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EP-4736606-A1 - SUPERCONDUCTING QUBIT DEVICES AND METHODS FOR FABRICATING SUPERCONDUCTING QUBIT DEVICES

EP4736606A1EP 4736606 A1EP4736606 A1EP 4736606A1EP-4736606-A1

Abstract

According to an aspect, the present description relates to a superconducting qubit (200) comprising at least one capacitor (202) and at least one Josephson junction (204) electrically coupled in parallel to the capacitor (202) The capacitor (202) comprises a first ferroelectric material (206), a first electrode (208) of a first superconducting material, and a second electrode (210) of a second superconducting material. The first electrode (208) and the second electrode (210) are in electric contact with the first ferroelectric material (206). The Josephson junction (204) comprises a first region (212) of a third superconducting material and a second region (214) of a fourth superconducting material coupled to said first region (212) by a third region (216) of a fifth material.

Inventors

  • CANO, Andrès
  • DONAIRE, Manuel

Assignees

  • Centre National de la Recherche Scientifique

Dates

Publication Date
20260506
Application Date
20240626

Claims (15)

  1. 1. A superconducting qubit (200) comprising: at least one capacitor (202) comprising a first ferroelectric material (206), a first electrode (208) of a first superconducting material, and a second electrode (210) of a second superconducting material, wherein the first electrode (208) and the second electrode (210) are in electric contact with the first ferroelectric material (206); and at least one Josephson junction (204) electrically coupled in parallel to the capacitor (202) wherein said Josephson junction (204) comprises a first region (212) of a third superconducting material and a second region (214) of a fourth superconducting material coupled to said first region (212) by a third region (216) of a fifth material.
  2. 2. The superconducting qubit (200) according to claim 1, wherein the fifth material of the third region (216) is a superconducting material with a critical temperature that is different from critical temperatures of the third and fourth superconducting materials.
  3. 3. The superconducting qubit (200) according to claim 1, wherein the fifth material of the third region (216) is a second ferroelectric material.
  4. 4. The superconducting qubit (200) according to any of the preceding claims, wherein the first superconducting material is NbN.
  5. 5. The superconducting qubit (200) according to any of the preceding claims, wherein the superconducting qubit (200) is configured to operate in the regime of incipient ferroelectricity.
  6. 6. The superconducting qubit (200) according to any of claims 1 to 4, wherein the superconducting qubit (200) is configured to operate in the regime of ferroelectricity.
  7. 7. The superconducting qubit (200) according to any of the preceding claims, wherein the first ferroelectric material is strontium titanate doped with calcium Sri, v Ca v TiO3, wherein x is 0, 1, or a value between 0 and 1.
  8. 8. The superconducting qubit (200) according to any of the preceding claims, wherein under operation the total energy comprises a quadratic term function of a normalized charge n and a non-quadratic term function of the normalized charge n, and wherein an absolute value of a ratio between a coefficient of the quadratic term and a coefficient of the nonquadratic term is less than or equal to 10.
  9. 9. The superconducting qubit (200) according to any of the preceding claims, wherein under operation a charging energy (E c ) of the superconducting qubit is less than or equal to an energy (£)) of the Josephson junction (204).
  10. 10. A method of manufacturing a superconducting qubit, comprising: depositing, adjacent to a substrate (1102), a layer of a first ferroelectric material (206), and at least one of a first layer of a first superconducting material (1104) and a second layer of a second superconducting material (1402); forming two electrodes (208, 210) by patterning at least one of the layer of the first ferroelectric material (206) and the at least one of the first layer of a first superconducting material (1104) and the second layer of a second superconducting material (1402); forming at least one capacitor (202) by electrically coupling the electrodes (208, 210) with the layer of the first ferroelectric material (206); forming, adjacent to the electrodes (208, 210), a first region (212) of a third superconducting material and a second region (214) of a fourth superconducting material; forming at least one Josephson junction (204) by coupling the first region (212) and the second region (214) by a third region (216) of a fifth material; and electrically coupling each of the first region (212) and the second region (214) to different electrode (208, 210).
  11. 11. The method according to claim 10, wherein forming the first region (212) and the second region (214) comprises patterning the layer of the first superconducting material (1104).
  12. 12. The method according to claim 10 or claim 11, wherein forming the two electrodes (208, 210) comprises patterning using contact lithography.
  13. 13. The method according to any of claims 10 to 12, wherein forming the first region (212) and the second region (214) comprises depositing said first region (212) and second region (214) by double angle evaporation.
  14. 14. The method according to any of claims 10 to 13, wherein forming the first region (212) and the second region (214) comprises patterning using electron-beam lithography.
  15. 15. The method according to any of claims 10 to 14, wherein the layer of the first ferroelectric material (206) is deposited between two layers of the first superconducting material (1104).

Description

Superconducting qubit devices and methods for fabricating superconducting qubit devices PRIOR ART Technical field [001] The present description relates to superconducting qubit devices. The present description also relates to methods for fabricating superconducting qubit devices. Background of the Invention [002] A quantum processor is a device that is configured to process input data by using quantum states of physical systems in order to implement an algorithm on said input data and generate output data. For example, algorithms may comprise a sequence of quantum operations that are implemented using quantum mechanical systems configured to store a quantum information. Conventionally, quantum information may be stored using quantum bits, referred to as “qubits,” which are typically quantum mechanical systems exhibiting two or more states, called “basis states”. The states of a qubit can be used to encode quantum information. For example, a qubit may be realized as a quantum system that has a ground state and an excited state, and these two basic states may be used to denote quantum bit values of 0 and 1. The quantum nature of a qubit is related to its ability to exist in a coherent superposition of basis states, and for the state of the qubit to have a phase. A qubit will retain this ability to exist as a coherent superposition of basis states when the qubit is sufficiently isolated from sources of decoherence [Ref. 1], [003] A quantum property that a qubit must exhibit is the ability to store energy in discrete values and an uneven spacing between the different energy levels. The uneven spacing of energy levels of a qubit is known as anharmonicity. [004] A small electric circuit built using superconducting wires may become a qubit with discrete energy levels. For some materials the conduction electrons are bounded together by interactions forming Cooper pairs. Unlike individual electrons, multiple Cooper pairs may be in the same quantum state, giving rise to the phenomenon of superconductivity [Ref. 2], [005] In order to keep its superconducting properties, preventing the superconducting circuit from warming up is desired. Two types of elements in a superconducting circuit do not produce heat: capacitors and inductors. A capacitor comprises two metallic elements that store electric charge, while an inductor comprises a wire that stores a magnetic field when a current passes through. A capacitor-inductance circuit with superconducting wires may have discrete energy levels. However, such energy levels are evenly spaced, making it impossible to isolate only two specific states. The solution to the problem of uneven spacing is provided by a Josephson junction. [006] A Josephson junction comprises two superconducting materials separated by a barrier of a different material [Ref. 2], The supercurrent going through the barrier is reduced but not completely stopped because of a “tunnel effect”. Cooper pairs can tunnel through the barrier and couple the superconducting wave functions on either side of the barrier. By replacing the inductor of the circuit with a Josephson junction, the energy levels of the superconducting circuit become unevenly spaced. Thanks to this anharmonicity, the qubit can be made to operate with two basic states, a ground state and first excited state. [007] Among different physical implementations, superconducting qubits are nowadays considered as one of the most promising technologies for quantum processing. In particular, superconducting qubits have been used to demonstrate quantum supremacy, z.e., the possibility of solving problems that no classical computer can solve in any feasible amount of time. [008] Fig. 1 shows a diagram of a superconducting qubit 100 including a capacitor 102 and a Josephson junction 104. Such superconducting qubit 100 generally includes one or more Josephson junctions 104 connected to one or more other circuit elements. The circuit elements may be one or more capacitors 102 (for charge qubits, see Fig. 1) or superconducting loops (for flux qubits). The charging energy Ec of such superconducting qubit is defined as (2e)2/(2Q, where e is the electron charge and C is the capacitance of the capacitor. The Josephson energy, £}, is a characteristic parameter of the Josephson junction 104 defined as £} = Ich/(4ne), where Ic is the maximum supercurrent that the Josephson junction can support and h is the Planck constant. [009] When used in a quantum processor, the qubit 100 may be connected to a gate capacitor so that it can receive external signals. However, a small change or noise in the gate charge can lead to changes in the qubit energy levels (that is, to changes in the relative energy between the ground state and the first excited state of the superconducting circuit). This issue is addressed in superconducting qubits of the prior art by increasing the size of the capacitor and thereby the capacitance C of the qubit (and thus, decreasing the charging energy Ec