, during the first period, the ground state of the energy potential of the coupling magnetic flux qubit becomes one. A method for controlling a quantum device, comprising applying a magnetic flux to the first magnetic flux application unit such that the ground state of the energy potential of the coupling magnetic flux qubit becomes two during a second period different from the first period.

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