Phase-Coherent Dynamics of a Superconducting Flux Qubit with Capacitive-Bias Readout

TL;DR

This study investigates the phase-coherent behavior of a superconducting flux qubit using a capacitive-bias readout, demonstrating faster measurement and insights into noise sources affecting qubit coherence.

Contribution

It introduces a capacitive-bias readout method for flux qubits, improving speed and noise management, and analyzes dominant noise contributions affecting qubit coherence.

Findings

Capacitive-bias readout achieves four times faster switching pulses.

White noise limits energy relaxation and dephasing far from the degeneracy point.

1/f-noise dominates dephasing near the optimal point.

Abstract

We present a systematic study of the phase-coherent dynamics of a superconducting three-Josephson-junction flux qubit. The qubit state is detected with the integrated-pulse method, which is a variant of the pulsed switching DC SQUID method. In this scheme the DC SQUID bias current pulse is applied via a capacitor instead of a resistor, giving rise to a narrow band-pass instead of a pure low-pass filter configuration of the electromagnetic environment. Measuring one and the same qubit with both setups allows a direct comparison. With the capacitive method about four times faster switching pulses and an increased visibility are achieved. Furthermore, the deliberate engineering of the electromagnetic environment, which minimizes the noise due to the bias circuit, is facilitated. Right at the degeneracy point the qubit coherence is limited by energy relaxation. We find two main noise contributions. White noise is limiting the energy relaxation and contributing to the dephasing far from the degeneracy point. 1/f-noise is the dominant source of dephasing in the direct vicinity of the optimal point. The influence of 1/f-noise is also supported by non-random beatings in the Ramsey and spin echo decay traces. Numeric simulations of a coupled qubit-oscillator system indicate that these beatings are due to the resonant interaction of the qubit with at least one point-like fluctuator, coupled especially strongly to the qubit.