Models of environment and T_1 relaxation in Josephson Charge Qubits

TL;DR

This paper offers a theoretical model explaining the nonmonotonic T_1 relaxation behavior in Josephson Charge Qubits, emphasizing the role of Andreev fluctuators as a source of background charge noise.

Contribution

It introduces the Andreev fluctuator model as a promising explanation for T_1 relaxation phenomena in Josephson Charge Qubits, aligning with experimental observations.

Findings

The model explains the linear dependence of T_1 on E_J.

It accounts for irregular fluctuations in T_1.

Resonant peaks are attributed to fluctuator interactions.

Abstract

A theoretical interpretation of the recent experiments of Astafiev et. al. on the T_1-relaxation rate in Josephson Charge Qubits is proposed. The experimentally observed reproducible nonmonotonic dependence of T_1 on the splitting E_J of the qubit levels suggests further specification of the previously proposed models of the background charge noise. From our point of view the most promising is the ``Andreev fluctuator'' model of the noise. In this model the fluctuator is a Cooper pair that tunnels from a superconductor and occupies a pair of localized electronic states. Within this model one can naturally explain both the average linear T_1(E_J) dependence and the irregular fluctuations. The role of fluctuators in the formation of strong resonant peaks in this dependence is also discussed.