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EP-4736605-A1 - A MULTI-LAYER STRUCTURE, A SUPERCONDUCTING APPARATUS, AND A METHOD FOR MANUFACTURING A SUPERCONDUCTING APPARATUS

EP4736605A1EP 4736605 A1EP4736605 A1EP 4736605A1EP-4736605-A1

Abstract

A multi-layer structure (102) for reducing the quasiparticle density of a superconducting device (106), said multi-layer structure comprises at least a first superconducting material layer (108), at least one trapping material layer (118), at least one insulating layer (114), and at least one tunneling layer (116), wherein the tunneling layer is provided between the trapping material layer and the first superconducting material layer. Said multi-layer structure is configured to be coupled to a superconducting device via the insulating layer such that the insulating layer is provided between the superconducting device and the first superconducting material layer.

Inventors

  • HOSSEINKHANI, Amin
  • NATH, Jayshankar
  • HSU, HAO

Assignees

  • IQM Finland Oy

Dates

Publication Date
20260506
Application Date
20230630

Claims (19)

  1. 1. A multi-layer structure (102) to reduce quasiparticle density in a superconducting device (106), said multi-layer structure (102) being configured to be coupled to a superconducting device, wherein the multi-layer structure (102) comprises at least a first superconducting material layer (108), at least one trapping material layer (118), at least one insulating layer (114), and at least one tunneling layer (116), wherein the tunneling layer (116) is provided between the trapping material layer (118) and the first superconducting material layer (108), characterized in that the multi-layer structure (102) is configured to be coupled to the superconducting device (106) via the insulating layer (114) such that the insulating layer (114) is provided between the superconducting device (106) and the first superconducting material layer (108).
  2. 2. The multi-layer structure (102) of claim 1 , wherein a thickness of the tunneling layer (116) is inferior to a thickness of the insulating layer (114).
  3. 3. The multi-layer structure (102) of any one of claims 1 or 2, wherein a resistance of the tunneling layer (116) is configured to be lower than a resistance of the insulating layer (114), preferably 1 to 2 orders of magnitude lower.
  4. 4. The multi-layer structure (102) of any previous claim, wherein the at least one trapping material layer (118) comprises a normal-conductivity metal material and/or a superconducting material, said superconducting material having a lower superconducting energy gap A3 than the superconducting material of the first superconducting material layer (108).
  5. 5. The multi-layer structure (102) of any previous claim, wherein the first superconducting material layer (108) is configured to provide a superconducting energy gap A1 that is smaller than a superconducting energy gap A2 of a superconducting material element (104) of the superconducting device (106) that is configured to be coupled to the multi-layer structure (102).
  6. 6. The multi-layer structure (102) of any previous claim, wherein the first superconducting material layer (108) comprises the same superconducting material as the superconducting material element (104) of the superconducting device (106).
  7. 7. The multi-layer structure (102) of any previous claim, wherein a thickness of the at least first superconducting material layer (108) and/or a thickness of the trapping material layer (118) is comprised between 20 to 500 nm, preferably between 80 to 200 nm.
  8. 8. A superconducting apparatus (300) comprising at least one superconducting device (106) comprising at least a superconducting material element (104), and at least one multi-layer structure (102) of any of claims 1-7, wherein the at least one multi-layer structure (102) is coupled to the at least one superconducting device (106), in particular to the superconducting material element (104) of the at least one superconducting device (106), more in particular to a portion of the superconducting material element (104), via the insulating layer (114).
  9. 9. The superconducting apparatus (300) of claim 8, wherein the insulating layer (114) of the at least one multi-layer structure (102) is sandwiched between the at least first superconducting material layer (108) and the superconducting material element (104).
  10. 10. The superconducting apparatus (300) of previous claim 8 or 9, wherein the multilayer structure (102) is configured to provide a tunneling rate of quasiparticles from the at least first superconducting material layer (108) to the trapping material layer (118) that is superior to a tunneling rate of quasiparticles from the superconducting device (106) to the at least first superconducting material layer (108) of the multilayer structure (102).
  11. 11. The superconducting apparatus (300) of any of previous claims 8-10, wherein the multi-layer structure (102) is configured to reduce a tunneling rate of quasiparticles from the trapping material layer (118) to the superconducting device (106).
  12. 12. The superconducting apparatus (300) of any of previous claims 8-11 , wherein the multi-layer structure (102) is configured to provide a net trapping rate of quasiparticles that is higher than the net trapping rate of quasiparticles of a trapping material layer (118) coupled to a superconducting device (106), such that the tunneling of quasiparticles from the trapping material layer (118) to the at least first superconducting layer (108) is reduced.
  13. 13. The superconducting apparatus (300) of any one of claims 8-12, wherein said at least one superconducting device (106) comprises or is connectable to at least one Josephson junction (302).
  14. 14. The superconducting apparatus (300) of claim 13, wherein the at least one multilayer structure (102) is positioned adjacent to the Josephson junction along a longitudinal axis of the superconducting device (106), preferably such that a distance between the multi-layer structure (102) and the Josephson junction (302) is 5 to 10 times the coherence length of the superconducting material of the superconducting element (104) of the superconducting device (106).
  15. 15. The superconducting apparatus (300) of claims 8 to 14, comprising a plurality of multi-layer structures (102) provided next to each other laterally on at least one of the at least one superconducting device (106), in particular a plurality of multi-layer structures (102) according to any of claims 1-7.
  16. 16. A method of manufacturing a superconducting apparatus (300) according to any one of claims 8 to 15, the method comprising: providing at least one superconducting device (106) comprising at least a superconducting material element (104) on a substrate (002), and providing at least one multi-layer structure (102) according to any one of claims 1 to 7, characterized in that the at least one multi-layer structure (102) is provided on a surface (106a) of the at least one superconducting device (106).
  17. 17. The method according to claim 16, wherein the step of providing the at least one multi-layer structure (102) comprises a step of providing an insulating layer (114) on at least a portion of a superconducting material element (104) of the at least one superconducting device (106).
  18. 18. The method according to any of claims 16 or 17, wherein the step of providing the at least one multi-layer structure (102) comprises providing all the layers of the multilayer structure (102) on the same side of the superconducting device (106), in particular the side of the superconducting device (106) where the superconducting element (104) is provided.
  19. 19. The method according to any of claims 16 to 18, wherein the step of providing at least one multi-layer structure (102) comprises providing more than one multi-layer structure (102), wherein the multi-layer structures (102) are provided side-by-side on at least one of the at least one superconducting device (106), in particular on the superconducting material element (104) of at least one of the at least one superconducting devices (106).

Description

A MULTI-LAYER STRUCTURE, A SUPERCONDUCTING APPARATUS, AND A METHOD FOR MANUFACTURING A SUPERCONDUCTING APPARATUS TECHNICAL FIELD OF THE INVENTION The invention relates to superconducting devices in general. More specifically, the invention relates to a multi-layer structure configured to be coupled to a superconducting device, and to a superconducting apparatus comprising such multi-layer structure. The invention also relates to a method of manufacturing such superconducting apparatus. BACKGROUND OF THE INVENTION Superconducting devices have many potential uses including e.g. quantum devices, sensor devices, and in connection with cryogenic applications. The performance of many of these superconducting devices is hindered by the presence of quasiparticles in the device. Quasiparticles result from the breaking apart of Cooper pairs, which are present in superconductors, when one of the electrons in the pair is excited to a higher energy level. At low temperatures, i.e. well below the critical temperature of superconducting materials, only Cooper pairs should be found in the superconducting materials. However, it has been demonstrated that non-equilibrium quasiparticles, i.e. unpaired electrons or quasiparticle excitations (also simply termed quasiparticles herein), are found in superconducting materials at temperatures well below their critical temperature. The reasons for such excitations are currently at least partially unknown, although there have been some indications that environmental radioactive materials and cosmic-ray bursts may generate the quasiparticles. Thus, the generation of quasiparticles may be difficult to avoid in devices in any practical solutions, and it would be advantageous to find solutions for reducing the density or mitigation of quasiparticles in superconducting devices. The non-equilibrium quasiparticles negatively affect the functioning of superconducting devices. For example, the quasiparticle reduces the quality factor of superconducting resonators. In the case of superconducting devices being qubits and comprising a Josephson junction, for instance, quasiparticles can tunnel to the Josephson Junction and cause qubit energy decay and decoherence, which limits the lifetime and stability of the qubits. Furthermore, it is known that one side effect of the Single Flux Quantum (SFQ) pulses, used for scaling up the qubits, is the generation of quasiparticles that can then limit the gate fidelities. As further disadvantages related to qubits, it is known that the presence of quasiparticles affect energy levels of qubits, leading to shifts of qubit frequencies and that quasiparticles may limit the relaxation time of superconducting qubits. In superconducting qubits, it has been firmly established both theoretically and experimentally that quasiparticle tunneling causes qubit energy decay and dephasing. The mitigation of effects of quasiparticles on qubits may provide more stable and long-lived qubits, which can facilitate the further development of quantum computing devices. Another example of superconducting devices where quasiparticles may cause problems are Josephson-junction based sensor devices, where measurements dependent on current flowing through a Josephson junction. Here, tunnelling quasiparticles may cause these devices to not function as desired. There have been attempts to reduce the number of quasiparticles in superconducting devices, by using methods or structures known as quasiparticle traps. It is known that vortices may be used for such purposes, but this requires cooling in a magnetic field, which makes the method unsuitable for many devices, such as 2D transmons. Vortices are also difficult to control and a large number of them could negatively influence the performance of a superconducting device. Other prior art quasiparticle traps may comprise a normal-conductivity metal layer that is coupled to the superconducting device, such that they are separated by an insulator layer to provide a superconductor - insulator - normal-conductivity metal (SIN) junction. These traps may evacuate quasiparticles from a superconducting material of a superconducting device by quasiparticles tunneling through to the metal material, and upon relaxation of the quasiparticle below the superconducting gap in the normal-conductivity metal layer, the density of quasiparticles present in the superconducting device is thus reduced. This results in a more stable and long-lived superconducting device. Such prior art quasiparticles traps are known from e.g. A. Hosseinkhani, R.-P. Riwar, R. J. Shoelkopf, L. I. Glazman, G. Catelani, Phys. Rev. App. 8, 064028, 2017, A. Hosseinkhani and G. Catelani, Phys. Rev. B 97, 054513 (2018), and R.-P. Riwar, A. Hosseinkhani, L. D. Burkhart, Y. Y. Gao, R. J. Schoelkopf, L. I. Glazman, G. Catelani, Phys. Rev. B 94, 104516 (2016). However, it is known that quasiparticle back tunneling is a problem with the abovedescribed SIN structure of normal-conductivity m