Quantum computing circuits and couplers

Abstract

This allows for relatively precise adjustment of the interaction strength of qubit elements. [Solution] The quantum computing circuit comprises a qubit element and a coupler that causes two or more of the qubit elements to interact, the coupler comprising a loop having a plurality of Josephson junctions, each having critical current values ​​different in at least two Josephson junctions, and a magnetic field application means for applying a magnetic field to the loop.

Description

[Technical Field]

[0001] This disclosure relates to quantum computing circuits and couplers. [Background technology]

[0002] In configurations where multiple qubit elements are coupled together, the strength of the interaction between the multiple qubit elements (the coupling strength of the qubit elements) may be adjusted. For example, Patent Document 1 describes a method of coupling two or four Josephson parametric oscillators and varying the coupling strength by varying the relative phase between the pump signals supplied to these Josephson parametric oscillators for parametric oscillation. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-076272 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] It is preferable to be able to adjust the strength of the interaction between multiple qubit elements with the highest possible precision.

[0005] One example of the purpose of this disclosure is to provide a quantum computing circuit and coupler that can solve the above-mentioned problems. [Means for solving the problem]

[0006] According to a first aspect of the present disclosure, a quantum computing circuit comprises a qubit element and a coupler for interacting two or more of the qubit elements, the coupler comprising a loop having a plurality of Josephson junctions, each having different critical current values ​​in at least two Josephson junctions, and a magnetic field applying means for applying a magnetic field to the loop.

[0007] According to a second aspect of the present disclosure, the coupler comprises a loop having a plurality of Josephson junctions, each having different critical current values ​​in at least two Josephson junctions, and a magnetic field applying means for applying a magnetic field to the loop, thereby causing two or more qubit elements to interact. [Effects of the Invention]

[0008] According to this disclosure, the strength of the interaction between qubit elements can be adjusted with relatively high precision. [Brief explanation of the drawing]

[0009] [Figure 1] This figure shows an example of the configuration of an information processing device according to at least one embodiment. [Figure 2] This figure shows an example of a many-body coupling implementation in a quantum computing circuit 100 according to at least one embodiment. [Figure 3] This figure shows an example of the relationship between the critical current value of a Josephson junction loop and the Kerr nonlinearity of the coupler. [Figure 4] This figure shows an example of the relationship between the magnetic field applied to a Josephson junction loop, the coupler's resonant frequency, and the Kerr nonlinearity. [Figure 5] This figure shows a first example of the configuration of a coupler according to at least one embodiment. [Figure 6] This figure shows a second example of the configuration of a coupler according to at least one embodiment. [Figure 7] This figure shows a third example of the configuration of a coupler according to at least one embodiment. [Figure 8] This figure shows an example of the configuration of a quantum computing circuit according to at least one embodiment. [Figure 9] This figure shows an example of the configuration of a coupler according to at least one embodiment. [Figure 10] This is a schematic block diagram showing the configuration of a computer according to at least one embodiment. [Modes for carrying out the invention]

[0010] Hereinafter, embodiments will be described with reference to the drawings.

[0011] <First Embodiment> FIG. 1 is a diagram showing an example of the configuration of an information processing apparatus according to at least one embodiment. In the configuration shown in FIG. 1, the information processing apparatus 1 includes a quantum computing circuit 100, a control unit 200, and an observation unit 300. The quantum computing circuit 100 includes a quantum bit element 110 and a coupler 120.

[0012] The information processing apparatus 1 performs quantum computing. For example, the information processing apparatus 1 may perform quantum annealing. The information processing apparatus 1 can also be referred to as a quantum computer. When the information processing apparatus 1 performs quantum annealing, the information processing apparatus can also be referred to as a quantum annealing machine.

[0013] The quantum computing circuit 100 executes quantum computing according to the control by the control unit 200. The quantum bit element 110 is an element for expressing a quantum bit value. The quantum bit element 110 may be configured using a Josephson parametric oscillator (JPO), but is not limited thereto.

[0014] The coupler 120 causes two or more quantum bit elements 110 to interact. The coupler 120 may cause three or more quantum bit elements 110 to interact in a many-body manner. Here, the interaction of the quantum bit elements 110 means that the quantum bit values expressed by the quantum bit elements 110 are correlated. The interaction of the quantum bit elements 110 can also be referred to as the interaction between the quantum bit elements 110. The interaction of the quantum bit elements 110 can also be referred to as the interaction of the quantum bits or the interaction between the quantum bits. The interaction of the quantum bit elements 110 can also be referred to as the coupling of the quantum bit elements, the coupling between the quantum bit elements, the coupling of the quantum bits, or the coupling between the quantum bits. The coupler 120 may be configured using a nonlinear coupler, such as a Josephson parametric oscillator.

[0015] The control unit 200 controls the quantum computing circuit 100 to perform quantum computing. For example, the control unit 200 sets parameter values ​​for quantum computing, such as the strength of the many-body interaction of the qubit elements 110 by the coupler 120, depending on the problem to be solved by quantum computing, such as a combinatorial optimization problem. The control unit 200 also controls the transition of the state of the qubit elements 110 (quantum state in the qubit elements 110) by changing the time of the magnetic field input to the qubit elements 110.

[0016] The observation unit 300 reads the qubit value as a result of quantum computing. Specifically, after a predetermined time has elapsed since the start of quantum computing, the observation unit 300 observes the output signal of the qubit element 110 to detect the quantum state.

[0017] Figure 2 shows an example of the implementation of many-body coupling (many-body interaction) in the quantum computing circuit 100. Figure 2 shows an example of four-body coupling, where four qubit elements 110 are connected to one coupler 120. However, the number of qubit elements 110 that the coupler 120 interacts with is not limited to four; it can be two or more.

[0018] The qubit element 110 includes a Josephson junction loop 111, an inductor 112, and a capacitor 113. The Josephson junction loop 111 and the capacitor 113 are arranged on a loop. The loop including the Josephson junction loop 111 and the capacitor 113 is also called a resonator 114. The presence of a Josephson junction in a loop can also be described as the loop having a Josephson junction.

[0019] The Josephson junction loop 111 is a superconductor in which a Josephson junction is provided forms a loop (closed circuit). For example, the Josephson junction loop 111 may be a SQUID (Superconducting Quantum Interference Device) which is a loop having two Josephson junctions, but is not limited to this.

[0020] The inductor 112 generates a magnetic field when current flows through it, and applies this magnetic field to the Josephson junction loop 111. By applying a variable-intensity magnetic field to the Josephson junction loop 111, the inductor 112 can make the Josephson junction loop 111 function as a variable inductor (an inductor with variable inductance). The strength of a magnetic field can also be referred to as the magnitude of the magnetic field.

[0021] Capacitor 113 represents the capacitance of the resonator 114. The structural capacitor of the resonator 114 may function as capacitor 113, or a component designated as capacitor 113 may be provided. The capacitance of capacitor 113 can also be considered as the capacitance of the qubit element 110. The Josephson junction loop 111 functions as a variable inductor, making the resonator 114 a loop with a variable resonant frequency. This allows the qubit element 110 to operate as a parametric oscillator. The resonant frequency of the resonator 114 can also be referred to as the resonant frequency of the coupler 120.

[0022] The coupler 120 includes a Josephson junction loop 121, an inductor 122, and a capacitor 123. The Josephson junction loop 121 and the capacitor 123 are arranged on a loop. The loop including the Josephson junction loop 121 and the capacitor 123 is also referred to as the resonator 124.

[0023] The Josephson junction loop 121 is formed by a superconductor containing a Josephson junction forming a loop. For example, the Josephson junction loop 121 may be a SQUID, but is not limited to this. Various nonlinear elements can be used as the Josephson junction loop 121. Because the Josephson junction loop 121 is constructed using a nonlinear element, three or more qubit elements 110 can be made to interact.

[0024] Here, the Hamiltonian H of a linear resonator can be expressed as shown in equation (1) using capacitance C, inductance L, charge Q, and magnetic flux Φ.

[0025]

number

[0026] In contrast, the Hamiltonian of a nonlinear resonator includes terms of order 3 or higher of Φ.

[0027] The Josephson junction loop 121 is assumed to have multiple asymmetric Josephson junctions. Symmetrical Josephson junctions mean that their critical current values ​​are equal. Asymmetrical Josephson junctions mean that there are Josephson junctions with critical current values ​​different from those of other Josephson junctions. If the Josephson junction loop 121 has three or more Josephson junctions, the critical current values ​​of all Josephson junctions may be different from each other, or some of the critical current values ​​may be the same. A Josephson junction loop that has an asymmetric Josephson junction is also called an asymmetric Josephson junction loop.

[0028] The inductor 122 generates a magnetic field when current flows through it, and applies this magnetic field to the Josephson junction loop 121. By applying a variable-intensity magnetic field to the Josephson junction loop 121, the inductor 122 can make the Josephson junction loop 121 function as a variable inductor. The inductor 122 is an example of a means for applying a magnetic field.

[0029] Capacitor 123 represents the capacitance of the resonator 124. The structural capacitor of the resonator 124 may function as capacitor 123, or a component designated as capacitor 123 may be provided. The capacitance represented by capacitor 123 can also be considered as the capacitance of the coupler 120.

[0030] The Josephson junction loop 121 functions as a variable inductor, making the resonator 124 a loop with a variable resonant frequency. By adjusting the resonant frequency of the coupler 120 and causing it to interact with each of the qubit elements 110, the four qubit elements 110 can be made to interact with each other. Because the inductance of the Josephson junction loop 121 is variable, the strength of the interaction between the coupler 120 and the qubit elements 110 can be adjusted.

[0031] Here, we will explain the relationship between the asymmetric nature of the Josephson junction loop in the coupler and the adjustment of the coupling strength of the qubit elements. Figure 3 shows an example of the relationship between the critical current value of a Josephson junction loop and the Kerr nonlinearity of the coupler. Figure 3 shows an example where the Josephson junction loop is a four-body coupler with a single junction and a resonant frequency of 10 gigahertz (GHz). Here, a Josephson junction loop is considered to be a single junction if it contains only one Josephson junction. In the graph in Figure 3, the horizontal axis represents the critical current value, and the vertical axis represents the intensity of Kerr nonlinearity. The stronger the Kerr nonlinearity, the stronger the four-body interaction originating from the coupler. The intensity of Kerr nonlinearity can also be referred to as the magnitude of Kerr nonlinearity. The intensity of the interaction can also be referred to as the magnitude of the interaction.

[0032] In the example shown in Figure 3, the smaller the critical current value, the stronger the Kerr nonlinearity. Regardless of whether there are four interacting qubit elements or only one Josephson junction in the Josephson junction loop, there is a correlation where the smaller the critical current value, the stronger the interaction between qubit elements.

[0033] Furthermore, the resonant frequency f of the coupler is given by equation (2).

[0034]

number

[0035] π represents the ratio of a circle's circumference to its diameter (pi). C represents the structural capacitance of the resonator (a loop containing a Josephson junction loop and a capacitor). Here, the structural capacitance of the resonator is the capacitance of the resonator. L J This shows the inductance of the Josephson junction loop. L represents the structural inductance of the resonator. Here, the structural inductance of the resonator is the inductance of the resonator excluding the inductance of the Josephson junction loop. Therefore, the structural inductance of the resonator L and the inductance of the Josephson junction loop L J The sum of L + L J This indicates the inductance of the resonator.

[0036] As shown in equation (2), the smaller the capacitance C, the larger the resonant frequency f. If we want to reduce the capacitance C to strengthen the interaction of the qubit elements while keeping the resonant frequency f constant, we can reduce the structural inductance L of the resonator or the inductance L of the Josephson junction loop. J At least one of these needs to be increased.

[0037] Here, there is a correlation where increasing the structural inductance L weakens the nonlinearity (the nonlinearity decreases). As the nonlinearity weakens, the interaction of the qubit elements also weakens. To strengthen the interaction of qubit elements while keeping the resonant frequency f constant, the capacitance C should be reduced, and the inductance L of the Josephson junction loop should be reduced. J It is possible to enlarge it. Inductance L of a Josephson junction loop J It is proportional to the reciprocal of the critical current value of the Josephson junction. Therefore, the inductance L of the Josephson junction loop is proportional to the reciprocal of the critical current value of the Josephson junction. J To increase this, it is necessary to decrease the critical current value of the Josephson junction.

[0038] In contrast, fabricating Josephson junctions with small critical current values ​​is generally difficult. Furthermore, qubit devices often utilize Josephson junctions with relatively large critical current values, typically several hundred nanoamperes (nA). By using the same Josephson junctions as in the qubit device for the coupler, both the qubit device and the coupler can be fabricated efficiently.

[0039] Therefore, we consider adjusting the resonant frequency by applying a magnetic field to the Josephson junction loop. Specifically, we consider fabricating a coupler using a Josephson junction with a large critical current value, similar to that of a qubit element, and then lowering the resonant frequency by applying a magnetic field to the Josephson junction loop.

[0040] In this case, adjusting the applied magnetic field to reduce the resonant frequency increases the nonlinearity, resulting in a stronger interaction. In the case of a coupler with a Josephson junction loop, where multiple symmetrical Josephson junctions are provided, adjusting the applied magnetic field reduces the resonant frequency to zero.

[0041] When the resonant frequency decreases to zero, the gradient of the resonant frequency with respect to the magnetic field becomes steeper compared to when the resonant frequency does not decrease to zero, which may make it difficult to adjust the resonant frequency with high precision. Therefore, as described above, a Josephson junction loop 121 of the coupler 120 is used, which is provided with multiple asymmetric Josephson junctions. In this case, the minimum value of the resonant frequency becomes greater than 0, and the gradient of the resonant frequency with respect to the magnetic field becomes relatively gentle. In particular, near the magnetic field where the resonant frequency is at its minimum value, the gradient of the resonant frequency with respect to the magnetic field becomes relatively gentle, and it is expected that the resonant frequency can be adjusted with relatively high precision using a range of resonant frequencies in which the interaction of the qubit elements is relatively strong. For example, it is expected that the resonant frequency can be kept approximately constant against changes in the magnetic field due to factors such as noise, and that resistance to magnetic field fluctuations can be provided.

[0042] Figure 4 shows an example of the relationship between the magnetic field applied to a Josephson junction loop and the coupler's resonant frequency and Kerr nonlinearity. Figure 4 shows an example for a four-body coupler equipped with a SQUID. The horizontal axis of the graph in Figure 4 represents the magnetic field strength, normalized by the magnetic flux quantum Φ0. The vertical axis represents the resonant frequency and the strength of the Kerr nonlinearity.

[0043] Line L111 shows an example of the relationship between magnetic field strength and resonant frequency in the case of an asymmetric SQUID. Line L112 shows an example of the relationship between magnetic field strength and resonant frequency for a symmetric SQUID. Line L121 shows an example of the relationship between magnetic field strength and Kerr nonlinearity strength in the case of an asymmetric SQUID.

[0044] For a symmetrical SQUID, the minimum resonant frequency is 0, and the gradient of the resonant frequency with respect to the magnetic field is relatively steep, as shown by line L112. In contrast, for an asymmetric SQUID, the minimum resonant frequency is greater than 0, and the gradient of the resonant frequency with respect to the magnetic field becomes relatively gentle, as shown by line L111. Because the gradient of the resonant frequency with respect to the magnetic field is relatively gentle, it is expected that the resonant frequency can be adjusted with relatively high precision.

[0045] Furthermore, as shown by line L121, the intensity of Kerr nonlinearity is strongest (maximum) at the magnetic field strength that minimizes the resonant frequency. By adjusting the strength of the magnetic field applied to the Josephson junction loop, the intensity of Kerr nonlinearity can be adjusted according to the resonant frequency. There is a relationship where the Kerr nonlinearity becomes stronger as the resonant frequency decreases, and as the slope of the resonant frequency becomes gentler, the slope of the Kerr nonlinearity also becomes gentler. With the coupler 120, in this respect, the Kerr nonlinearity can be adjusted with relatively high precision, and it is expected that the strength of the interaction of the qubit elements 110 can be adjusted with relatively high precision.

[0046] In particular, the inductor 122 may apply a magnetic field to the Josephson junction loop 121 in a variable strength within a range that includes the strength at which the resonant frequency of the coupler 120 is minimized. That is, the control unit 200 may adjust the strength of the interaction between the qubit elements by the coupler 120 by setting the current value flowing through the inductor 122 to a current value within a range that includes the current value at which the resonant frequency of the coupler 120 is minimized.

[0047] This allows the control unit 200 to adjust the strength of the interaction within a range where the interaction by the coupler 120 is relatively strong. Furthermore, as illustrated in Figure 4, near the magnetic field where the resonant frequency of the coupler 120 takes its minimum value, the gradient of the interaction strength with respect to the magnetic field strength is particularly gentle, and it is expected that the control unit 200 can adjust the interaction strength with relatively high precision. The control unit 200 adjusting the strength of the interaction by the coupler 120 can also be understood as the coupler 120 adjusting the strength of the interaction in accordance with the control by the control unit 200.

[0048] It is expected that the coupler 120 can adjust the interaction strength of the qubit elements 110 with relatively high precision, not only when there are two Josephson junctions in the Josephson junction loop 121, but also when there are three or more Josephson junctions. It is expected that the coupler 120 can adjust the strength of the interaction between the qubit elements 110 with relatively high precision, not only when there are four qubit elements 110 to interact, but also when there are two or three qubit elements 110, and even when there are five or more qubit elements 110 to interact.

[0049] As described above, the Josephson junction loop 121 of the coupler 120 may be a SQUID, but is not limited to this. Figure 5 shows a first example of the configuration of the coupler 120. In the example in Figure 5, the Josephson junction loop 121 is constructed using an asymmetric SQUID. As described above, a SQUID is a loop having two Josephson junctions.

[0050] A Josephson junction is also referred to as a Josephson junction 125. In the example in Figure 5, the two Josephson junctions are also referred to as Josephson junctions 125a and 125b. Josephson junctions 125a and 125b are asymmetrical; that is, Josephson junctions 125a and 125b have different critical current values. As shown in the example in Figure 5, a Josephson junction loop 121 constructed using a SQUID is also referred to as a Josephson junction loop 121a. A coupler 120 in which the Josephson junction loop 121 is constructed using an asymmetric SQUID is also referred to as a coupler 120a.

[0051] Figure 6 shows a second example of the configuration of the coupler 120. In the example in Figure 6, the Josephson junction loop 121 is configured as a loop having three or more asymmetric Josephson junctions 125. Figure 6 shows an example where the Josephson junction loop 121 has three Josephson junctions 125. However, the Josephson junction loop 121 may have four or more Josephson junctions 125. Also, the number of Josephson junctions to the left of the Josephson junction loop 121b (the number of Josephson junctions in the row of Josephson junctions 125c) in Figure 6 is not limited to one, and multiple Josephson junctions may be provided.

[0052] In the example in Figure 6, the three Josephson junctions are also referred to as Josephson junctions 125c, 125d, and 125e. Josephson junctions 125c, 125d, and 125e are asymmetric. That is, at least two of Josephson junctions 125c, 125d, and 125e have different critical current values. It is also possible that all critical current values ​​of Josephson junctions 125c, 125d, and 125e are different from each other.

[0053] As shown in the example in Figure 6, a Josephson junction loop 121 having three or more asymmetric Josephson junctions 125 is also referred to as a Josephson junction loop 121b. A coupler 120 having a Josephson junction loop 121 with three or more asymmetric Josephson junctions 125 is also referred to as a coupler 120b. Even if four or more Josephson junctions 125 are provided in the Josephson junction loop 121b, it is sufficient that the critical current value of one or more of those four or more Josephson junctions 125 is different from the critical current values ​​of the other Josephson junctions 125.

[0054] Here, if the Josephson junction loop 121 is constructed using a SQUID, additional operations such as signal application are required to make an odd number of qubit elements 110 interact, such as a three-body interaction or a five-body interaction. In contrast, since the Josephson junction loop 121 is constructed as a loop having three or more Josephson junctions, it is expected that an odd number of qubit elements 110 can be made to interact with relatively high precision without the need for additional operations.

[0055] Figure 7 shows a third example of the configuration of the coupler 120. In the example in Figure 7, the Josephson junction loop 121 is constructed using a combination of two loops. The two loops are also referred to as loops L11 and L12. Furthermore, in the example shown in Figure 7, the Josephson junction loop 121 has four asymmetric Josephson junctions 125. Each of the four Josephson junctions 125 is also referred to as Josephson junction 125f, 125g, 125h, and 125i.

[0056] Of the four Josephson junctions 125, Josephson junction 125f is found only in loop L11 of loops L11 and L12. Josephson junctions 125g and 125h are common Josephson junctions 125 found in both loops L11 and L12. That is, Josephson junctions 125g and 125h are found in both loops L11 and L12. Josephson junction 125i is found only in loop L12 of loops L11 and L12.

[0057] Josephson junctions 125f, 125g, 125h, and 125i are asymmetric. That is, at least two of the Josephson junctions 125f, 125g, 125h, and 125i have different critical current values. It is also possible that all of the critical current values ​​of Josephson junctions 125f, 125g, 125h, and 125i are different from each other.

[0058] However, a Josephson junction loop 121 may consist of three or more loops. Furthermore, the number of Josephson junctions 125 in a Josephson junction loop 121 consisting of multiple loops is not limited to a specific number. Each loop may have a Josephson junction 125 that is unique to that loop, and there may also be a Josephson junction 125 common to multiple loops.

[0059] As shown in the example in Figure 7, a Josephson junction loop 121 in which multiple asymmetric Josephson junctions 115 are provided in multiple loops is also called a Josephson junction loop 121c. A coupler 120 having a Josephson junction loop 121 in which multiple asymmetric Josephson junctions 115 are provided in multiple loops is also called a coupler 120c. Even if the Josephson junction loop 121c has four or more Josephson junctions 125, it is sufficient that the critical current value of one or more of those four or more Josephson junctions 125 is different from the critical current values ​​of the other Josephson junctions 125.

[0060] Since the Josephson junction loop 121 is configured as a loop having three or more Josephson junctions, it is expected that an odd number of qubit elements 110 can be interacted with relatively high precision. Furthermore, since the Josephson junction loop 121 is configured using a combination of multiple loops, the magnetic field applied to each loop can be adjusted, and it is expected that the interaction of the qubit elements 110 can be executed with relatively high precision.

[0061] As shown in the examples in Figure 6 and Figure 7, the Josephson junction loop 121 is configured as a loop having three or more Josephson junctions 125, which is expected to allow an odd number of qubit elements 110 to interact with relatively high precision without the need for additional operations.

[0062] On the other hand, as explained with reference to Figures 3 and 4, in the region where the gradient of the resonant frequency with respect to the magnetic field strength is relatively gentle, a higher sharpness in the graph of the part where the resonant frequency is minimum results in a narrower region, while a lower sharpness results in a wider region. Furthermore, when there are three or more Josephson junctions 125 in the Josephson junction loop 121, the sharpness of the graph is higher compared to the case with two junctions.

[0063] Therefore, as shown in the example in Figure 5, when the Josephson junction loop 121 has two Josephson junctions 125, the gradient of the resonant frequency with respect to the magnetic field strength becomes gentler compared to when the Josephson junction loop 121 has three or more Josephson junctions 125. In this respect, when the Josephson junction loop 121 has two Josephson junctions 125, it is expected that the resonant frequency can be adjusted with relatively high precision.

[0064] Furthermore, when the Josephson junction loop 121 has two Josephson junctions 125, the gradient of the interaction strength of the qubit element 110 with respect to the magnetic field strength becomes gentler compared to when the Josephson junction loop 121 has three or more Josephson junctions 125. In this respect, when the Josephson junction loop 121 has two Josephson junctions 125, it is expected that the interaction strength of the qubit element 110 can be adjusted with relatively high precision.

[0065] As described above, the Josephson junction loop 121 is provided with multiple Josephson junctions 125, each having at least two Josephson junctions 125 with different critical current values. Inductor 122 applies a magnetic field to the Josephson junction loop 121. The coupler 120 interacts two or more qubit elements.

[0066] In the quantum computing circuit 100, the minimum value of the resonance frequency of the coupler 120 becomes larger than 0, and the gradient of the resonance frequency with respect to the magnetic field becomes relatively gentle. According to the quantum computing circuit 100, in this regard, the resonance frequency of the coupler 120 can be adjusted with relatively high precision, and thus it is expected that the intensity of the interaction between the qubit elements by the coupler 120 can be adjusted with relatively high precision.

[0067] Further, the inductor 122 applies a magnetic field of an intensity near the intensity at which the resonance frequency of the coupler 120 takes the minimum value to the Josephson junction loop 121. For example, when the resonance frequency f is a function f(Φ) of the magnetic field intensity Φ (intensity normalized by the magnetic flux quantum Φ0), within the range where the second derivative coefficient of f is a positive value, that is, within the range shown by Equation (3), d 2 f / dΦ 2 >0 it is preferable that the value of Φ is set within the range where this holds.

[0068]

Number

[0069] This can also be rephrased as follows. Let the value of f that satisfies Equation (4) be f1.

Number

[0070] In FIG. 4, the point where Equation (4) holds is the inflection point of the graph. The value of Φ corresponding to the inflection point in this case is at two locations, on the smaller side (left side towards the graph) and the larger side (right side towards the graph), with respect to the value of Φ corresponding to the minimum resonance frequency f min When these are denoted as Φ 1a and Φ 1b respectively, it is preferable that the magnetic field intensity Φ applied to the Josephson junction loop 121 is set to a value within the range shown by Equation (5).

number

[0071] Also, f1 and f min Let f² represent the midpoint value (the value at the midpoint, or the mean). That is, let it be expressed as shown in equation (6).

number

[0072] Similar to the case of the value of Φ corresponding to f1 (i.e., the case of the inflection point), the value of Φ corresponding to f2 is the minimum resonant frequency f min For the value of Φ corresponding to , there are two locations: on the smaller side (left side of the graph) and on the larger side (right side of the graph). These are respectively Φ 2a and Φ 2b In this case, it is even more preferable that the magnetic field strength Φ applied to the Josephson junction loop 121 be set to a value within the range shown in equation (7).

number

[0073] Furthermore, f2 and f min Let f3 represent the midpoint value (the value at the midpoint, or the mean). That is, let it be expressed as shown in equation (8).

number

[0074] Similar to the case of the value of Φ corresponding to f2, the value of Φ corresponding to f3 is the minimum resonant frequency f min For the value of Φ corresponding to , there are two locations: on the smaller side (left side of the graph) and on the larger side (right side of the graph). These are respectively Φ 3a and Φ 3b In this case, it is even more preferable that the magnetic field strength Φ applied to the Josephson junction loop 121 be set to a value within the range shown in equation (9).

[0075]

number

[0076] According to the quantum computing circuit 100, the strength of the interaction can be adjusted within a range where the interaction of the qubit elements 110 is relatively strong. Furthermore, according to the quantum computing circuit 100, the interaction strength can be adjusted within a range where the gradient of the interaction strength with respect to the magnetic field strength is relatively gentle, and in this respect, it is expected that the interaction strength can be adjusted with relatively high precision.

[0077] Furthermore, the Josephson junction loop 121 is provided with two Josephson junctions that have different critical current values. According to the quantum computing circuit 100, the gradient of the resonant frequency with respect to the magnetic field strength becomes gentler compared to the case where the Josephson junction loop 121 has three or more Josephson junctions 125. In this respect, the quantum computing circuit 100 is expected to allow for relatively high-precision adjustment of the resonant frequency.

[0078] Furthermore, according to the quantum computing circuit 100, the gradient of the interaction strength of the qubit element 110 with respect to the magnetic field strength becomes gentler compared to the case where the number of Josephson junctions 125 in the Josephson junction loop 121 is three or more. In this respect, the quantum computing circuit 100 is expected to allow for relatively high-precision adjustment of the interaction strength of the qubit element 110.

[0079] Furthermore, the Josephson junction loop 121 is provided with three or more Josephson junctions 115, each having at least two Josephson junctions 115 with different critical current values. According to the quantum computing circuit 100, it is expected that an odd number of qubit elements 110 can be made to interact with relatively high precision.

[0080] <Second Embodiment> Figure 8 shows an example of the configuration of a quantum computing circuit according to at least one embodiment. In the configuration shown in Figure 8, the quantum computing circuit 610 comprises a qubit element 611 and a coupler 612. The coupler 612 comprises a loop 613 and a magnetic field application unit 615. The loop 613 is provided with a plurality of Josephson junctions 614.

[0081] The coupler 612 causes two or more qubit elements 611 to interact. Of the multiple Josephson junctions 614, the critical current values ​​of at least two Josephson junctions 614 are different from each other. The magnetic field application unit 615 applies a magnetic field to the loop 613. The magnetic field application unit 615 is an example of a magnetic field application means.

[0082] In the quantum computing circuit 610, the minimum resonant frequency of the coupler 612 becomes greater than 0, resulting in a relatively gentle slope of the resonant frequency with respect to the magnetic field. According to the quantum computing circuit 610, the resonant frequency of the coupler 612 can be adjusted with relatively high precision, and this is expected to allow for relatively high-precision adjustment of the strength of the interaction between the qubit elements by the coupler 612.

[0083] <Third Embodiment> Figure 9 shows an example of the configuration of a coupler according to at least one embodiment. In the configuration shown in Figure 9, the coupler 620 comprises a loop 621 and a magnetic field application section 623. The loop 621 is provided with a plurality of Josephson junctions 622.

[0084] Of the multiple Josephson junctions 622, the critical current values ​​in at least two of them are different from each other. The magnetic field application unit 623 applies a magnetic field to the loop 621. The coupler 620 interacts two or more qubit elements. The magnetic field application unit 623 is an example of a magnetic field application means.

[0085] In the coupler 620, the minimum resonant frequency becomes greater than 0, and the gradient of the resonant frequency with respect to the magnetic field becomes relatively gentle. With the coupler 620, the resonant frequency can be adjusted with relatively high precision, and it is expected that the strength of the interaction between the qubit elements by the coupler 620 can be adjusted with relatively high precision.

[0086] Figure 10 is a schematic block diagram showing the configuration of a computer according to at least one embodiment. In the configuration shown in Figure 10, the computer 700 comprises a CPU 710, a main memory 720, an auxiliary memory 730, an interface 740, a non-volatile recording medium 750, and a quantum device 760.

[0087] The above-described information processing device 1 or a part thereof may be implemented in a computer 700. In that case, the quantum computing circuit 100 may be used as the quantum device 760. The operation of the control unit 200 and the observation unit 300 may be stored in the auxiliary storage device 730 in the form of a program. The CPU 710 reads the program from the auxiliary storage device 730, loads it into the main memory 720, and executes the above processing according to the program. The CPU 710 also reserves memory in the main memory 720 for the control unit 200 and the observation unit 300 to perform processing according to the program. Furthermore, the interface 740 outputs control signals to the quantum device 760 and reads signals output by the quantum device 760, in accordance with the control of the CPU 710. The interface 740 also has a port for the non-volatile recording medium 750, and reads information from the non-volatile recording medium 750 and writes information to the non-volatile recording medium 750. Communication between the information processing device 1 and other devices is performed by the interface 740 having a communication function and performing communication according to the control of the CPU 710. Interaction between the information processing device 1 and the user is performed by the interface 740 having an input device and an output device, presenting information to the user via the output device and accepting user operations via the input device according to the control of the CPU 710.

[0088] One or more of the above-mentioned programs may be recorded on the non-volatile recording medium 750. In this case, the interface 740 may read the program from the non-volatile recording medium 750. The CPU 710 may then either directly execute the program read by the interface 740, or temporarily save it in the main memory 720 or auxiliary memory 730 before executing it.

[0089] Alternatively, the processing of each unit may be performed by recording a program for executing all or part of the processing performed by the control unit 200 and the observation unit 300 on a computer-readable recording medium, and then loading and executing the program recorded on this recording medium into a computer system. The term "computer system" here includes hardware such as the operating system (OS) and peripheral devices. Furthermore, "computer-readable recording media" refers to portable media such as flexible disks, magneto-optical disks, ROMs (Read Only Memory), CD-ROMs (Compact Disc Read Only Memory), and storage devices such as hard disks built into computer systems. The above-mentioned program may be intended to implement only a part of the functions described above, and may also be able to implement the above-mentioned functions in combination with programs already recorded in the computer system.

[0090] Although the present disclosure has been described above with reference to embodiments, the present disclosure is not limited to the embodiments described above. Various modifications to the structure and details of the present disclosure can be made as understandable to those skilled in the art within the scope of the present disclosure. Furthermore, the embodiments described above may be combined with other embodiments as appropriate.

[0091] Some or all of the above embodiments may also be described as follows, but are not limited to the following:

[0092] (Note 1) The system comprises a qubit element and a coupler that causes two or more of the qubit elements to interact, The aforementioned coupler is, A loop comprising multiple Josephson junctions, where the critical current values ​​of at least two Josephson junctions are different from each other, A magnetic field application means for applying a magnetic field to the aforementioned loop, Equipped with, Quantum computing circuit.

[0093] (Note 2) The magnetic field applying means applies a magnetic field to the loop with an intensity near the intensity at which the resonance frequency of the coupler takes its minimum value. The quantum computing circuit described in Appendix 1.

[0094] (Note 3) The aforementioned loop is provided with two Josephson junctions having different critical current values. The quantum computing circuit described in Appendix 1 or Appendix 2.

[0095] (Note 4) The loop is provided with three or more Josephson junctions, each having critical current values ​​that are different from those of at least two Josephson junctions. The quantum computing circuit described in Appendix 1 or Appendix 2.

[0096] (Note 5) A loop comprising multiple Josephson junctions, where the critical current values ​​of at least two Josephson junctions are different from each other, A magnetic field application means for applying a magnetic field to the aforementioned loop, Equipped with, Interacting two or more qubit elements combiner.

[0097] (Note 6) The magnetic field applying means applies a magnetic field to the loop with an intensity near the intensity at which the resonance frequency of the coupler takes its minimum value. The coupler described in Appendix 5.

[0098] (Note 7) The aforementioned loop is provided with two Josephson junctions having different critical current values. The coupler described in Appendix 5 or Appendix 6.

[0099] (Note 8) The loop is provided with three or more Josephson junctions, each having critical current values ​​that are different from those of at least two Josephson junctions. The coupler described in Appendix 5 or Appendix 6. [Explanation of Symbols]

[0100] 1. Information Processing Device 100, 610 quantum calculation circuit 110, 611 qubit elements 111, 121 Josephson junction loop 112, 122 inductors 113, 123, 130 Capacitors 114, 124 resonator 115, 125, 614, 622 Josephson junctions 120, 612, 620 combiner 200 Control Unit 300 Observation Unit 613, 621 Loop 615, 623 Magnetic field application section

Claims

1. The system comprises a qubit element and a coupler that causes two or more of the qubit elements to interact, The aforementioned coupler is, A loop comprising multiple Josephson junctions, where the critical current values ​​of at least two Josephson junctions are different from each other, A magnetic field application means for applying a magnetic field to the aforementioned loop, Equipped with, Quantum computing circuit.

2. The resonant frequency of the coupler is expressed as a function of the magnetic field strength, The magnetic field application means applies a magnetic field to the loop with an intensity in a range such that the second derivative of the resonant frequency is a positive value. The quantum computing circuit according to claim 1.

3. The aforementioned loop is provided with two Josephson junctions having different critical current values. A quantum computing circuit according to claim 1 or claim 2.

4. The loop is provided with three or more Josephson junctions, each having critical current values ​​that are different from those of at least two Josephson junctions. A quantum computing circuit according to claim 1 or claim 2.

5. A loop comprising multiple Josephson junctions, where the critical current values ​​of at least two Josephson junctions are different from each other, A magnetic field application means for applying a magnetic field to the aforementioned loop, Equipped with, Interacting two or more qubit elements combiner.

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