Quantum device and quantum device production method

Abstract

This quantum device has: a qubit (10) that is provided with a Josephson junction element (3); a signal source (1) that is connected to the qubit; and a resistance element (5) that is connected between a ground line and a signal path (8) between the signal source and the qubit. The quantum device can be used in, for example, quantum computing.

Description

Quantum device and method for manufacturing the same

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to quantum devices and methods for manufacturing quantum devices.

[0002] One type of quantum bit used in quantum computers is known to be a quantum bit equipped with a Josephson junction device.

[0003] JP 2018-513580 A JP 2007-516610 A U.S. Patent Application Publication No. 2019 / 0081629

[0004] Greater precision is desired for the manipulation of qubits.

[0005] An object of the present disclosure is to provide a quantum device and a method for manufacturing the quantum device that can improve the accuracy of quantum bit manipulation.

[0006] According to one aspect of the present disclosure, there is provided a quantum device including a quantum bit having a Josephson junction element, a signal source connected to the quantum bit, and a resistive element connected between a signal path between the signal source and the quantum bit and a ground line.

[0007] According to the present disclosure, the accuracy of manipulating quantum bits can be improved.

[0008] FIG. 1 is a circuit diagram showing a model used in the simulation. FIG. 2 is a diagram showing the results of the first simulation. FIG. 3 is a diagram showing the results of the second simulation (part 1). FIG. 4 is a diagram showing the results of the second simulation (part 2). FIG. 5 is a diagram showing a quantum device according to the first embodiment. FIG. 6 is a diagram showing a quantum device according to the second embodiment. FIG. 7 is a diagram showing a quantum device according to the third embodiment. FIG. 8 is a diagram showing a quantum device according to the fourth embodiment. FIG. 9 is a diagram showing a quantum device according to the fifth embodiment. FIG. 10 is a schematic diagram (part 1) showing a method of forming a quantum bit substrate according to the fifth embodiment. FIG. 11 is a schematic diagram (part 2) showing a method of forming a quantum bit substrate according to the fifth embodiment. FIG. 12 is a schematic diagram (part 3) showing a method of forming a quantum bit substrate according to the fifth embodiment. FIG. 13 is a schematic diagram (part 4) showing a method of forming a quantum bit substrate according to the fifth embodiment. FIG. 14 is a schematic diagram (part 5) showing a method of forming a quantum bit substrate according to the fifth embodiment. 15 is a schematic diagram (part 6) showing a method for forming a quantum bit substrate according to the fifth embodiment. FIG. 16 is a diagram showing a quantum bit substrate according to a modification of the fifth embodiment.

[0009] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant description may be omitted.

[0010] (Basic Principles of the Present Disclosure) First, the basic principles of the present disclosure will be described. Josephson junction devices are known to behave as nonlinear inductors. Therefore, the present inventors considered modeling a quantum bit having a Josephson junction device so that the quantum bit can be handled in a superconducting circuit simulator while treating the Josephson junction device as a nonlinear inductor. In this modeling, the behavior of the quantum bit when an external signal having a resonant frequency is applied to the quantum bit, such as during gate operation, was examined. The signal having the resonant frequency was assumed to be a signal that causes a perturbation while the quantum bit remains in the ground state, rather than transitioning to an excited state. Using such a signal, a model is obtained in which an internal current flows that is smaller than the zero-point oscillation at the ground state, has the resonant frequency, and is zero when averaged over time. For example, when this model is applied to a transmon, a type of gate-type quantum bit, the quantum bit remains at the bottom of the ground state, ensuring that the transmon region is satisfied in the simulation. Furthermore, when a signal with a resonant frequency is applied, the changes that occur in the qubit and in the system coupled to the qubit can be observed as internal currents in the circuit using a superconducting circuit simulator. In this way, by regarding the Josephson junction as a nonlinear inductor and including a capacitor in the qubit equivalent circuit, the qubit can be handled using a superconducting circuit simulator.

[0011] Furthermore, in quantum devices in which two quantum bits are capacitively coupled to each other, inductance exists in the capacitive coupling line. Therefore, the inventors of the present application decided to adopt a model in which, taking into account the inductance of this line, there is a resistance between the end of the line that becomes the open end for capacitive coupling and ground. For example, for transmons, they decided to adopt a model in which a Josephson junction element, a capacitor, and a resistor are connected in parallel. Figure 1 is a circuit diagram showing the model used in the simulation.

[0012] As shown in Fig. 1, this model has a signal source 1, a capacitor 2, a Josephson junction device 3, a capacitor 4, a resistor 5, and a capacitor 6. A quantum bit 10 has the Josephson junction device 3, the capacitor 4, and the resistor 5. The Josephson junction device 3 and the capacitor 4 are connected in parallel with each other, and the capacitor 6 is connected between a node 7 of one of the Josephson junction device 3 and the capacitor 4 and ground. In addition, the resistor 5 is connected between a node 8 of the other of the Josephson junction device 3 and the capacitor 4 and ground. The signal source 1 is connected between a node 9 and ground, and the capacitor 2 is connected between the node 8 and the node 9. The capacitance of the capacitor 6 corresponds to the capacitance between the quantum bit 10 and ground.

[0013] The inventors of the present application performed a simulation using the model shown in Figure 1, and were able to quantitatively estimate the effects of the capacitors that make up the quantum bit, the critical current of the Josephson junction devices, and the coupling between quantum bits through calculations using a circuit simulator. It was also revealed that connecting a resistive element in parallel to the quantum bit makes it easier for the signal supplied from the signal source to be accurately reflected in the quantum bit state.

[0014] Here, two examples of simulations performed by the inventors of the present invention will be described.

[0015] (First Simulation) In the first simulation, the relationship between the resistance value of the resistive element 5 and the current (internal current) flowing inside the quantum bit 10 was calculated. The results are shown in Fig. 2. As shown in Fig. 2, when the resistance value R of the resistive element 5 is 1000Ω or more, the internal current Ip becomes stable.

[0016] (Second Simulation) In the second simulation, the change in the potential of the node 8 when the resistance value of the resistive element 5 was set to 100 Ω or 1000 Ω was calculated. The results are shown in FIGS. 3 and 4. FIG. 3 shows the change in the potential of the node 8 when the resistance value of the resistive element 5 was set to 100 Ω, and FIG. 4 shows the change in the potential of the node 8 when the resistance value of the resistive element 5 was set to 1000 Ω. As shown in FIGS. 3 and 4, when the resistance value of the resistive element 5 was 100 Ω, the waveform stabilized quickly.

[0017] Based on the results of these simulations, the inventors of the present invention have come up with the following various configurations in which a resistance element is actually connected in parallel to a quantum bit.

[0018] First Embodiment First, a first embodiment will be described. The first embodiment relates to a quantum device. Fig. 5 is a diagram showing a quantum device according to the first embodiment.

[0019] As shown in FIG. 5 , the quantum device 100 according to the first embodiment includes a quantum bit substrate 119, a signal source 120, a resistor 130, a capacitor 140, a dilution refrigerator 150, and a sample holder 155. The quantum bit substrate 119 includes a quantum bit 110 and a capacitor 115. The quantum bit 110 includes a first electrode 111, a second electrode 112, and a Josephson junction element 113. The Josephson junction element 113 is disposed between the first electrode 111 and the second electrode 112 and is connected to the first electrode 111 and the second electrode 112. The first electrode 111 and the second electrode 112 are electrically insulated from each other, and a capacitor 114 is located between the first electrode 111 and the second electrode 112. The capacitor 115 is connected between the second electrode 112 and a ground line.

[0020] The quantum bit substrate 119 is supported by a sample holder 155. That is, the sample holder 155 supports the quantum bit 110. The sample holder 155, for example, sandwiches the quantum bit substrate 119 from above and below, blocking the influence of external electromagnetic fields on the quantum bit 110. The sample holder 155 may be provided with contact probes such as pogo pins. The contact probes can be used for inputting and outputting signals to and from the quantum bit 110. The sample holder 155 is an example of a holder.

[0021] Dilution refrigerator 150 accommodates sample holder 155 and cools quantum bits 110 to an extremely low temperature. Dilution refrigerator 150 cools quantum bits 110 to a temperature of, for example, about 10 mK.

[0022] Signal source 120, resistive element 130, and capacitor 140 are provided outside dilution refrigerator 150. Capacitor 140 is connected between signal source 120 and first electrode 111, and signal source 120 is capacitively coupled to first electrode 111 via capacitor 140. Resistive element 130 is connected between a signal path between signal source 120 and quantum bit 110 and a ground line. More specifically, resistive element 130 is connected between node 121 between capacitor 140 and signal source 120 and a ground line. An electrical control signal is supplied from signal source 120 to first electrode 111. The resistance value of resistive element 130 is not limited and may be, for example, 100 Ω or more and 1000 Ω or less, or 1000 Ω or more and 10,000 Ω or less.

[0023] In the first embodiment, resistive element 130 is connected between the ground line and the signal path between signal source 120 and quantum bit 110. Therefore, as has become clear from simulations performed by the inventors of the present application, the signal supplied from signal source 120 is more likely to be accurately reflected in the state of quantum bit 110, thereby improving the accuracy of the operation of quantum bit 110. In other words, the fidelity of the state of quantum bit 110 can be improved.

[0024] Furthermore, because resistive element 130 is provided outside dilution refrigerator 150, it is possible to suppress the influence of thermal noise caused by a temperature rise in resistive element 130 on the state of quantum bit 110. For example, if there is a time lag between the timing at which the state of quantum bit 110 is manipulated and the timing at which cooling of the heat generated in resistive element 130 is completed, there is a risk that the operation of quantum bit 110 will be hindered by thermal noise, but it is possible to suppress the hindering of the operation of quantum bit 110.

[0025] Second Embodiment Next, a second embodiment will be described. The second embodiment differs from the first embodiment mainly in the connection of the resistance elements. Fig. 6 is a diagram showing a quantum device according to the second embodiment.

[0026] As shown in FIG. 6 , in the quantum device 200 according to the second embodiment, the dilution refrigerator 150 has a first temperature region 151 and a second temperature region 152. The first temperature region 151 is a region cooled to a temperature of 100 mK or less, preferably 20 mK or less, and more preferably 10 mK or less, and the second temperature region 152 is a region cooled to a temperature of 100 mK or less and to a temperature higher than the temperature of the first temperature region 151. The resistive element 130 is connected to a node 122 between the capacitor 140 and the first electrode 111. The node 122 is located in the second temperature region 152. The sample holder 155 is accommodated in the first temperature region 151.

[0027] Other configurations of the second embodiment are similar to those of the first embodiment.

[0028] According to the second embodiment, the signal supplied from signal source 120 is also more likely to be accurately reflected in the state of quantum bit 110, thereby improving the accuracy of the operation of quantum bit 110. In other words, it is possible to improve the fidelity of the state of quantum bit 110. Furthermore, because resistive element 130 is provided outside dilution refrigerator 150, it is possible to suppress the influence of thermal noise caused by a temperature rise in resistive element 130, etc., on the state of quantum bit 110.

[0029] Third Embodiment Next, a third embodiment will be described. The third embodiment differs from the second embodiment mainly in the connection of the resistance elements. Fig. 7 is a diagram showing a quantum device according to the third embodiment.

[0030] 7 , in a quantum device 300 according to the third embodiment, a dilution refrigerator 150 has a first temperature region 151 and a second temperature region 152. The resistive element 130 is connected to a node 123 between the capacitor 140 and the first electrode 111. The node 123 is within the first temperature region 151.

[0031] Other configurations of the third embodiment are similar to those of the second embodiment.

[0032] According to the third embodiment, the signal supplied from signal source 120 is also more likely to be accurately reflected in the state of quantum bit 110, thereby improving the accuracy of the operation of quantum bit 110. In other words, it is possible to improve the fidelity of the state of quantum bit 110. Furthermore, because resistive element 130 is provided outside dilution refrigerator 150, it is possible to suppress the influence of thermal noise caused by a temperature rise in resistive element 130, etc., on the state of quantum bit 110.

[0033] Fourth Embodiment Next, a fourth embodiment will be described. The fourth embodiment differs from the third embodiment mainly in the connection of the resistance elements. Fig. 8 is a diagram showing a quantum device according to the fourth embodiment.

[0034] As shown in Figure 8, in quantum device 400 according to the fourth embodiment, dilution refrigerator 150 has first temperature region 151 and second temperature region 152. Resistance element 130 is housed in dilution refrigerator 150, which cools quantum bit 110 and resistance element 130. Resistance element 130 is supported by sample holder 155. That is, sample holder 155 supports quantum bit 110 and resistance element 130. Resistance element 130 is connected to node 124 between capacitor 140 and first electrode 111 within sample holder 155. Therefore, quantum bit 110 and resistance element 130 are cooled to the same temperature within dilution refrigerator 150.

[0035] Other configurations of the fourth embodiment are similar to those of the third embodiment.

[0036] According to the fourth embodiment, the signal supplied from signal source 120 is also more likely to be accurately reflected in the state of quantum bit 110, thereby improving the accuracy of the operation of quantum bit 110. In other words, it is possible to improve the fidelity of the state of quantum bit 110. Furthermore, since resistive element 130 is supported by sample holder 155, resistive element 130 can be cooled together with quantum bit 110.

[0037] Resistance element 130 may be accommodated in second temperature region 152. That is, within dilution refrigerator 150, quantum bit 110 may be cooled to a temperature lower than that of resistance element 130. Furthermore, resistance element 130 may be provided apart from sample holder 155 within dilution refrigerator 150.

[0038] Fifth Embodiment Next, a fifth embodiment will be described. The fifth embodiment differs from the fourth embodiment mainly in the connection of the resistive elements. Fig. 9 is a diagram showing a quantum device according to the fifth embodiment.

[0039] As shown in FIG. 9 , in a quantum device 500 according to the fifth embodiment, a dilution refrigerator 150 has a first temperature region 151 and a second temperature region 152. The resistive element 130 is provided on a quantum bit substrate 119 together with the quantum bit 110. That is, the quantum bit substrate 119 has the quantum bit 110, the resistive element 130, and a capacitor 115. The resistive element 130 is connected to a first superconductor film 501 (see FIG. 11 ) that is connected to a first electrode 111 in the quantum bit substrate 119. The first superconductor film 501 constitutes a Josephson junction element 113. Therefore, in the fifth embodiment as well, the quantum bit 110 and the resistive element 130 are cooled to the same temperature within the dilution refrigerator 150.

[0040] Here, a method for forming the quantum bit substrate 119 in the fifth embodiment will be described. Figures 10 to 15 are schematic diagrams showing a method for forming the quantum bit substrate 119 in the fifth embodiment.

[0041] 10, a first electrode 111 and a second electrode 112 are formed on a substrate 510 such as a silicon substrate. The first electrode 111 and the second electrode 112 are formed by forming, for example, a titanium nitride (TiN) film or a niobium (Nb) film and patterning it by dry etching.

[0042] 11, a first superconductor film 501 is formed to be connected to the first electrode 111. In forming the first superconductor film 501, oblique shadow deposition of an aluminum (Al) film is performed using a mask.

[0043] 12, the surface of the first superconductor film 501 is oxidized to form an insulating film 502 on the surface of the first superconductor film 501. The insulating film 502 is, for example, an aluminum oxide film. At this time, portions of the first superconductor film 501 that are not oxidized and have conductivity are left.

[0044] Next, as shown in FIG. 13 , a second superconductor film 503 connected to the second electrode 112 is formed. The second superconductor film 503 is formed so that a portion thereof is located on the insulating film 502. The second superconductor film 503 is formed by oblique shadow deposition of an aluminum film using a mask. The first superconductor film 501 and the second superconductor film 503 can be formed using the same mask by changing the oblique shadow deposition method. The Josephson junction device 113 is composed of the overlapping first superconductor film 501, insulating film 502, and second superconductor film 503. After the second superconductor film 503 is formed, the masks used to form the first superconductor film 501 and the second superconductor film 503 are removed. The materials for the first superconductor film 501 and the second superconductor film 503 are also deposited on the mask, but the deposits are removed when the mask is removed. That is, lift-off is performed.

[0045] Next, as shown in Fig. 14, an aluminum film 504 is formed to be connected to the first superconductor film 501. The aluminum film 504 is formed to a thickness that allows the entire film to be oxidized by natural oxidation. In forming the aluminum film 504, the aluminum film is vapor-deposited using a mask, and then the mask is removed. The aluminum film is an example of a metal film.

[0046] 15, the entire aluminum film 504 is oxidized by natural oxidation to form the resistance element 130. Thereafter, the resistance element 130 and the ground electrode portion are connected by wire bonding.

[0047] In this manner, the quantum bit substrate 119 of the fifth embodiment can be formed.

[0048] 16, the second electrode 112 may be formed so as to be in contact with the resistance element 130 and may be grounded. In this case, the quantum bit substrate 119 does not need to have the capacitor 115.

[0049] Quantum devices according to the present disclosure can be used, for example, in quantum computing.

[0050] Although the preferred embodiments have been described in detail above, the present disclosure is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims.

[0051] 100, 200, 300, 400, 500: quantum device 110: quantum bit 111: first electrode 112: second electrode 113: Josephson junction element 114, 115, 140: capacitor 119: quantum bit substrate 120: signal source 130: resistive element 150: dilution refrigerator 151: first temperature region 152: second temperature region 155: sample holder 501: first superconductor film 502: insulating film 503: second superconductor film 504: aluminum film 510: substrate

Claims

1. A quantum device comprising: a quantum bit having a Josephson junction element; a signal source connected to the quantum bit; and a resistive element connected between a signal path between the signal source and the quantum bit and a ground line.

2. The quantum device according to claim 1, further comprising a dilution refrigerator for cooling said quantum bit and said resistance element.

3. The quantum device according to claim 2, wherein the quantum bit and the resistance element are cooled to the same temperature in the dilution refrigerator.

4. The quantum device of claim 3, further comprising a holder for supporting said quantum bit and said resistive element.

5. The quantum device according to claim 3, further comprising a holder for supporting the quantum bit, and the resistive element is provided at a distance from the holder.

6. The quantum device of claim 2, wherein the quantum bit is cooled to a temperature lower than that of the resistive element in the dilution refrigerator.

7. The quantum device according to claim 6, further comprising a capacitor provided in the signal path, and the resistive element is connected between the ground line and a portion of the signal path between the capacitor and the signal source.

8. The quantum device according to claim 1, further comprising a dilution refrigerator for cooling the quantum bit, and the resistance element is provided outside the dilution refrigerator.

9. The quantum device according to claim 1, wherein the resistance value of the resistive element is not less than 100 Ω and not more than 1000 Ω.

10. The quantum device according to any one of claims 1 to 8, wherein the resistance value of the resistive element is not less than 1000 Ω and not more than 10,000 Ω.

11. A method for manufacturing a quantum device, comprising the steps of: forming a Josephson junction element on a substrate, the Josephson junction element having a first superconductor film, a second superconductor film, and an insulating film between the first superconductor film and the second superconductor film; forming a metal film on the substrate, the metal film being electrically connected to the first superconductor film; and oxidizing the entire metal film to form a resistor element.

12. The method for manufacturing a quantum device according to claim 11, wherein the metal film is an aluminum film.

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