US-12627295-B2 - Persistent current switch

US12627295B2US 12627295 B2US12627295 B2US 12627295B2US-12627295-B2

Abstract

A persistent current switch is disclosed for controlling, e.g. initiating, a persistent current in a superconductor loop. The persistent current switch comprises a piece of superconductor material that is part of the superconductor loop. Further, the persistent current switch comprises an illumination system that is configured to direct light onto the piece of superconductor material for influencing an electrical resistance of the piece of superconductor material. The illumination system is configured such that the light impinging on the piece of superconductor material substantially does not heat the piece of superconductor material.

Inventors

Ryoichi Ishihara
Salahuddin Nur
Jaime Oscar Tenorio Pearl

Assignees

TECHNISCHE UNIVERSITEIT DELFT

Dates

Publication Date: 20260512
Application Date: 20230102
Priority Date: 20220103

Claims (18)

1 . A persistent current switch for controlling a persistent current in a superconductor loop, the persistent current switch comprising: a piece of superconductor material that is part of the superconductor loop, and an illumination system that is configured to direct, using a light-guiding system, light onto the piece of superconductor material for influencing an electrical resistance of the piece of superconductor material, wherein the illumination system is configured such that the light impinging on the piece of superconductor material substantially does not heat the piece of superconductor material, and wherein the light impinging on the piece of superconductor material has a power of less than 1 μW.
2 . The persistent current switch according to claim 1 , wherein the illumination system comprises a light source for generating the light.
3 . The persistent current switch according to claim 1 , wherein the light-guiding system is an on-chip photonic waveguide.
4 . The persistent current switch according to claim 1 , wherein the piece of superconductor material and/or the illumination system and/or the light is and/or are configured such that the light impinging on the piece of superconductor material prevents the piece of superconductor material from adopting a superconducting state.
5 . The persistent current switch according to claim 4 , wherein the piece of superconductor material and/or the illumination system and/or the light is and/or are configured such that the light impinging on the piece of superconductor material breaks Cooper pairs that are present in the piece of superconductor material.
6 . The persistent current switch according to claim 1 , further comprising a control system for controlling the persistent current switch, the control system being configured to perform steps of: causing the illumination system to direct light onto the piece of superconductor material so that the piece of superconductor material does not adopt a superconducting state, and causing the illumination system to not direct light onto the piece of superconductor material, so that the piece of superconductor material adopts the superconducting state.
7 . The persistent current switch according to claim 1 , wherein the persistent current switch is configured to operate at temperatures below 50 Kelvin.
8 . The persistent current switch according to claim 1 , wherein the superconductor loop and/or the piece of superconductor material comprises niobium, Nb, and/or niobium nitride, NbN, and/or niobium titanium nitride, NbTiN.
9 . A quantum computing system comprising the persistent current switch according to claim 1 .
10 . The quantum computing system according to claim 9 , further comprising a DC power source for energizing the superconductor loop.
11 . The persistent current switch according to claim 2 , wherein the light source is a laser.
12 . The persistent current switch according to claim 7 , wherein the persistent current switch is configured to operate at temperatures below 10 Kelvin.
13 . A chip having integrated thereon a persistent current switch for controlling a persistent current in a superconductor loop, the persistent current switch comprising: a piece of superconductor material that is part of the superconductor loop, and an illumination system that is configured to direct, using a light-guiding system, light onto the piece of superconductor material for influencing an electrical resistance of the piece of superconductor material, wherein the illumination system is configured such that the light impinging on the piece of superconductor material substantially does not heat the piece of superconductor material, and wherein the light impinging on the piece of superconductor material has a power of less than 1 μW.
14 . The chip according to claim 13 , wherein the persistent current switch is configured to operate at temperatures below 50 Kelvin.
15 . The chip according to claim 13 , wherein the illumination system comprises a light source for generating the light and wherein the light-guiding system is an on-chip photonic waveguide.
16 . A method for initiating a persistent current in a superconductor loop comprising a piece of superconductor material, the method comprising cooling the superconductor loop to below a critical temperature, and causing an illumination system to direct light onto the piece of superconductor material so that the piece of superconductor material does not adopt a superconducting state, wherein the illumination system is configured such that the light impinging on the piece of superconductor material substantially does not heat the piece of superconductor material and wherein the light impinging on the piece of superconductor material has a power of less than 1 μW, and energizing the superconductor loop comprising causing an electrical current through at least part of the superconductor loop, and while the electrical current is flowing through at least part of the superconductor loop, causing the illumination system to not direct light onto the piece of superconductor material so that the piece of superconductor material adopts the superconductive state so that the superconductor loop becomes superconductive and conducts the persistent current.
17 . The method according to claim 16 , wherein the illumination system comprises a light source for generating the light and wherein the step of causing the illumination system to not direct light onto the piece of superconductor material comprises switching off the light source.
18 . The method according to claim 16 , wherein causing the illumination system to not direct light onto the piece of superconductor material comprises controlling a light guiding system to not direct light onto the piece of superconductor material.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS The present application is a Section 371 National Stage Application of International Application No. PCT/NL2023/050001, filed Jan. 2, 2023, and further claims priority to Dutch patent application no. 2030396, filed Jan. 3, 2022. FIELD OF THE INVENTION This disclosure relates to a persistent current switch for controlling a persistent current in a superconductor loop. Further, this disclosure relates to a chip and a quantum computing system comprising such switch. This disclosure also relates to a method for initiating a persistent current in a superconductor loop. BACKGROUND Quantum computing devices need high precision magnetic bias signals to accurately set the operating point of the qubits. Since very low heat dissipation is required, low power operation of control electronics is necessary. The current fluctuation must also be very low to avoid decohering the qubit. Because the qubit is operated at temperatures in a few Kelvin regime, using conventional wires and driving electronics for generating the required control magnetic field will heat the refrigerator considerably and inject undesirable noise into the device. The cryostat has a limited cooling power and hence it is important to keep the power dissipation low in order to keep the temperature low and/or reduce thermal noise. One way to solve these problems is to generate a magnetic field with a superconducting loop controlled by a persistent current switch (PCS). A persistent current switch can be regarded as a variable resistance. Typically, a PCS is connected in parallel with an external DC power supply and the superconducting loop, usually forming a coil. During what is called the charging or energizing process, the PCS preferably has a relatively high resistance so that the external power supply can energize the superconducting loop. After the loop has been sufficiently charged, the PCS switches to a zero-resistant state to short-circuit the loop. As a result of this short circuit, a closed superconductive loop is formed in which an electrical current is flowing. Due to the superconductive state of the loop, this electrical current will continue to flow forever, in principle, without requiring a continuous driving current. A disadvantage of thermally controlled persistent current switches is that they require the generation of heat in order to control the switch. In particular, a part of the superconductor loop is heated so that that part is not superconductive. However, heat is actually undesired as quantum chips are operated at very low temperature. Further, it takes some time for the heated part of the superconductor loop to cool down again and adopt a superconductive state. Sometimes this “recover time” can take several seconds even. Such a long recovery time is undesired because the current that is present in the superconductor loop will be consumed by the resistance of the switch during the recovery, leading to lower persistent currents and/or higher power losses. In light of the above, there is a need in the art for a persistent current switch that enables to efficiently initiate a persistent current in a superconductive loop. SUMMARY To that end, a persistent current switch is disclosed for controlling, e.g. initiating, a persistent current in a superconductor loop. The persistent current switch comprises a piece of superconductor material that is part of the superconductor loop. Further, the persistent current switch comprises an illumination system that is configured to direct light onto the piece of superconductor material for influencing an electrical resistance of the piece of superconductor material. The illumination system is configured such that the light impinging on the piece of superconductor material substantially does not heat the piece of superconductor material. In particular, the illumination system may be configured as such in that a light source of the illumination system is configured to generate light having a certain radiant flux. The disclosed persistent current switch enables to control a persistent current in a highly efficient manner. The switch is based on a variable resistance of the piece of superconductor, the value of which depends on whether the piece of superconductor receives light or not from the illumination system. The disclosed persistent current switch may be regarded as a so-called photonic switch. In principle, the electrical resistance of the piece of superconductor increases upon the light interacting with the piece of superconductor. It is believed that suitable light breaks so-called Cooper pairs that are present in the piece of superconductor material which prevents the piece of superconductor material to become superconductive. Breaking the Cooper pairs namely lowers the critical current for superconductivity for the piece of superconductor material. Advantageously, no heat generation is required for controlling the disclosed switch rendering it suitab