Superconducting device

Abstract

To improve the uniformity of characteristics of a Josephson junction.SOLUTION: A first unevenness 102 is formed on the first surface 101a of the first superconductor 101, and the insulating layer 111 is formed on the first superconductor 101 to cover the first unevenness 102. In addition, second irregularities 104 are formed on a second surface 103a of the second superconductor 103. The first superconductor 101 and the second superconductor 103 are arranged such that the first surface 101a and the second surface 103a face each other. In the superconducting device according to the embodiment, the first unevenness 102 of the first superconductor 101 on which the insulating layer 111 is formed is fitted into the second unevenness 104 of the second superconductor 103 to be integrated, thereby forming a Josephson junction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to superconducting devices. [Background technology]

[0002] Since the operating characteristics of superconducting devices that handle quantum information depend heavily on the magnitude of the critical current of the Josephson junction, it is important to align the characteristics of the Josephson junctions. In devices that handle discrete digital information, the information is judged by whether it exceeds a reference value, so some variation in characteristics is acceptable. However, devices that handle quantum information use a continuous superposition of 0 and 1 states, so the tolerance for device variation is narrower, making it important to align the characteristics.

[0003] The methods described in Non-Patent Documents 1 and 2 have been used to fabricate Josephson junctions in superconducting quantum devices, but it is difficult to achieve the required precision for integrated devices, such as achieving uniform critical current and other characteristics. In these methods, a resist material is first applied to a substrate, and a specific pattern is then drawn on the applied resist layer and developed to create a structure. Superconducting material is then evaporated onto this structure at different angles, forming the first superconductor 301, as shown in Figure 3. Next, oxygen is introduced to oxidize the material, creating an insulating layer 303. Furthermore, superconducting material is evaporated from a different angle than the first evaporation, forming the second superconductor 302. A Josephson junction is fabricated using these steps.

[0004] These techniques are useful for fabricating Josephson junctions and have been widely used. To commercialize superconducting devices that handle quantum information, integration of Josephson junctions with uniform characteristics is necessary. However, with conventional techniques, it is difficult to align the characteristics with the precision required for integrated devices due to the variation in the characteristics of Josephson junctions from element to element. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] GJ Dolan, "Offset masks for lift-off photoprocessing", Applied Physics Letters, vol. 31, pp. 337-339, 1977. [Non-patent document 2] A. Potts et al., "Novel fabrication methods for submicrometer Josephson junction qubits", Journal of Materials Science: Materials in Electronics, vol. 12, pp. 289-293, 2001. Summary of the Invention [Problem to be solved by the invention]

[0006] Suppressing the variation in the critical current of Josephson junctions is a challenge for integrating superconducting quantum circuits. However, even when multiple Josephson junctions are fabricated on a wafer under the same conditions to achieve a uniform critical current, variations in the critical current still occur.

[0007] There are various causes for the variation in the critical current value of Josephson junctions. For example, variation in the thickness of the insulating layer 303 contributes to the variation in the critical current value. Variations in the position of the edge of the junction also cause the variation in the critical current value. When the variation is due to differences in the thickness of the insulating layer 303 for each junction, the magnitude of the variation in the critical current value is proportional to the junction area. Furthermore, when the variation is due to local differences in the thickness of the insulating layer 303 depending on the location within a single junction, the variation in the critical current value is proportional to the square root of the junction area. When the edge position of the junction varies, the variation in the critical current value may depend on the edge length of the junction, not the junction area.

[0008] The variation in critical current values ​​with current technology is greater than the uniformity of elements required for the integration of superconducting quantum circuits, which is known to cause problems such as the inability to control the frequency of quantum bits with sufficient precision. In order to put superconducting devices that handle quantum information into practical use, technology is needed to improve the uniformity of the characteristics of the Josephson junctions that are fabricated.

[0009] The present invention has been made to solve the above problems, and has as its object to improve the uniformity of the characteristics of Josephson junctions. [Means for solving the problem]

[0010] The superconducting device of the present invention comprises a first superconductor made of a superconducting material, a first irregularity formed on a first surface of the first superconductor, an insulating layer formed on the first surface to cover the first irregularity, a second superconductor made of a superconducting material, and a second irregularity formed on a second surface of the second superconductor, and the first irregularity of the first superconductor on which the insulating layer is formed is inserted into and integrated with the second irregularity of the second superconductor to form a Josephson junction. [Effects of the Invention]

[0011] As described above, according to the present invention, the first concave-convex portion of the first superconductor having an insulating layer formed thereon is inserted into the second concave-convex portion of the second superconductor to form a Josephson junction as a whole, thereby improving the uniformity of the characteristics of the Josephson junction. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional view (a) and a plan view (b) showing the configuration of a superconducting device according to an embodiment of the present invention. [Figure 2A] FIG. 2A is an explanatory diagram illustrating a method for manufacturing a superconducting device according to an embodiment of the present invention. [Figure 2B] FIG. 2B is an explanatory diagram illustrating a method for manufacturing a superconducting device according to an embodiment of the present invention. [Figure 2C]FIG. 2C is an explanatory diagram illustrating a method for manufacturing a superconducting device according to an embodiment of the present invention. [Figure 3] FIG. 3 shows a cross-sectional view (a) and a plan view (b) of a conventional Josephson junction. DETAILED DESCRIPTION OF THE INVENTION

[0013] A superconducting device according to an embodiment of the present invention will be described below with reference to Fig. 1. This superconducting device includes a first superconductor 101 made of a superconducting material, an insulating layer 111, and a second superconductor 103 made of a superconducting material.

[0014] A first asperity 102 is formed on a first surface 101a of the first superconductor 101, and an insulating layer 111 is formed on the surface of the first superconductor 101, covering the first asperity 102. A second asperity 104 is formed on a second surface 103a of the second superconductor 103. The first superconductor 101 and the second superconductor 103 are arranged such that the first surface 101a and the second surface 103a face each other. In the superconducting device according to the embodiment, the first asperity 102 of the first superconductor 101, on which the insulating layer 111 is formed, is fitted into and integrated with the second asperity 104 of the second superconductor 103, thereby forming a Josephson junction.

[0015] Here, the first asperity 102 can be formed at a plurality of locations on the surface of the first superconductor 101, and the second asperity 104 can be formed at a plurality of locations on the surface of the second superconductor 103. Moreover, the first asperity 102 can be formed at equal intervals at a plurality of locations on the surface of the first superconductor 101, and the second asperity 104 can be formed at the same intervals as the first asperity 102 at a plurality of locations on the surface of the second superconductor 103. The first asperity 102 is a groove formed extending in a predetermined direction, and the second asperity 104 is a groove formed extending in a predetermined direction.

[0016] In this superconducting device, a first superconductor 101 has a first asperity 102 that is comb-tooth shaped in a cross-sectional view, and a second superconductor 103 also has a second asperity 104 that is comb-tooth shaped in a cross-sectional view. The comb-tooth portion of the first superconductor 101 and the comb-tooth portion of the second superconductor 103 are fitted together, and an insulating layer 111 is formed at the fitted interface.

[0017] The critical current value of a Josephson junction is proportional to the area of ​​the junction and also varies depending on the thickness of the insulating layer. Therefore, even if the junction area is increased, the same critical current value can be obtained by changing the thickness of the insulating layer. By increasing the area, the variation in critical current caused by unevenness in the junction area during the fabrication of the Josephson junction becomes relatively smaller, so improved uniformity can be expected. However, simply increasing the junction area increases the size of the device, which goes against the goal of integration.

[0018] In the embodiment, the junction area is increased without increasing the size of the device. According to the above-described embodiment, the area of ​​the insulating layer 111 between the superconductor layers is increased without changing the area occupied by the junction itself, thereby improving the uniformity of the junction.

[0019] The variation in the critical current of a Josephson junction can be evaluated by normalizing the variation in the critical current by the magnitude of the critical current. Since the critical current itself is proportional to the junction area, contributions other than the variation proportional to the first power or more of the junction area can be averaged out and made relatively small by increasing the junction area.

[0020] By using a multilayer comb-like structure in which superconductors and insulating layers are alternately stacked in the junction region, the area of ​​the insulating layers can be increased without increasing the area occupied by the Josephson junction itself. This structure increases the effective junction area, averaging out variations in critical current and improving the uniformity of characteristics.

[0021] Next, the fabrication of a superconducting device according to an embodiment will be described with reference to FIGS. 2A to 2C. In the following, aluminum oxide (AlO x) as an insulating layer. Similar to the Dolan bridge method described in Non-Patent Document 1, a bridge structure 201 is fabricated using resist. First, as shown in FIG. 2A, Al is vacuum-deposited from the upper right corner of the page to form a first Al layer 202. At this time, an Al layer 202a is also formed on the bridge structure 201. Next, oxygen is introduced to oxidize the surface of the first Al layer 202, forming an insulating layer, thereby creating a region 211 that functions as an insulating layer in the Josephson junction.

[0022] Next, as shown in FIG. 2B, Al is vacuum-deposited from the upper left of the page to form the second Al layer 203. At this time, an Al layer 203a is also formed on the bridge structure 201. Next, oxygen is introduced to oxidize the surface of the second Al layer 203, forming an insulating layer, creating a region 211a that functions as an insulating layer in the Josephson junction. Next, as shown in FIG. 2C, Al is vacuum-deposited from the upper right of the page to form the third Al layer 204. At this time, an Al layer 204a is also formed on the bridge structure 201.

[0023] As described above, by repeatedly depositing Al at different angles on even and odd layers sandwiching an insulating layer, the insulating layer between the left and right Al layers takes on a comb-like structure, resulting in an insulating layer with a large area. As described above, increasing the number of Al layers increases the area of ​​the insulating layer, which is expected to improve the uniformity of the Josephson junction. If the thickness of the Al layer is negligible compared to the size of the junction, the area of ​​the insulating layer of a Josephson junction made with the Nth Al layer will be (N-1) times that of a junction made with two ordinary Al layers. This makes it possible to increase the junction area without increasing the occupied area, thereby improving the uniformity of the junction characteristics.

[0024] As described above, according to the embodiment of the present invention, the first concave-convex of the first superconductor on which an insulating layer is formed is inserted into the second concave-convex of the second superconductor to form a Josephson junction as a whole, thereby improving the uniformity of the characteristics of the Josephson junction.

[0025] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention. [Explanation of symbols]

[0026] 101...first superconductor, 101a...first surface, 102...first irregularity, 103...second superconductor, 103a...second surface, 104...second irregularity, 111...insulating layer.

Claims

1. a first superconductor made of a superconducting material; a first asperity formed on a first surface of the first superconductor; an insulating layer formed on the first surface to cover the first irregularities; a second superconductor made of a superconducting material; a second asperity formed on a second surface of the second superconductor; Equipped with The first concave-convex portion of the first superconductor on which the insulating layer is formed is fitted into the second concave-convex portion of the second superconductor, and they are integrated to form a Josephson junction. Superconducting devices.

2. 10. The superconducting device of claim 1, the first irregularities are formed at a plurality of locations on the first surface, A superconducting device in which the second irregularities are formed at a plurality of locations on the second surface.

3. 3. The superconducting device of claim 2, the first irregularities are formed at a plurality of locations on the first surface at equal intervals, A superconducting device in which the second irregularities are formed at a plurality of locations on the second surface at the same intervals as the first irregularities.

4. The superconducting device according to any one of claims 1 to 3, the first irregularities are formed by grooves extending in a predetermined direction, A superconducting device in which the second irregularities are formed of grooves extending in a predetermined direction.

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