Superconducting quantum circuit and superconducting quantum circuit device

Abstract

To provide a superconducting quantum circuit having a small output error.SOLUTION: A superconducting quantum circuit 100 according to one aspect of the present invention is a superconducting quantum circuit 100 to which a bias current is applied and which outputs a voltage, the superconducting quantum circuit 100 including a main circuit section 10 having one or more effective Josephson junctions 30 having a critical current capable of generating a voltage in accordance with the bias current and extending linearly in a plan view, and a power supply circuit section 20 having a dummy Josephson junction 40 having a critical current larger than that of the effective Josephson junction 30 so as not to generate a voltage due to the bias current and supplying the bias current to the main circuit section 10 through the dummy Josephson junction 40.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a superconducting quantum circuit and a superconducting quantum circuit device. [Background technology]

[0002] A Josephson junction is a type of junction in which a pair of superconducting materials is joined via a thin barrier layer made of an insulator, semiconductor, normal conductor, etc., and a tunneling current flows through the barrier layer due to the Josephson effect. A Josephson junction can pass a superconducting current up to a certain current value even without a voltage difference. The maximum current value of such a Josephson junction that does not require a voltage difference is called the critical current.

[0003] When microwaves are applied to a Josephson junction, the voltage across the junction increases in a step-like pattern at regular intervals relative to the current value, known as a Shapiro step. The output voltage V [V] of this Josephson junction is given by the microwave frequency f [GHz], an integer n, and the Josephson constant K J = 483597.8 [GHz / V], V = nf / K J Therefore, by controlling the frequency and current value of the microwaves applied to the Josephson junction, the output voltage of the Josephson junction can be accurately determined. The current that controls the voltage of the Josephson junction in this way is called the bias current. Note that the current value corresponding to each n has a certain range (operating point), but if the bias current value goes outside the range of each range, an error in the output voltage will occur.

[0004] The voltage that a single Josephson junction can output is only a few tens of microvolts. For this reason, a superconducting quantum circuit that can obtain a desired voltage is formed by connecting a large number of Josephson junctions in series to form an array structure. It has also been proposed to construct a highly accurate analog-to-digital conversion device by dividing a Josephson junction array into multiple segments and controlling the bias current for each segment (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-224476 Summary of the Invention [Problem to be solved by the invention]

[0006] As a result of research by the present applicants, it has been confirmed that in superconducting quantum circuits including Josephson junctions, when the current path is bent, the designed output voltage may not be obtained. This is thought to be due to the current bias caused by the Meissner effect and other factors, which reduces the effective area where the AC Josephson effect is obtained, narrowing the width of the Shapiro step, i.e., narrowing the range of bias current (operating point) at which the expected Josephson junction output voltage is obtained. Such output voltage errors are likely to occur when using niobium nitride, etc., which have a relatively small Josephson penetration depth. In view of this situation, an object of the present invention is to provide a superconducting quantum circuit and a superconducting quantum circuit device with small output error by increasing the operating point of the Josephson junction. [Means for solving the problem]

[0007] (1) A superconducting quantum circuit according to one aspect of the present invention is a superconducting quantum circuit to which a bias current is applied and which outputs a voltage, the superconducting quantum circuit comprising: a main circuit section having one or more effective Josephson junctions with critical currents capable of generating a voltage in response to the bias current and extending linearly in a planar view; and a power supply circuit section having dummy Josephson junctions with critical currents larger than those of the effective Josephson junctions so as not to generate a voltage in response to the bias current and supplying the bias current to the main circuit section through the dummy Josephson junctions.

[0008] (2) In the above-described superconducting quantum circuit, the dummy Josephson junction may have a junction area larger than that of the effective Josephson junction.

[0009] (3) In the above-described superconducting quantum circuit, the dummy Josephson junction may be divided into a plurality of electrically parallel portions.

[0010] (4) In the above-described superconducting quantum circuit, the power supply circuit section may be bent beyond the dummy Josephson junction with respect to the main circuit section.

[0011] (5) A superconducting quantum circuit device according to one aspect of the present invention includes the above-described superconducting quantum circuit and a current supply circuit that applies the bias current to the main circuit unit via the power supply circuit unit. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a superconducting quantum circuit and a digital-to-analog conversion device in which the output error of the Josephson junction is small. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic plan view of a superconducting quantum circuit according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a schematic side view of the superconducting quantum circuit of FIG. [Figure 3] 1 is a graph showing a current distribution in an effective Josephson junction. [Figure 4] 10 is a graph showing a current distribution in an effective Josephson junction when the second wiring member is arranged perpendicular to the first wiring member. [Figure 5] 10 is a graph showing a current distribution in a dummy Josephson junction when a third wiring member is disposed perpendicular to a second wiring member. [Figure 6] FIG. 10 is a schematic plan view of a superconducting quantum circuit according to a second embodiment of the present invention. [Figure 7] FIG. 10 is a schematic plan view of a superconducting quantum circuit according to a third embodiment of the present invention. [Figure 8] 1 is a circuit diagram of a digital-to-analog conversion device according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description of the embodiments, components similar to those in the previously described embodiments will be designated by the same reference numerals, and redundant description will be omitted. Furthermore, for convenience, hatching and component reference numerals may be omitted, and in such cases, other drawings should be referred to. Furthermore, the dimensions of various components in the drawings have been adjusted for clarity.

[0015] [First embodiment] FIG. 1 is a schematic plan view of a superconducting quantum circuit 100 according to a first embodiment of the present invention, and FIG. 2 is a schematic side view of the superconducting quantum circuit 100. The superconducting quantum circuit 100 receives a bias current and outputs a voltage. The superconducting quantum circuit 100 includes a main circuit unit 10 that extends linearly in a plan view, and a pair of power supply circuits 20 that supply a bias current to the main circuit unit 10. The main circuit unit 10 includes one or more effective Josephson junctions 30 that have a critical current that can generate a voltage in response to the bias current. Meanwhile, the power supply circuit unit 20 includes dummy Josephson junctions 40 that each have a critical current larger than that of the effective Josephson junctions 30 so as not to generate a voltage in response to the bias current, and supplies the bias current to the main circuit unit 10 through the dummy Josephson junctions 40.

[0016] In this embodiment, the main circuit unit 10 includes a plurality of first wiring members 50 arranged in two rows facing each other and shifted by a half period in side view (overlapping in plan view), two second wiring members 60 arranged side by side at the ends of the rows of first wiring members 50 and shared with the power supply circuit unit 20, and a plurality of effective barrier materials 70 interposed between the opposing first wiring members 50 and between the second wiring members 60 and the first wiring members 50, respectively, to form effective Josephson junctions 30. In other words, two effective barrier materials 70 are laminated on each first wiring member 50. The power supply circuit unit 20 also includes a second wiring member 60 shared with the main circuit unit 10, a third wiring member 80 that partially overlaps the second wiring member 60 in plan view, and a dummy barrier material 90 interposed between the second wiring member 60 and the third wiring member 80 to form dummy Josephson junctions 40. It is understood that the portion of the second wiring member 60 on the side of the first wiring member 50 is included in the main circuit portion 10, and the portion of the second wiring member 60 on the side of the first wiring member 50 is included in the main circuit portion 10.

[0017] The first wiring member 50 is formed of a superconducting material such as niobium nitride or a lead alloy, and extends linearly in the main circuit direction. The first wiring member 50 may be a band-shaped, planar wiring pattern formed on the main surface of a substrate (not shown) disposed on the outside in a side view. The thickness of the first wiring member 50 may be, for example, 100 nm to 1000 nm. The width of the first wiring member 50 may be, for example, 10 μm to 100 μm. The length of the first wiring member 50 is determined so that, when effective barrier materials 70 are disposed at both ends of the first wiring member 50 and effective Josephson junctions 30 are formed between the first wiring member 50 and the two adjacent first wiring members 50 facing each other, an insulating distance can be ensured between the first wiring member 50 and the two adjacent first wiring members 50 facing each other. The first wiring member 50 may have a structure in which the area where the effective barrier materials 70 are stacked protrudes to prevent short-circuiting between the facing first wiring members 50. The protruding region of the first wiring member 50 may be formed from a separate member. In other words, the first wiring member 50 may be formed by laminating one of a pair of superconducting materials that form the effective Josephson junction 30 and a superconducting material that provides an electrical path for supplying current to the effective Josephson junction 30.

[0018] The second wiring member 60 is formed from the same superconducting material as the first wiring member 50 and extends linearly in the main circuit direction. Like the first wiring member 50, the second wiring member 60 may be a band-shaped planar wiring pattern, and may be formed on the main surface of an additional substrate (not shown) disposed on the outside in side view. The second wiring member 60 is disposed at one end of a row of first wiring members 50 so as to partially face the end of each first wiring member 50, and extends from the first wiring member 50 in the main circuit direction in plan view. The thickness and width of the second wiring member 60 may be the same as those of the first wiring member 50. The length of the second wiring member 60 is determined to be a length that allows effective Josephson junctions 30 and dummy Josephson junctions 40 to be formed between the second wiring member 60 and the opposing first wiring member 50 and third wiring member 80, respectively, and ensures an insulating distance between the opposing first wiring member 50 and third wiring member 80. Like the first wiring material 50, the second wiring material 60 may have protruding regions where the effective Josephson junctions 30 and the dummy Josephson junctions 40 are formed, and may be formed by joining a superconducting material that provides an electrical path with another superconducting material that forms the effective Josephson junctions 30 and the dummy Josephson junctions 40.

[0019] The third wiring member 80 is formed from the same superconducting material as the first wiring member 50 and the second wiring member 60. Like the first wiring member 50 and the second wiring member 60, the third wiring member 80 may also be a planar wiring pattern that may be formed on an external substrate (not shown). Unlike the first wiring member 50 and the second wiring member 60, which are formed linearly in the main circuit direction, the third wiring member 80 can have any planar shape required by the circuit design, except that it overlaps with the second wiring member 60 in a planar view so that dummy Josephson junctions 40 can be formed between the third wiring member 80 and the second wiring member 60. As shown in FIG. 1 , the third wiring member 80 may extend from the second wiring member 60 in a direction different from the main circuit direction, or it may extend in the main circuit direction and then bend. In other words, the power supply circuit section 20 may be bent beyond the dummy Josephson junctions 40. In the present invention, the term "bent" includes a partially bent shape, such as a branched shape.

[0020] The effective barrier material 70 is disposed between the first wiring members 50 and between the first wiring member 50 and the second wiring member 60, forming a Josephson junction that can generate a voltage in response to a bias current. In other words, the effective Josephson junction 30 formed by joining the first wiring members 50 together or the first wiring member 50 and the second wiring member 60 via the effective barrier material 70 has a critical current lower than the applied bias current. Depending on the bias current value, the effective Josephson junction 30 can transition between a state in which it passes the bias current without generating a voltage and a state in which it outputs a voltage determined by the Shapiro step. The effective barrier material 70 can be formed from a normal-conducting material such as titanium nitride or aluminum nitride, a semiconductor material, an insulating material, or the like. The thickness of the effective barrier material 70 can be, for example, 0.5 nm or more and 50 nm or less. The planar shape of the effective barrier material 70 can be, for example, a relatively small square or circle. The critical current of a Josephson junction depends on the materials of the first wiring member 50, the second wiring member 60, and the effective barrier material 70, but the larger the junction area of ​​the effective barrier material 70, the larger the critical current of the Josephson junction that is formed. For this reason, the size of the effective barrier material 70 is designed to achieve the desired critical current of the effective Josephson junction 30 that is formed.

[0021] The dummy barrier material 90 is disposed between the second wiring member 60 and the third wiring member 80 to form a dummy Josephson junction 40 that has a critical current significantly larger than that of the effective Josephson junction 30 and does not generate voltage regardless of the value of the bias current. The dummy barrier material 90 can be formed from the same material as the effective barrier material 70. The dummy barrier material 90 may be formed from a different material from the effective barrier material 70 to increase the critical current, or may be formed from the same material as the effective barrier material 70 so that it can be disposed simultaneously with the effective barrier material 70. The thickness of the dummy barrier material 90 depends on the material, but may be the same as that of the effective barrier material 70. The dummy barrier material 90 preferably has a larger junction area than the effective barrier material 70 to increase the critical current. In particular, when the dummy barrier material 90 is formed simultaneously with the effective barrier material 70 using the same material, it is essential to increase the junction area to increase the critical current. As a specific example, the junction area of ​​the dummy Josephson junctions 40 can be 10 to 100 times the junction area of ​​the effective Josephson junctions 30. Furthermore, the dummy barrier material 90 is preferably arranged to extend in the main circuit direction along which the second wiring member 60 extends, so as not to reduce the area efficiency of the circuit layout by increasing the widths of the second wiring member 60 and the third wiring member 80. When area efficiency is important, the dummy barrier material 90 can be formed into a rectangular shape that is long in the extension direction of the first wiring member 50, as shown in FIG.

[0022] In the superconducting quantum circuit 100 having the above configuration, the dummy Josephson junctions 40 are formed near the bending position of the power supply circuit unit 20, with the linear main circuit unit 10 having the effective Josephson junctions 30 as the reference. By passing through the dummy Josephson junctions 40, bias caused by the bending of the power supply circuit unit 20 is alleviated, and the bias current flows evenly into the effective Josephson junctions 30 of the main circuit unit 10. This prevents the width of each Shapiro step of the effective Josephson junctions 30 (the width of the current value at the operating point) from being reduced due to current bias, and the output voltage of the effective Josephson junctions 30 can be made to accurately match the design voltage, resulting in small output voltage errors.

[0023] FIG. 3 shows the results of a simulation of the current distribution in the effective Josephson junction 30 formed between the first wiring member 50 and the second wiring member 60 extending in the main circuit direction. FIG. 4 shows the results of a simulation of the current distribution in the effective Josephson junction 30 when the second wiring member 60 is arranged perpendicular to the main circuit direction. FIG. 5 shows the results of a simulation of the current distribution in the dummy Josephson junction 40, i.e., when the planar dimensions of the second wiring member 60 and the effective barrier material 70 in the simulation of FIG. 4 are increased. These simulations assume that the first wiring member 50, the second wiring member 60, and the third wiring member 80 are sufficiently long and linear, and that there are no factors that could cause current imbalance other than the bending of the circuit due to the extension direction of the first wiring member 50, the second wiring member 60, and the third wiring member 80. As shown in FIG. 3, in a typical effective Josephson junction 30, the current density tends to be higher at the four corners. However, the current distribution is symmetrical, and the current flows relatively dispersedly. On the other hand, when the circuit is bent as shown in Fig. 4, the current density at the edge of the effective Josephson junction 30 in the direction in which the first wiring member 50 and the second wiring member 60 extend increases continuously, and the deviation in current density can reduce the effective area of ​​the Josephson junction. In contrast, as shown in Fig. 5, in the dummy Josephson junction 40, the current density increases at the edge of the dummy Josephson junction 40 on the side closer to the second wiring member 60 (the main circuit section 10 side), but the deviation in current density in the width direction of the second wiring member 60 is relatively small. In other words, by providing the dummy Josephson junction 40 near the bend position of the circuit with respect to the effective Josephson junction 30, the deviation in the width direction of the current flowing into the effective Josephson junction 30 is eliminated, and a current distribution with little deviation can be achieved as shown in Fig. 3.

[0024] [Second embodiment] FIG. 6 is a schematic plan view of a superconducting quantum circuit 101 according to a second embodiment of the present invention. The superconducting quantum circuit 101 receives a bias current and outputs a voltage. The superconducting quantum circuit 101 includes eight main circuit units 10 that extend linearly in a plan view and are arranged parallel to one another; a seven-circuit intermediate feed circuit unit 21 that electrically connects the eight main circuit units 10 in series; and a two-circuit edge feed circuit unit 22 that is connected to both ends of the series-connected eight main circuit units 10. The main circuit unit 10 includes multiple effective Josephson junctions 30 each having a critical current that can generate a voltage in response to a bias current. Meanwhile, the intermediate feed circuit unit 21 and the edge feed circuit unit 22 each include dummy Josephson junctions 41 that have a critical current greater than that of the effective Josephson junctions 30 so as not to generate a voltage in response to the bias current.

[0025] The intermediate power feed circuit section 21 is bent in a U-shape in plan view so as to connect adjacent main circuit sections 10. The intermediate power feed circuit section 21 includes two second wiring members 60 shared by the respective main circuit sections 10, third wiring members 81 each having a U-shape in plan view and extending in the main circuit direction at both ends so as to overlap with the second wiring members 60, and dummy barrier materials 91 interposed between the second wiring members 60 and each of the third wiring members 81.

[0026] The dummy barrier materials 91 are each divided into a plurality of portions, and thus the dummy Josephson junctions 41 are formed as a plurality of divided portions arranged electrically in parallel. This makes it possible to suppress a decrease in the uniformity of current distribution due to an increase in the junction area of ​​the dummy Josephson junctions 41. Note that the area of ​​the dummy barrier materials 90, i.e., the junction area of ​​the dummy Josephson junctions 41, in this case means the total area of ​​the portions that are electrically parallel and can be regarded as a single element. The dummy barrier materials 91 are preferably arranged in line in the main circuit direction from the second wiring member 60 so as not to increase the widths of the second wiring member 60 and the third wiring member 80 and thereby reduce the area efficiency of the circuit layout.

[0027] The edge feed circuit section 22 has second wiring members 60 shared with the main circuit section 10, third wiring members 82 each having one end overlapping with the second wiring member 60 in a plan view and extending from the second wiring member 60 toward the main circuit, and dummy barrier materials 91 interposed between the second wiring members 60 and each of the third wiring members 82. The third wiring members 82 shown in the figure are straight, but may be bent beyond the dummy Josephson junctions 41.

[0028] The superconducting quantum circuit 101 of the present embodiment having the above-described configuration includes an intermediate feed circuit unit 21 that connects the main circuit units 10, i.e., an intermediate feed circuit unit 21 that supplies a bias current to one main circuit unit 10 through another main circuit unit 10. By providing this intermediate feed circuit unit 21 with a dummy Josephson junction 41, the bias of the bypass current caused by bending of the intermediate feed circuit unit 21 is alleviated, and it is possible to increase the number of effective Josephson junctions 30 connected in series while suppressing an error in the output voltage.

[0029] [Third embodiment] FIG. 7 is a schematic plan view of a superconducting quantum circuit 102 according to a third embodiment of the present invention. The superconducting quantum circuit 102 receives a bias current and outputs a voltage. The superconducting quantum circuit 102 includes two main circuit units 10 that extend linearly in a plan view and are arranged coaxially side by side; a common feed circuit unit 23 that is arranged between the two main circuit units 10 and supplies a bias current to the two main circuit units 10; and individual feed circuit units 24 that are connected to the other ends of the two main circuit units 10. The main circuit unit 10 includes multiple effective Josephson junctions 30 each having a critical current that can generate a voltage in response to the bias current. Meanwhile, the common feed circuit unit 23 and the individual feed circuit unit 24 each include dummy Josephson junctions 40 that have a critical current larger than that of the effective Josephson junctions 30 so as not to generate a voltage in response to the bias current.

[0030] The common power supply circuit 23 connects the two main circuit units 10 and a bias current supply circuit (not shown). The main circuit units 10 of the common power supply circuit 23 include two second wiring members 60 shared by each of the main circuit units 10, a third wiring member 82 formed in a Y-shape in plan view and having ends overlapping with the two second wiring members 60, and a dummy barrier material 90 interposed between the second wiring member 60 and the third wiring member 82 to form a dummy Josephson junction 40. The third wiring member 82 includes a U-shaped connection portion 83 whose both ends overlap with the second wiring members 60, and a trunk portion 84 connected to the center of the connection portion 83. The individual power supply circuit section 24 has a second wiring member 60 shared with the main circuit section 10, a third wiring member 82 having one end overlapping with the second wiring member 60 in a planar view and extending from the second wiring member 60 in the main circuit direction, and a dummy barrier material 90 interposed between the second wiring member 60 and each of the third wiring members 82.

[0031] In the superconducting quantum circuit 102 of this embodiment having the above-mentioned configuration, by forming dummy Josephson junctions 40 between each main circuit unit 10 and the branch points where the current of the T-shaped common feeder circuit unit 23 bends, the bias of the bypass current is alleviated, and the error in the output voltage can be reduced.

[0032] [Superconducting quantum circuit device] 8 shows the configuration of a digital-analog conversion device, which is one embodiment of a superconducting quantum circuit device according to the present invention. The digital-analog conversion device includes a superconducting quantum circuit 103, a current supply circuit 200 that applies a bias current to the superconducting quantum circuit 103, and a microwave application circuit 300 that applies microwaves to the superconducting quantum circuit 103.

[0033] The superconducting quantum circuit 103 has a plurality of segments S1, S2, ... Sn connected in series, each corresponding to one bit of the input. nThe main circuit units 10 of the circuits may be connected in series by intermediate feed circuit units 21 of (2n-1) circuits. Adjacent segments S1, S2, ... Sn are connected by a common feed circuit unit 23, and are fed with bypass current from a current supply circuit 200. Individual feed circuit units 24 are connected to the outer ends of the segments S1, Sn at both ends, and are fed with bypass current from the current supply circuit 200, and the sum of the voltages of all segments S1, S2, ... Sn is output to analog output terminals T1, T2.

[0034] The current supply circuit 200 applies no current to each segment S1, S2, ... Sn of the superconducting quantum circuit 103 if the corresponding input bit is 0, and applies a bias current corresponding to the Shapiro step if the corresponding input bit is 1.

[0035] The microwave application circuit 300 includes a transmitter 301 that transmits microwaves, a termination 302 that prevents microwave leakage, and a high-frequency power supply 303. The microwave application circuit 300 applies microwaves of a predetermined frequency to all of the effective Josephson junctions 30 and dummy Josephson junctions 40 of the superconducting quantum circuit 103. The frequency of the microwaves applied by the microwave application circuit 300 is set to a frequency at which the Shapiro step voltage of the effective Josephson junctions 30 becomes a predetermined voltage.

[0036] The digital-analog conversion device according to this embodiment includes a superconducting quantum circuit 103 with a small error in the output voltage, and therefore can output an accurate analog voltage in response to a digital signal input.

[0037] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and various modifications and variations are possible. For example, if some power supply circuit sections extend linearly in the main circuit direction until they are sufficiently separated from the main circuit section, the third wiring material and dummy barrier material (dummy Josephson junction) may be omitted from that power supply circuit section, and the second wiring material may be extended instead. The superconducting quantum circuit device according to the present invention is not limited to a digital-to-analog conversion device, and may be, for example, a voltage standard or the like. [Explanation of symbols]

[0038] 100,101,102,103 Superconducting quantum circuits 200 Current supply circuit 300 Microwave application circuit 301 Transmission Unit 302 Termination 303 High frequency power supply section 10 Main circuit section 20, 21, 22, 23, 24 Power supply circuit section 30 Effective Josephson Junction 40,41 Dummy Josephson junction 50,52 1st wiring material 60 2nd wiring material 70 Effective barrier material 80,81,82 3rd wiring material 83 Connection 84 Executive 90,91 Dummy barrier material S1, S2, Sn segments T1, T2 analog output terminals

Claims

1. A superconducting quantum circuit to which a bias current is applied and which outputs a voltage, a main circuit section having one or more effective Josephson junctions with a critical current capable of generating a voltage in response to the bias current, the main circuit section extending linearly in a plan view; a power supply circuit section having a dummy Josephson junction having a critical current larger than that of the effective Josephson junction so as not to generate a voltage due to the bias current, and supplying the bias current to the main circuit section through the dummy Josephson junction; A superconducting quantum circuit comprising:

2. The superconducting quantum circuit of claim 1 , wherein the dummy Josephson junction has a junction area larger than that of the effective Josephson junction.

3. 3. The superconducting quantum circuit according to claim 1, wherein the dummy Josephson junction is formed by dividing it into a plurality of electrically parallel portions.

4. 3. The superconducting quantum circuit according to claim 1, wherein the power supply circuit section is bent beyond the dummy Josephson junction with respect to the main circuit section.

5. The superconducting quantum circuit according to claim 1 or 2; a current supply circuit that applies the bias current to the main circuit section via the power supply circuit section; A superconducting quantum circuit device comprising:

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