Effects of external driving on the coherence time of a Josephson junction qubit in a bath of two level fluctuators

TL;DR

This paper investigates how external driving influences the noise and coherence time of Josephson-junction qubits by analyzing the behavior of two-level fluctuators, revealing potential methods to enhance qubit coherence.

Contribution

It introduces a Bloch-Redfield approach to study the effects of external driving on TLS noise spectra and coherence, proposing low-frequency driving as a means to improve qubit coherence.

Findings

External driving increases TLS fluctuations depending on energy splitting.

Strong driving reduces low-frequency noise, potentially increasing qubit dephasing time.

High driving frequencies suppress the beneficial effects on coherence.

Abstract

We study the effect of external driving on the two level systems (TLSs) assumed to be a major obstacle in increasing the coherence time of solid state Josephson-junction qubits. We find, by use of a Bloch-Redfield approach, that external driving has two major effects on the TLS. The first is increased fluctuations between the two states of the TLS, the significance of this effect compared to thermal fluctuations depend on the energy splitting of the TLS compared to temperature. The second effect is a reduction in the intensity of the noise spectrum at low frequencies, and at the same time an increase in intensity around the renormalized Rabi frequency of the TLS, the driving frequency and at beatings between these two frequencies. Finally we study the ensemble averaged noise spectrum for a typical distribution of TLSs known to give origin to $\propto 1/f$ noise. We find that strong driving leads to reduced noise at low frequencies, and therefore to an increased dephasing time $T_2^Q$ of the qubit. However this effect is exponentially suppressed when the driving frequency is large compared to temperature, as we typically find for Josephson qubits. We suggest that external driving at frequencies much lower than the qubit frequency might be used in order to enhance the the qubit coherence time.