Comparison of coherence times in three dc SQUID phase qubits

TL;DR

This study compares coherence times in three different dc SQUID phase qubits, revealing that flux noise is not the primary decoherence source, with variations observed based on device design and material.

Contribution

It provides the first comparative analysis of coherence times across different dc SQUID phase qubit geometries and materials, highlighting the impact of device design on decoherence.

Findings

Coherence times ranged from 4 to 10 ns across devices.

Rabi oscillation decay times were longer in the Al device.

Flux noise was not the dominant decoherence mechanism.

Abstract

We report measurements of spectroscopic linewidth and Rabi oscillations in three thin-film dc SQUID phase qubits. One device had a single-turn Al loop, the second had a 6-turn Nb loop, and the third was a first order gradiometer formed from 6-turn wound and counter-wound Nb coils to provide isolation from spatially uniform flux noise. In the 6 - 7.2 GHz range, the spectroscopic coherence times for the gradiometer varied from 4 ns to 8 ns, about the same as for the other devices (4 to 10 ns). The time constant for decay of Rabi oscillations was significantly longer in the single-turn Al device (20 to 30 ns) than either of the Nb devices (10 to 15 ns). These results imply that spatially uniform flux noise is not the main source of decoherence or inhomogenous broadening in these devices.