Dephasing of a superconducting flux qubit

TL;DR

This study investigates decoherence in superconducting flux qubits by measuring relaxation and echo times, revealing that energy relaxation limits coherence at optimal points and flux noise causes dephasing away from optimal conditions.

Contribution

It provides experimental evidence linking flux noise with dephasing and demonstrates that energy relaxation dominates decoherence at the optimal point.

Findings

At the optimal point, $T_2^{ m echo} \\approx 2T_1$ indicating relaxation-limited decoherence.

Moving away from the optimal point, phase relaxation rate increases linearly with magnetic flux.

Flux noise with a $1/f$ spectrum explains the observed dephasing behavior.

Abstract

In order to gain a better understanding of the origin of decoherence in superconducting flux qubits, we have measured the magnetic field dependence of the characteristic energy relaxation time ($T_1$) and echo phase relaxation time ($T_2^{\rm echo}$) near the optimal operating point of a flux qubit. We have measured $T_2^{\rm echo}$ by means of the phase cycling method. At the optimal point, we found the relation $T_2^{\rm echo}\approx 2T_1$. This means that the echo decay time is {\it limited by the energy relaxation} ($T_1$ process). Moving away from the optimal point, we observe a {\it linear} increase of the phase relaxation rate ($1/T_{2}^{\rm echo}$) with the applied external magnetic flux. This behavior can be well explained by the influence of magnetic flux noise with a $1/f$ spectrum on the qubit.