Decoherence in dc SQUID phase qubits

TL;DR

This study measures coherence times and Rabi oscillations in Al and Nb dc SQUID phase qubits, revealing insights into their decoherence mechanisms and the impact of device materials and coupling on quantum coherence.

Contribution

It provides the first detailed comparison of coherence times in Al and Nb dc SQUID phase qubits under similar conditions, highlighting material-dependent decoherence characteristics.

Findings

Al qubit T2* ranged from 3 to 7 ns

Nb devices showed T2* of 4 to 6 ns

Decoherence was not primarily due to bias line noise

Abstract

We report measurements of Rabi oscillations and spectroscopic coherence times in an Al/AlOx/Al and three Nb/AlOx/Nb dc SQUID phase qubits. One junction of the SQUID acts as a phase qubit and the other junction acts as a current-controlled nonlinear isolating inductor, allowing us to change the coupling to the current bias leads in situ by an order of magnitude. We found that for the Al qubit a spectroscopic coherence time T2* varied from 3 to 7 ns and the decay envelope of Rabi oscillations had a time constant T' = 25 ns on average at 80 mK. The three Nb devices also showed T2* in the range of 4 to 6 ns, but T' was 9 to 15 ns, just about 1/2 the value we found in the Al device. For all the devices, the time constants were roughly independent of the isolation from the bias lines, implying that noise and dissipation from the bias leads were not the principal sources of dephasing and inhomogeneous broadening.