Electronic structure of superposition states in flux qubits

Abstract

Flux qubits, small superconducting loops interrupted by Josephson junctions, are successful realizations of quantum coherence for macroscopic variables. Superconductivity in these loops is carried by $\sim 10^6$ -- $10^{10}$ electrons, which has been interpreted as suggesting that coherent superpositions of such current states are macroscopic superpositions analogous to Schrödinger's cat. We provide a full microscopic analysis of such qubits, from which the macroscopic quantum description can be derived. This reveals that the number of microscopic constituents participating in superposition states for experimentally accessible flux qubits is surprisingly but not trivially small. The combination of this relatively small size with large differences between macroscopic observables in the two branches is seen to result from the Fermi statistics of the electrons and the large disparity between the values of superfluid and Fermi velocity in these systems.

Minor cosmetic changes. Published version.

References in corpus (5)

• Coherent Quantum Dynamics of a Superconducting Flux Qubit
• Dephasing of a superconducting qubit induced by photon noise
• Density of states in a superconductor carrying a supercurrent
• A measurement-based measure of the size of macroscopic quantum superpositions
• Measuring the size of a Schroedinger cat state

Cited by in corpus (12)

• Leggett-Garg Inequalities
• Macroscopic quantum states: measures, fragility and implementations
• Robust Mesoscopic Superposition of Strongly Correlated Ultracold Atoms
• The size of macroscopic superposition states in flux qubits
• Measurement- and comparison-based sizes of Schrödinger cat states of light
• Protected cat states from kinetic driving of a boson gas
• Engineering mesoscopic superpositions of superfluid flow
• Insufficiency of avoided crossings for witnessing large-scale quantum coherence in flux qubits
• Macroscopicity of quantum superpositions on a one-parameter unitary path in Hilbert space
• Entanglement of condensed magnons via momentum-space fragmentation
• Circuit Quantisation from First Principles
• Attraction Induced by Mutual Quantum Measurements of Velocity