Coupler and computer

Abstract

A coupler according to an embodiment has a variable transmon, a second capacitor, a first resonator, a third capacitor, a second resonator, a fourth capacitor, an end of a fifth capacitor, and an end of a sixth capacitor. The variable transmon includes a first capacitor, a first Josephson junction, and a second Josephson junction. A closed circuit surrounded by the first Josephson junction and the second Josephson junction is configured. The other end of the fifth capacitor can be electrically connected to a first quantum bit. The other end of the sixth capacitor can be electrically connected to a third quantum bit.

Description

Couplers and calculators

[0001] Embodiments of the present invention relate to a coupler and a computer.

[0002] For example, in computers that utilize multiple qubits, couplers are used to connect multiple qubits. Improved controllability is desired in computers.

[0003] Japanese Patent Publication No. 2023-20041 Japanese Patent Publication No. 2024-8057

[0004] H. Goto, “Double-Transmon Coupler: Fast Two-Qubit Gate with No Residual Couling for Highly Detuned Superconducting Qubits” Phys. Rev. Appl. 18, 034038 (2022)N. Shirai et al., “All-Microwave Manipulation of Superconducting Qubits with a Fixed-Frequency Transmon Coupler”Phys. Rev. Lett. 130, 260601 (2023)

[0005] The problem that this invention aims to solve is to provide a coupler and a computer with improved controllability.

[0006] The coupler according to the embodiment has a variable transmon, a second capacitor, a first resonator, a third capacitor, a second resonator, a fourth capacitor, the end of a fifth capacitor, and the end of a sixth capacitor. The variable transmon includes a first capacitor including the end and the other end, a first Josephson junction including the end and the other end, wherein the end of the first Josephson junction is electrically connected to the end of the first capacitor, and the other end of the first Josephson junction is electrically connected to the other end of the first capacitor, and a second Josephson junction including the end and the other end, wherein the end of the second Josephson junction is electrically connected to the end of the first Josephson junction, and the other end of the second Josephson junction is electrically connected to the other end of the first Josephson junction, and a closed circuit exists enclosed by the first Josephson junction and the second Josephson junction. The second capacitor includes an end and the other end, the other end of the second capacitor is electrically connected to the end of the first capacitor, and the end of the second capacitor is electrically connected to the first resonator. The third capacitor includes an end and the other end, the other end of the third capacitor is electrically connected to the end of the first capacitor, and the end of the third capacitor is electrically connected to the second resonator. The fourth capacitor includes an end and the other end, the end of the fourth capacitor is electrically connected to the end of the second capacitor, and the other end of the fourth capacitor is electrically connected to the end of the third capacitor. The end of the fifth capacitor is electrically connected to the end of the second capacitor, the end of the fourth capacitor, and the first resonator, and the fifth capacitor further includes the other end which can be electrically connected to the first qubit. The end of the sixth capacitor is electrically connected to the end of the third capacitor, the other end of the fourth capacitor, and the second resonator, and the sixth capacitor further includes the other end which can be electrically connected to the second qubit.

[0007] Figure 1 is a diagram illustrating an example configuration of a computer system according to this embodiment. Figure 2 is a diagram illustrating a simplified circuit configuration of a computer according to the first embodiment. Figure 3 is a diagram illustrating a detailed circuit configuration of a computer according to the first embodiment. Figure 4 is a diagram illustrating other circuit configurations of the first and second resonators. Figure 5 is a diagram illustrating other circuit configurations of the first and second resonators. Figure 6 is a diagram illustrating an example of the frequency arrangement of five transmons according to this embodiment. Figure 7 is a diagram illustrating another example of the frequency arrangement of five transmons according to this embodiment. Figure 8 is a diagram illustrating the simulation results of the energy spectrum and ZZ coupling strength. Figure 9 is a diagram illustrating the simulation results of the ZZ coupling strength at zero magnetic field in the first and second frequency arrangements. Figure 10 is a diagram illustrating the circuit configuration of a computer according to the second embodiment. Figure 11 is a diagram showing the simulation results of a two-qubit gate. Figure 12 is a diagram illustrating a simplified circuit configuration of a computer according to the third embodiment. Figure 13 is a diagram illustrating a detailed circuit configuration of a computer according to the third embodiment. Figure 14 is a diagram illustrating the detailed circuit configuration of the computer according to the fourth embodiment. Figure 15 is a schematic plan view of a quantum processor implementing nine qubits according to the fifth embodiment.

[0008] The coupler and computer according to this embodiment will be described below with reference to the drawings. The drawings schematically or conceptually illustrate the embodiment, and the relationship between the thickness and width of each part, the ratio of the sizes between parts, etc., are not necessarily the same as those in reality. Even when representing the same part, the dimensions and ratios may be represented differently in the drawings.

[0009] Figure 1 shows an example of the configuration of the computer system 1 according to this embodiment. As shown in Figure 1, the computer system 1 according to this embodiment has a classical computer 100 and a quantum computer 200 connected to each other by an information and communication network such as a LAN (Local Area Network).

[0010] The classical computer 100 transmits information about the quantum circuit to be executed to the quantum computer 200. The quantum circuit information is assumed to be data of the circuit diagram and / or program code of the quantum circuit. The quantum computer 200 has a control device 210 and a quantum processor 220. The quantum processor 220 is quantum hardware equipped with a computer having qubits and couplers for executing the quantum circuit. The quantum processor 220 is assumed to be a superconducting circuit type. The control device 210 receives the quantum circuit information from the classical computer 100 and converts the received quantum circuit information into a control sequence optimized for the quantum processor 220 of the quantum computer 200, for example. The optimization includes, for example, conversion to a primitive quantum circuit, scheduling of quantum gates, mapping of qubits, etc. Then, the control device 210 operates the quantum processor 220 by supplying various control signals, etc. to the quantum processor 220 according to the control sequence. As a result, the quantum circuit to be executed is executed. The data of the execution result of the quantum circuit is transmitted from the control device 210 to the classical computer 100. The classical computer 100 utilizes the received execution results for various purposes.

[0011] Hereinafter, the computer mounted on the quantum processor 220 according to this embodiment will be described in several embodiments.

[0012] (First Embodiment) Figure 2 is a diagram illustrating a simplified circuit configuration of the computer 230A according to the first embodiment. In Figure 2, circuit elements are depicted with multiple line types, but this is for visually distinguishing adjacent circuit elements and is unrelated to the actual configuration. As shown in Figure 2, the computer 230A is a superconducting circuit having at least a triple transmon coupler (TTC), a first qubit (Q1), and a second qubit (Q2). The TTC, Q1, and Q2 are mounted on a substrate made of silicon, sapphire, or the like. The TTC is a coupler containing three transmons. The TTC has a single transmon coupler (STC) C5, a first resonator C3, and a second resonator C4. C5 is a frequency-tunable transmon and includes a transmon with a closed circuit (loop) for adjusting the magnetic flux. The TTC is placed between Q1 and Q2 and couples Q1 and Q2. The strength of the coupling between Q1 and Q2 can be adjusted by controlling the magnetic flux Φ passing through the closed circuit. C5, C4, C3, Q1, and Q2 are each connected to a fixed potential (GND), for example.

[0013] The "transmon" in this embodiment is a circuit element including a capacitor and a Josephson junction connected in parallel. The transmon functions as a nonlinear resonant circuit. The "capacitor" in this embodiment is a circuit element including two conductors arranged opposite each other. The conductors are formed from conductive materials such as Al, Nb, NbN, TiN, NbTiN, and Ta. For example, one of the two conductors constitutes one end of the capacitor, and the other constitutes the other end of the capacitor. The "Josephson junction" in this embodiment is a circuit element including two superconductors arranged opposite each other and a thin film connecting the two superconductors. The "Josephson junction" is also called a Josephson element. Each superconductor is a conductor formed from a superconducting material such as Al, Nb, NbN, TiN, NbTiN, and Ta. The thin film is Al ２ O ３ Nb ２ O ５ NbO ２It is formed from insulating materials such as NbO and AlN. A normal conducting metal or semiconductor may be used as the thin film. For example, one of the two superconductors forms one end of the Josephson junction, and the other forms the other end.

[0014] Figure 3 is a diagram illustrating the detailed circuit configuration of the computer 230A according to the first embodiment. The circuit configuration shown in Figure 3 is equivalent to the circuit configuration shown in Figure 2. As shown in Figure 3, the triple transmon coupler TTC has a variable transmon C5, a second capacitor 22, a first resonator C3, a third capacitor 23, a second resonator C4, a fourth capacitor 24, a fifth capacitor terminal 251, and a sixth capacitor terminal 261.

[0015] The variable transmon C5 is a single transmon coupler and includes a transmon with a closed circuit (loop) 14 for adjusting magnetic flux. Specifically, the variable transmon C5 includes a first capacitor 11, a first Josephson junction 12, and a second Josephson junction 13. The first capacitor 11 includes one end 111 and the other end 112. The first Josephson junction 12 includes one end 121 and the other end 122. One end 121 is electrically connected to the one end 111 of the first capacitor 11. The other end 122 is electrically connected to the other end 112 of the first capacitor 11. The second Josephson junction 13 includes one end 131 and the other end 132. One end 131 is electrically connected to the one end 121 of the first Josephson junction 12. The other end 132 is electrically connected to the other end 122 of the first Josephson junction 12. The first Josephson junction 12 and the second Josephson junction 13 form a closed circuit 14. The control device 210 applies magnetic flux to the space inside the closed circuit 14.

[0016] The second capacitor 22 includes one end 221 and the other end 222. The other end 222 is electrically connected to the end 111 of the first capacitor 11. The end 221 is electrically connected to the first resonator C3.

[0017] The first resonator C3 has one fixed frequency transmon. Specifically, the first resonator C3 includes a seventh capacitor 31 and a third Josephson junction 32. The third Josephson junction 32 includes an end 321 and the other end 322. The seventh capacitor 31 includes an end 311 and the other end 312. End 311 is electrically connected to end 321 of the third Josephson junction 32. The other end 312 is electrically connected to the other end 322 of the third Josephson junction 32.

[0018] The third capacitor 23 includes one end 231 and the other end 232. The other end 232 is electrically connected to the end 111 of the first capacitor 11. The end 231 is electrically connected to the second resonator C4.

[0019] The second resonator C4 has one fixed frequency transmon. Specifically, the second resonator C4 includes a seventh capacitor 33 and a third Josephson junction 34. The seventh capacitor 33 includes one end 331 and the other end 332. The third Josephson junction 34 includes one end 341 and the other end 342. The end 331 of the seventh capacitor 33 is electrically connected to the end 341 of the third Josephson junction 34. The other end 332 of the seventh capacitor 33 is electrically connected to the other end 342 of the third Josephson junction 34.

[0020] The end 251 of the fifth capacitor 25 is electrically connected to the end 221 of the second capacitor 22, the end 241 of the fourth capacitor 24, and the first resonator C3. The fifth capacitor 25 further includes another end 252. The other end 252 of the fifth capacitor 25 can be electrically connected to the first qubit Q1.

[0021] The end 261 of the sixth capacitor 26 is electrically connected to the end 231 of the third capacitor 23, the other end 242 of the fourth capacitor 24, and the second resonator C4. The sixth capacitor 26 further includes another end 262. The other end 262 of the sixth capacitor 26 can be electrically connected to the second qubit Q2.

[0022] The first qubit Q1 is a fixed-frequency transmon qubit comprising an eighth capacitor 41 and a fourth Josephson junction 42. The eighth capacitor 41 includes an end 411 and the other end 412. The fourth Josephson junction 42 includes an end 421 and the other end 422. End 421 is electrically connected to end 411 of the eighth capacitor 41. The other end 422 is electrically connected to the other end 412 of the eighth capacitor 41.

[0023] The second qubit Q2 is a fixed-frequency transmon qubit comprising an eighth capacitor 43 and a fourth Josephson junction 44. The eighth capacitor 43 includes an end 431 and the other end 432. The fourth Josephson junction 44 includes an end 441 and the other end 442. End 441 is electrically connected to end 431 of the eighth capacitor 43. The other end 442 is electrically connected to the other end 432 of the eighth capacitor 43.

[0024] The first resonator C3 and the second resonator C4 are connected via the second capacitor 22, the variable transmon C5, and the third capacitor 23, and are also directly capacitively connected by the fourth capacitor 24.

[0025] The circuit parameters of a TTC are designed according to the frequency difference (detuning) between the first qubit Q1 and the second qubit Q2. When the detuning is large, it is possible to suppress the crosstalk error between the first qubit Q1 and the second qubit Q2. Specifically, the circuit parameters of a TTC refer to the parameters of various circuit elements included in the TTC, such as capacitors and Josephson junctions. Examples of circuit element parameters include the capacitance of a capacitor and the critical current of a Josephson junction.

[0026] The two lowest energy levels of the multiple energy levels possessed by the first qubit Q1 and the second qubit Q2 correspond to the ground state and the first excited state. These two states of the qubits correspond to computational basis states. The first resonator C3 reads out the quantum state of the first qubit Q1 by distributed readout. The second resonator C4 reads out the quantum state of the second qubit Q2 by distributed readout.

[0027] As shown in Figure 3, the TTC is positioned between the first qubit Q1 and the second qubit Q2. The first qubit Q1 and the second qubit Q2 are not connected without the TTC; in other words, there is no direct coupling between the first qubit Q1 and the second qubit Q2. The strength of the coupling between the first qubit Q1 and the second qubit Q2 (hereinafter referred to as the Q1-Q2 coupling) can be adjusted by adjusting the magnetic flux Φ. For example, by adjusting the magnetic flux Φ, it is possible to switch the Q1-Q2 coupling between on and off. The Q1-Q2 coupling can be evaluated by the ZZ coupling.

[0028] When performing a gate operation on one qubit, the control device 210 adjusts the magnetic flux Φ to turn off the Q1-Q2 coupling and operates the first qubit Q1 and / or the second qubit Q2. By turning off the Q1-Q2 coupling, the crosstalk error between the first qubit Q1 and the second qubit Q2 is prevented or reduced, and a gate operation on one qubit becomes possible. When performing a gate operation on two qubits, the control device 210 adjusts the magnetic flux Φ to turn on the Q1-Q2 coupling, enabling a gate operation on two qubits, the first qubit Q1 and the second qubit Q2.

[0029] By adjusting the magnetic flux Φ applied to the closed circuit (loop) 14 for adjusting the magnetic flux, it is possible to substantially turn off the Q1-Q2 coupling. When the Q1-Q2 coupling is substantially off, it is possible to electrically separate the first qubit Q1 and the second qubit Q2. The value of the magnetic flux at which the Q1-Q2 coupling is turned off is determined by the circuit parameters of the TTC, and this value can be designed to cover a wide magnetic field range, including a zero magnetic field. With the above configuration, the TTC according to this embodiment can turn off the Q1-Q2 coupling of the first qubit Q1 and the second qubit Q2, which have a relatively large frequency difference, without a direct junction and in a substantially zero magnetic field. By achieving the Q1-Q2 coupling off in a substantially zero magnetic field, it is possible to enjoy advantages such as suppression of thermal noise and increased qubit lifetime.

[0030] Here, the TTC according to this embodiment is compared with various comparative examples. Comparative Example 1, as shown in Figure 6(a) of Non-Patent Document 1, has an STC placed between the first qubit and the second qubit, and the first qubit and the second qubit are capacitively connected via a capacitor, and includes a direct coupling between the first qubit and the second qubit. In Comparative Example 1, the range of the magnetic field that turns off the Q1-Q2 coupling can be adjusted by design, and it is also possible to turn off the Q1-Q2 coupling at zero magnetic field. However, in Comparative Example 1, when the detuning is increased, it is not possible to turn off the Q1-Q2 coupling, and typically the ZZ coupling strength ζ is 10 １ The frequency can only be reduced to around kHz, making it susceptible to quantum crosstalk. Furthermore, because it includes a direct coupling between the first and second qubits, it is also susceptible to microwave crosstalk. Comparative Example 2, as shown in Figure 1 of Non-Patent Document 1, has a DTC (double-transmon coupler) placed between the first and second qubits. In Comparative Example 2, it is possible to turn off the Q1-Q2 coupling even with a large degree of detuning. However, it is difficult to turn off the Q1-Q2 coupling in a nearly zero magnetic field.

[0031] On the other hand, as described above, the TTC according to this embodiment uses a triple transmon coupler (TTC) C3-C5(STC)-C4 as the coupler between the first and second qubits, instead of the STC in Comparative Example 1 or the DTC in Comparative Example 2. This makes it possible to turn off the Q1-Q2 coupling between the first qubit Q1 and the second qubit Q2, which have a relatively large detuning (frequency difference), in a nearly zero magnetic field without direct coupling. Because there is no direct coupling between the first qubit Q1 and the second qubit Q2, it is resistant to microwave crosstalk. There is no need to continuously flow DC current, which reduces power consumption and suppresses heat generation. Along with suppressing heat generation, it is also possible to suppress thermal noise. Furthermore, since a zero magnetic field is a sweet spot, an increase in the idle point phase relaxation time T2 can be expected. In addition, a net-zero (NZ) type CZ gate can be implemented, and pulse distortion can be expected to be reduced.

[0032] In the above description, it was assumed that both the first resonator C3 and the second resonator C4 are transmon-type resonators including capacitors 31 and 33 and Josephson junctions 32 and 34. However, this embodiment is not limited to this. Both the first resonator C3 and the second resonator C4 may be waveguide resonators as shown in Figure 4, or Josephson junction array-type resonators as shown in Figure 5. ZZ coupling can be reduced in any of the transmon-type resonators, waveguide resonators, and Josephson junction array-type resonators.

[0033] The waveguide resonator shown in Figure 4 includes a capacitor C and an inductor L. One end of capacitor C is electrically connected to one end of inductor L, and the other end of capacitor C is electrically connected to the other end of inductor L. Because waveguide resonators are anharmonic, they can perform qubit measurements with higher accuracy compared to transmon resonators and Josephson junction array resonators. Furthermore, since waveguide resonators are used as an element of a coupler, the absence of anharmonic is not a problem.

[0034] The transmon-type resonators shown in Figures 2 and 3, as described above, include a capacitor C, a Josephson junction, and another capacitor, and are anharmonic. Transmon-type resonators have a longer lifespan and require a smaller circuit size compared to waveguide resonators.

[0035] The Josephson junction array type resonator shown in Figure 5 includes a capacitor C, a Josephson junction array, and another capacitor C. The Josephson junction array includes two or more electrically connected Josephson junctions. For example, the Josephson junction array shown in Figure 5 has three Josephson junctions connected in series. One end of capacitor C is electrically connected to one end of the Josephson junction array, and the other end of capacitor C is electrically connected to the other end of the Josephson junction array. The Josephson junction array type resonator has properties intermediate between a waveguide resonator and a transmon type resonator. The more Josephson junctions there are, the closer the properties become to those of a waveguide resonator.

[0036] In the above explanation, it was assumed that both the first resonator C3 and the second resonator C4 are of the same type from among the transmon type resonators shown in Figures 2 and 3, the waveguide resonators shown in Figure 4, and the Josephson junction array type resonators shown in Figure 5. However, the first resonator C3 and the second resonator C4 may be different types from among the transmon type resonators, waveguide resonators, and Josephson junction array type resonators.

[0037] In the above description, it was assumed that both the first qubit Q1 and the second qubit Q2 are transmon qubits comprising the eighth capacitors 41, 43 and the fourth Josephson junctions 42, 44. However, this embodiment is not limited to this, and either the first qubit Q1 or the second qubit Q2 may be a transmon qubit comprising the eighth capacitors 41, 43 and the fourth Josephson junctions 42, 44. The first qubit Q1 or the second qubit Q2 that is not a transmon qubit may be a magnetic flux qubit, a charge qubit, a phase qubit, or any other superconducting qubit. Furthermore, both the first qubit Q1 and the second qubit Q2 do not have to be transmon qubits.

[0038] As shown in Figures 2 and 3, the computer 230A according to this embodiment includes a first qubit Q1 and a second qubit Q2, which are transmon qubits, as well as a TTC having three transmons as a coupler used to connect the first qubit Q1 and the second qubit Q2. That is, the computer 230A has a series connection of five transmons, and the two ends of this connection are qubits Q1 and Q2, forming a two-qubit system.

[0039] Here, we focus on the fact that the five transmons Q1, C3, C5, C4, and Q2 in this embodiment are frequency-fixed transmons, frequency-fixed transmons, frequency-tunable transmons, frequency-fixed transmons, and frequency-fixed transmons, respectively, and refer to them as the F-F-T-F-F type. "F" indicates that it is a frequency-fixed transmon, and "T" indicates that it is a frequency-tunable transmon. Here, we compare the F-F-T-F-F type in this embodiment with the F-F-F-F-F type, T-T-T-T-T type, T-F-T-F-T type, and F-T-F-T-F type which are different from this embodiment. In the F-F-F-F-F type, "F" is assumed to represent a qubit. In the T-T-T-T-T type, "T" is assumed to represent a qubit or an STC. Qubits and STCs are connected alternately to break the coupling between adjacent qubits. In the T-F-T-F-T and F-T-F-T-F types, "F" represents a qubit and "T" represents an STC. Qubits and STCs are connected alternately to break the coupling between adjacent qubits.

[0040] In the F-F-F-F-F type, all five transmons are fixed-frequency transmons, and therefore, the F-F-F-F-F type does not have any closed circuits for adjusting magnetic flux. In the T-T-T-T-T type, all five transmons are variable-frequency transmons with closed circuits, and therefore, the T-T-T-T-T type has five closed circuits for adjusting magnetic flux. Similarly, the T-F-T-F-T type has a closed circuit for adjusting magnetic flux every other transmon, and the same applies to the F-T-F-T-F type. Therefore, the F-F-T-F-F type in this embodiment differs from the other types in the position and number of closed circuits for adjusting magnetic flux.

[0041] The arrangement of the transmons in this embodiment is not limited to the F-F-T-F-F type, and any fixed-frequency transmon (F) may be replaced with a variable-frequency transmon (T). In this case, the arrangement of the transmons in this embodiment may be the same as the T-T-T-T-T type or T-F-T-F-T type in the comparative example. Even in this case, the TTC in this embodiment has a different frequency arrangement of the five transmons compared to the comparative example.

[0042] Next, the frequency arrangement of five transmon qubits in a zero magnetic field will be described. When the magnetic flux Φ passing through the closed loop 14 is zero, the triple transmon coupler C5 can resonate in multiple modes.

[0043] FIG. 6 is a diagram showing an example of the frequency arrangement when the magnetic flux Φ passing through the closed loop of five transmon qubits according to the present embodiment is zero (zero magnetic field). "Q1" in FIG. 6 is the resonance frequency ω of the first qubit. Ｑ１ "Q2" is the resonance frequency ω of the second qubit. Ｑ２ "A", "B", and "C" are three specific modes among the multiple modes generated by the mixing of the resonance frequencies of C3, C4, and C5. The three specific modes are the three modes with the lowest frequencies among the multiple modes. The resonance frequency of each of the three specific modes is represented as ω. ３ , ω ４ , ω ５ . The frequency arrangement shown in FIG. 6 represents an example of the frequency arrangement common to the F - F - T - F - F type and the T - T - T - T - T type when both ends are qubits. When the magnetic flux Φ passing through the closed loop is zero, as shown in FIG. 6, the resonance frequencies ω corresponding to "A", "B", and "C". ３ , ω ４ , ω ５ are all higher than the resonance frequencies ω of the qubits at both ends. Ｑ１ , ω Ｑ２ .

[0044] FIG. 7 is a diagram showing another example of the frequency arrangement when the magnetic flux Φ passing through the closed loop of five transmon qubits according to the present embodiment is zero. The frequency arrangement shown in FIG. 7 represents the frequency arrangement specific to the F - F - T - F - F type. When the magnetic flux Φ passing through the closed loop is zero, similar to FIG. FIG. 7 shows that the resonance frequencies ω. ３ , ω ４ , ω ５ are all higher than the resonance frequencies ω of the qubits. Ｑ１ , ω Ｑ２ . And when the magnetic flux Φ passing through the closed loop is zero, among the resonance frequencies of the multiple modes, the third lowest frequency ω. ５ is the resonance frequency ω of the variable transmon C5. Ｃ５ . As described above, the number of modes is actually more than three, and ω.Ｃ５ Many modes exist with higher frequencies than this. It is also possible to achieve the frequency configuration shown in Figure 7 through design in the T-T-T-T-T and T-F-T-F-T types. However, in the T-T-T-T-T and T-F-T-F-T types, the frequency configuration in Figure 7 presents a problem where the coupling of qubits (T or F) connected via T(STC) cannot be broken. Therefore, for the T-T-T-T-T and T-F-T-F-T types, the frequency configuration in Figure 7 degrades the performance of the computer.

[0045] Figure 8 shows the simulation results for the energy spectrum and ZZ bond strength. The upper part of Figure 8 shows the graph of the energy spectrum, with the vertical axis defined as frequency ω (GHz / 2π) and the horizontal axis as Θ. ｅｘ It is defined by / π. The phase Θ ｅｘ / π reduces the magnetic flux Φ to the magnetic flux quantum φ ０ It is a dimensionless quantity normalized by ω. Ｑ１Ｑ２ ω is the resonance frequency when both the first and second qubits are in the first excited state, specifically, ω Ｑ１Ｑ２ = ω Ｑ１ +ω Ｑ２ It is defined as +ζ. As shown in the upper part of Figure 8, when the magnetic flux Φ passing through the closed circuit 14 is zero, ω ５ = ω Ｃ５ >ω ３ ,ω ４ >ω Ｑ１ ,ω Ｑ２ This is the result.

[0046] The lower part of Figure 8 shows the simulation results for the ZZ coupling strength. The vertical axis is defined by the frequency strength ζ (MHz / 2π) of the ZZ coupling, and the horizontal axis is defined by the phase Θ. ｅｘ It is defined by / π. The frequency strength ζ of a ZZ coupling is given by ζ = ω Ｑ１Ｑ２ -ω Ｑ２ -ω Ｑ１ It is defined as follows. As shown in the lower part of Figure 8, when the magnetic flux Φ is zero (zero magnetic field), it is possible to make the ZZ coupling strength approximately zero. Also in Figure 8, Θ ｅｘ Not only π = 0, but also Θ ｅｘIn the region where / π is less than approximately 0.3, the absolute value of the ZZ coupling strength is approximately 10 kHz or less. In other words, the qubit coupling is broken over a wide range including the zero magnetic field. In particular, by selecting the zero magnetic field as the idle point, T2 can be increased. Also, due to symmetry, NZ-style gate operations can be performed. Since it is possible to make the ZZ coupling strength nearly zero at zero magnetic field, power consumption can be reduced and heat generation can be suppressed. Furthermore, the phase Θ ｅｘ A strong ZZ coupling strength is observed in the range of / π = 0.6 to 0.8. Therefore, a high-speed CZ gate can be implemented using Dc magnetic flux.

[0047] Figure 9 shows simulation results illustrating the ZZ coupling strength at zero magnetic field in the first and second frequency configurations. The left panel of Figure 9 shows a two-dimensional map of the ZZ coupling strength, with the vertical axis representing the resonance frequency ω of the first resonator C3. C3 The horizontal axis is defined as ω, the resonance frequency of the variable transmon C5, with the horizontal axis being ω. C5 The frequency is defined as ζ / 2π (GHz), and the pixel values ​​of the 2D map are defined by the frequency intensity ζ / 2π (kHz) of the ZZ coupling. The resonant frequency of the second resonator C4 is ω C4 / 2π (GHz) is ω C3 It was assumed to be equal to / 2π (GHz). The light-colored areas in the 2D map represent the regions where the ZZ coupling strength can be turned off. It can be seen that it is possible to turn off the coupling over a wide range of regions. Region 1 is the frequency region that satisfies the first frequency configuration, and region 2 is the frequency region that satisfies the second frequency configuration. The second frequency configuration corresponds to the frequency configuration illustrated in Figure 7, and specifically, i. When the magnetic flux Φ passing through the closed circuit 14 is zero, the resonant frequencies ω corresponding to modes "A", "B", and "C" ３ ,ω ４ ,ω ５ All of these have a resonant frequency ω Ｑ１ ,ω Ｑ２ ii. When the magnetic flux Φ passing through the closed circuit 14 is zero, the third lowest frequency ω among the resonant frequencies of the multiple modes is higher than ii. ５ The resonant frequency ω of the variable transmon C5 Ｃ５This frequency configuration satisfies both requirements i and ii. The first frequency configuration is obtained by removing the second frequency configuration from the frequency configuration shown in Figure 6. The frequency configuration shown in Figure 6 is i. When the magnetic flux Φ passing through the closed circuit 14 is zero, the resonant frequency ω ３ ,ω ４ ,ω ５ All of these have a resonant frequency ω Ｑ１ ,ω Ｑ２ This frequency configuration satisfies the requirement of being higher than [a certain frequency].

[0048] Here, the gap ω ｇａｐ = ω ３ -ω Ｑ２ Let's consider this. In region 1, the phase Θ ｅｘ Around / π = 0.5, the gap ω ｇａｐ There is a range where the phase Θ is relatively small. On the other hand, in region 2, compared to region 1, ｅｘ / π has a gap ω ｇａｐ The coefficient of the coupling is relatively large. Therefore, high-speed CZ gate implementation utilizing strong ZZ coupling strength is difficult in the first frequency configuration, but easy in the second frequency configuration.

[0049] (Second Embodiment) The computer according to the second embodiment has a conductor for applying a magnetic field to the space within the closed circuit 14. The computer according to the second embodiment will be described in detail below. In the following description, components having substantially the same function as those in the first embodiment will be denoted by the same reference numerals and will be described again only when necessary.

[0050] Figure 10 is a diagram illustrating the circuit configuration of a computer 230B according to the second embodiment. As shown in Figure 10, the computer 230B further includes a conductor 51. The conductor 51 is made of a conductive material and can apply a magnetic flux Φ to the space within the closed circuit 14. For example, the control device 210 supplies current to the conductor 51, generating a magnetic field, which is then applied to the space within the closed circuit 14. As a result, the magnetic flux of the magnetic field penetrates the space within the closed circuit 14. The strength of the magnetic flux Φ is adjusted by adjusting the strength of the current supplied to the conductor 51. By adjusting the strength of the magnetic flux Φ, it is possible to switch the coupling between the first qubit Q1 and the second qubit Q2 on and off. This makes it possible to perform gate operations on two qubits.

[0051] Figure 11 shows the simulation results for a two-qubit gate. The left figure in Figure 11 shows the simulation results for a CZ gate using a single-pole pulse, and the right figure shows the simulation results for a CZ gate using an NZ pulse. The vertical axis in each figure is Θ. ｅｘ The x-axis is defined by / π, and the x-axis is defined by time t (ns). The fidelity of a CZ gate using a single-pole pulse with a pulse peak of 0.7π and a gate time T = 52ns was 0.99992. The fidelity of a CZ gate using an NZ pulse with a pulse peak of 0.6915π and a gate time T = 72ns was 0.99982. As can be seen from the left and right figures of Figure 11, computer 230B is capable of performing high-speed and high-fidelity 2-qubit gate operations.

[0052] (Third Embodiment) The computer according to the third embodiment has a circuit configuration for performing gate operations by microwave mounting. The computer according to the third embodiment will be described in detail below. In the following description, components having substantially the same functions as those in the first and second embodiments will be denoted by the same reference numerals and will be described again only when necessary.

[0053] FIG. 12 is a diagram illustrating a simplified circuit configuration of a computer according to the third embodiment. As shown in FIG. 12, in the computer according to the third embodiment, power supplies V1, V2, V3, V4, and V5 are provided for the respective transmon Q1, Q2, C3, C4, and C5. The power supplies V1, V2, V3, V4, and V5 supply microwaves to the transmon Q1, Q2, C3, C4, and C5 in a predetermined control sequence, and execute gate operations according to the quantum circuit.

[0054] Here, the gate operation of two qubits will be described. It is assumed that the resonance microwave controls the oscillation between the first quantum state |Q １ , Q ２ , C ３ , C ４ , C ５ > and the second quantum state |Q' １ , Q' ２ , C' ３ , C' ４ , C' ５ . The drive term H Ｖｉ determined by the microwave applied from the power supply Vi and |<Q' １ , Q' ２ , C' ３ , C' ４ , C' ５ |H Ｖｉ |Q １ , Q ２ , C ３ , C ４ , C ５ > | value is determined. When this value is large, a high-speed oscillation occurs between the two states due to the resonance microwave. In CAS (coupler-assisted swap), at least one of the power supplies V ３ , V ４ and V ５ is used. The first quantum state is any one of the quantum computing states, and the second quantum state is assumed to be a state in which the coupler is excited rather than a quantum computing state. At this time, it is possible to add a phase of -1 to the first quantum state by the above rotation, and a CZ gate can be implemented. In the cross-resonance method, at least one of the power supplies V １ and V ２ is used.

[0055] Here, examples of the first CAS and examples of the second CAS are shown. The example according to the first CAS uses at least one of the power supplies V ３ , V ４ and V ５ . Assume a rotation between the first quantum state |1, 0, 0, 0, 0> which is a quantum computing state and at least one or more of the three patterns of the second quantum state where the coupler is excited but not in the quantum computing state.

[0056] The first quantum state |Q １ , Q ２ , C ３ , C ４ , C ５ > = |1, 0, 0, 0, 0> The second - 1 quantum state |Q' １ , Q' ２ , C' ３ , C' ４ , C' ５ > = |0, 1, 1, 0, 0> The second - 2 quantum state |Q' １ , Q' ２ , C' ３ , C' ４ , C' ５ > = |0, 1, 0, 1, 0> The second - 3 quantum state |Q' １ , Q' ２ , C' ３ , C' ４ , C' ５ > = |0, 1, 0, 0, 1>

[0057] The example according to the second CAS uses at least one of the power supplies V ３ , V ４ and V ５ . Assume using a rotation between the first quantum state |0, 1, 0, 0, 0> which is a quantum computing state and at least one or more of the three patterns of the second quantum state where the coupler is excited but not in the quantum computing state.

[0058] The first quantum state |Q １ , Q ２ , C ３ , C ４ , C ５ > = |0, 1, 0, 0, 0> The second - 1 quantum state |Q' １ , Q' ２ , C'３ , C' ４ , C' ５ >=|1,0,1,0,0> Quantum state 2-2 |Q' １ , Q' ２ , C' ３ , C' ４ , C' ５ >=|1,0,0,1,0> Second-third quantum state |Q' １ , Q' ２ , C' ３ , C' ４ , C' ５ >=|1,0,0,0,1>

[0059] Figure 13 illustrates the detailed circuit configuration of the computer 230C according to the third embodiment. Note that the power supplies for the first qubit Q1 and the power supplies for the second qubit Q2 are omitted. As shown in Figure 13, the computer 230C further includes a first power supply 61, a second power supply 62, a third power supply 63, capacitors 71, 72, and 73.

[0060] The first power supply 61 is connected to the variable transmon C5 via a capacitor 71. The capacitor 71 includes an end 711 and an other end 712. End 711 is connected to the other end 222 of the second capacitor 22 and the other end 232 of the third capacitor 23. The other end 712 is connected to the first power supply 61. The first power supply 61 supplies microwaves to the variable transmon C5.

[0061] The second power supply 62 is connected to the second resonator C4 via a capacitor 72. The capacitor 72 includes an end 721 and the other end 722. End 721 is connected to the end 261 of the sixth capacitor 26 and the other end 242 of the fourth capacitor 24. The other end 722 is connected to the second power supply 62. The second power supply 62 supplies microwaves to the second resonator C4.

[0062] The third power supply 63 is connected to the first resonator C3 via a capacitor 73. The capacitor 73 includes an end 731 and the other end 732. End 731 is connected to the end 251 of the fifth capacitor 25 and the end 241 of the fourth capacitor 24. The other end 732 is connected to the third power supply 63. The third power supply 63 supplies microwaves to the first resonator C3.

[0063] The first power supply 61, the second power supply 62, and the third power supply 63 are controlled by the control device 210. The control device 210 controls the first power supply 61, the second power supply 62, and the third power supply 63 according to a control sequence corresponding to the quantum circuit. The first power supply 61, the second power supply 62, and the third power supply 63 apply microwave voltages to the TTC transmons C3, C4, and C5 according to commands from the control device 210 and perform a two-qubit gate operation with respect to the first qubit Q1 and the second qubit Q2.

[0064] (Fourth Embodiment) The computer according to the fourth embodiment has a qubit measurement system. The computer according to the fourth embodiment will be described in detail below. In the following description, components having substantially the same function as those in the first, second, and third embodiments will be denoted by the same reference numerals and will be described again only when necessary.

[0065] Figure 14 is a diagram illustrating the detailed circuit configuration of the computer 230D according to the fourth embodiment. As shown in Figure 14, the computer 230D further includes a first filter 81, a capacitor 91, a first read line A1, a capacitor B1, a second filter 82, a capacitor 92, a second read line A2, and a capacitor B2.

[0066] As described above, it is possible to effectively turn off the coupling between the first qubit Q1 and the second qubit Q2. This state is called the idle state. In the idle state, it is possible to isolate the first resonator C3 and the second resonator C4. This allows the first resonator C3 to be used as the readout resonator for the first qubit Q1, and the second resonator C4 to be used as the readout resonator for the second qubit Q2.

[0067] Filter 81 is electrically connected to capacitor 91. Any filter, such as a parcel filter, can be used as filter 81. Capacitor 91 has one end 911 and the other end 912. End 911 is electrically connected to end 251 of the fifth capacitor 25 and end 241 of the fourth capacitor 24. The other end 912 is electrically connected to the first filter 81. The first read line A1 is electrically connected to capacitor B1. Capacitor B1 has one end B11 and the other end B12. End B11 is electrically connected to the first filter 81. The other end B12 is electrically connected to the first read line A1. The first read line A1 is controlled by the control device 210.

[0068] The measurement of the quantum state of the first qubit Q1 by the first readout line A1 is as follows. The control device 210 can read out the quantum state of the first qubit Q1 via the reflected signal of the signal input to the first readout line A1. Specifically, the control device 210 supplies a quantum state readout signal to the first readout line A1. Since the readout line A1 is connected to the first resonator C3 via capacitor B1, first filter 81, and capacitor 91, the reflected signal of the readout signal contains information about the first resonator C3 and information about the first qubit Q1 that interacts with the first resonator C3. The control device 210 reads the reflected signal from the first readout line A1 and measures the quantum state of the first qubit Q1.

[0069] Filter 82 is electrically connected to capacitor 92. Any filter, such as a parcel filter, can be used as filter 82. Capacitor 92 has one end 921 and the other end 922. End 921 is electrically connected to end 261 of the sixth capacitor 26 and the other end 242 of the fourth capacitor 24. The other end 922 is electrically connected to the second filter 82. The second read line A2 is electrically connected to capacitor B2. Capacitor B2 has one end B21 and the other end B22. End B21 is electrically connected to the second filter 82. The other end B22 is electrically connected to the second read line A2.

[0070] Measuring the quantum state of the second qubit Q2 using the second readout line A2 is similar to measuring the quantum state of the first qubit Q1 using the first readout line A1. That is, the control device 210 can read out the quantum state of the second qubit Q2 via the signal from the second readout line A2. Specifically, the control device 210 supplies a quantum state readout signal to the second readout line A2. Since the readout line A2 is connected to the second resonator C4 via capacitor B2 and the first filter 82 and capacitor 92, the reflected signal of the readout signal contains information about the second resonator C4 and information about the second qubit Q2 that interacts with the second resonator C4. The control device 210 reads the reflected signal from the second readout line A2 and measures the quantum state of the second qubit Q2.

[0071] (Fifth Embodiment) The computers according to the first to fourth embodiments described above have two qubits. The computer according to the fifth embodiment can be scaled up to any number of qubits, three or more, by combining the computers according to the first to fourth embodiments.

[0072] Figure 15 is a schematic plan view of a quantum processor 220 with nine qubits implemented according to the fifth embodiment. As shown in Figure 15, a total of nine qubits are provided on the substrate 240, three in the vertical direction and three in the horizontal direction. For convenience, let's assume that the central qubit is Q1. In this case, qubits Q2-i (where i is the qubit number) are connected to the central qubit Q1 via TTCs in the left, right, up, and down directions, respectively. Qubits Q2-i (where i is the qubit number) are also connected to adjacent qubits Q3-i (where i is the qubit number) via TTCs.

[0073] Each TTC includes a first resonator C3-i (where i is the TTC number), a second resonator C4-i, a variable transmon C5-i, etc., as described in the first to fourth embodiments. That is, four first resonators C3-1, C3-2, C3-3, and C3-4 are provided for the first qubit Q1. Therefore, the control device 210 can determine the quantum state of the first qubit Q1 based on statistics such as majority voting or averaging of the reflected signals from each of the four first resonators C3-1, C3-2, C3-3, and C3-4. Similarly, the quantum state of qubit Q2-i can be determined based on statistics such as majority voting or averaging of the reflected signals from each of the three resonators connected to qubit 2-i. Therefore, according to the fifth embodiment, it becomes possible to reduce measurement errors of the quantum states of Q1 and Q2-i.

[0074] Specifically, when using majority voting as a statistical method in the measurement of Q1, if errors occur in two or more of the four resonators connected to Q1, it results in a measurement error of the quantum state. If errors occur in two resonators, and the error rate of one resonator is p, then the error rate of the quantum state measurement is p. ２ Therefore, according to this embodiment, for example, p = 10 －２ In the case of 10 －４ This makes it possible to reduce measurement errors in quantum states to an extent that is achievable.

[0075] In Figure 15, nine qubits are shown on the substrate 240, but ten or more qubits may be provided. Furthermore, the arrangement of the qubits is not limited to the square shape (lattice) described above, but may be hexagonal, octagonal, or any other arbitrary arrangement.

[0076] Although the first to fifth embodiments have been described above, the first to fifth embodiments can be combined as appropriate. For example, the computer according to this embodiment may be designed to incorporate some or all of the circuit elements of the first to fourth embodiments.

[0077] Thus, according to the various embodiments described above, it becomes possible to provide couplers and computers with improved controllability.

[0078] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents.

[0079] 1...Computer system, 11...First capacitor, 111...End, 112...Other end, 12...First Josephson junction, 121...End, 122...Other end, 13...Second Josephson junction, 131...End, 132...Other end, 14...Closed circuit (loop), 22...Second capacitor, 221...End, 222...Other end, 23...Third capacitor, 231...End, 232...Other end, 24...Fourth capacitor, 241...End, 242...Other end, 25...Fifth capacitor, 251...End, 252...Other end, 26...Sixth capacitor, 261...End, 262...Other end, 31...Seventh capacitor, 311...End, 312...Other end, 32...Third Josephson junction, 321...End, 322... Other end, 33...7th capacitor, 331...end, 332...other end, 34...3rd Josephson junction, 341...end, 342...other end, 41...8th capacitor, 411...end, 412...other end, 42...4th Josephson junction, 421...end, 422...other end, 43...8th capacitor, 431...end, 432...other end, 44...4th Josephson junction, 441...end, 442...other end, 51...conductor, 61...1st power supply, 62...2nd power supply, 63...3rd power supply, 100...classical computer, 200...quantum computer, 210...control device, 220...quantum processor, 230A...computer, 230B...computer, 230C...computer, 230D...computer, 240...substrate.

Claims

1. A variable transmon comprising: a first capacitor including an end and an end; a first Josephson junction including an end and an end, wherein the end of the first Josephson junction is electrically connected to the end of the first capacitor, and the end of the first Josephson junction is electrically connected to the other end of the first capacitor; a second Josephson junction including an end and an end, wherein the end of the second Josephson junction is electrically connected to the end of the first Josephson junction, and the other end of the second Josephson junction is electrically connected to the other end of the first Josephson junction, and the variable transmon comprising a closed circuit enclosed by the first Josephson junction and the second Josephson junction; a second capacitor including an end and an end, wherein the other end of the second capacitor is electrically connected to the end of the first capacitor, and the end of the second capacitor is electrically connected to the first resonator; and the first resonator. A coupler comprising: a third capacitor including an end and an end, wherein the other end of the third capacitor is electrically connected to the end of the first capacitor, and the end of the third capacitor is electrically connected to a second resonator; a second resonator; a fourth capacitor including an end and an end, wherein the end of the fourth capacitor is electrically connected to the end of the second capacitor, and the other end of the fourth capacitor is electrically connected to the end of the third capacitor; an end of a fifth capacitor electrically connected to the end of the second capacitor, the end of the fourth capacitor and the first resonator, wherein the fifth capacitor further includes an end that can be electrically connected to a first qubit; and an end of a sixth capacitor electrically connected to the end of the third capacitor, the other end of the fourth capacitor and the second resonator, wherein the sixth capacitor further includes an end that can be electrically connected to a second qubit.

2. The coupler according to claim 1, wherein at least one of the first resonator and the second resonator is a waveguide resonator.

3. The coupler according to claim 1, wherein at least one of the first resonator and the second resonator is an array including an array end and an array end, the array including two or more electrically connected Josephson junctions; and a seventh capacitor including an array end and an array end, the seventh capacitor having the array end electrically connected to the array end and the array end electrically connected to the array end.

4. The coupler according to claim 1, wherein at least one of the first resonator and the second resonator includes a third Josephson junction having an A end and the other end, and a seventh capacitor having an A end and the other end, wherein the A end of the seventh capacitor is electrically connected to the A end of the third Josephson junction, and the other end of the seventh capacitor is electrically connected to the other end of the third Josephson junction.

5. The coupler according to claim 1, wherein both the first resonator and the second resonator comprise only a third Josephson junction having an A end and the other end, and a seventh capacitor having an A end and the other end, wherein the A end of the seventh capacitor is electrically connected to the A end of the third Josephson junction, and the other end of the seventh capacitor is electrically connected to the other end of the third Josephson junction.

6. The coupler according to any one of claims 1 to 5, wherein when the magnetic flux passing through the closed circuit is zero, the coupler is capable of resonating in multiple modes, and the resonant frequency of each of the multiple modes is higher than the resonant frequency of the first qubit and the resonant frequency of the second qubit.

7. The coupler according to claim 6, wherein the third lowest frequency among the resonant frequencies of each of the multiple modes is the resonant frequency of the variable transmon.

8. A computer comprising: the coupler according to claim 1; the other end of the fifth capacitor; the first qubit; the other end of the sixth capacitor; and the second qubit.

9. The computer according to claim 8, wherein at least one of the first qubit and the second qubit includes an eighth capacitor having an end and the other end, and a fourth Josephson junction having an end and the other end, wherein the end of the fourth Josephson junction is electrically connected to the end of the eighth capacitor, and the other end of the fourth Josephson junction is electrically connected to the other end of the eighth capacitor.

10. The computer according to claim 8, wherein both the first qubit and the second qubit comprise only an eighth capacitor having an end and the other end, and a fourth Josephson junction having an end and the other end, wherein the end of the fourth Josephson junction is electrically connected to the end of the eighth capacitor, and the other end of the fourth Josephson junction is electrically connected to the other end of the eighth capacitor.

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