Dephasing of a superconducting qubit induced by photon noise

TL;DR

This study investigates how photon noise causes dephasing in a superconducting flux qubit, demonstrating control over the environment and quantifying its impact on qubit coherence.

Contribution

It provides a controllable experimental setup to study photon-induced dephasing and introduces a simple model to explain the thermal fluctuation effects.

Findings

Dephasing rate reduces to zero when coupling is tuned to zero.

Achieved a spin-echo decay time of 4 microseconds at optimal conditions.

Thermal photon fluctuations are identified as the main dephasing source.

Abstract

We have studied the dephasing of a superconducting flux-qubit coupled to a DC-SQUID based oscillator. By varying the bias conditions of both circuits we were able to tune their effective coupling strength. This allowed us to measure the effect of such a controllable and well-characterized environment on the qubit coherence. We can quantitatively account for our data with a simple model in which thermal fluctuations of the photon number in the oscillator are the limiting factor. In particular, we observe a strong reduction of the dephasing rate whenever the coupling is tuned to zero. At the optimal point we find a large spin-echo decay time of $4 \mu s$.