Transport signatures of phase fluctuations in superconducting qubits

TL;DR

This paper investigates phase fluctuations in superconducting transmon qubits through transport measurements, revealing intrinsic phase diffusion effects linked to junction capacitance that impact qubit behavior.

Contribution

It provides experimental evidence of phase fluctuations in transmon qubits caused by intrinsic junction capacitance, enhancing understanding of decoherence mechanisms.

Findings

Reduced switching currents in small junctions

Finite resistance indicating phase diffusion

Phase fluctuations originate from intrinsic capacitance

Abstract

Josephson junctions supply the nonlinear inductance element in superconducting qubits. In the widely used transmon configuration, where the junction is shunted by a large capacitor, the low charging energy minimizes the sensitivity of the qubit to charge noise while maintaining the necessary anharmonicity to qubit states. We report here low-frequency transport measurements on small standalone junctions and identically fabricated capacitively-shunted junctions that show two distinct features normally attributed to small capacitance junctions near zero bias: reduced switching currents and prominent finite resistance associated with phase diffusion in the current-voltage characteristic. Our transport data reveals the existence of phase fluctuations in transmons arising from intrinsic junction capacitance.