A tunable rf SQUID manipulated as flux and phase qubit

TL;DR

This paper demonstrates a tunable rf SQUID functioning as both a flux and phase qubit, showcasing fast, coherent manipulations with high-frequency oscillations and comparable decay times, advancing quantum control techniques.

Contribution

It introduces dual manipulation methods for a tunable rf SQUID, enabling flux and phase qubit operations with high-frequency coherence and faster control capabilities.

Findings

Larmor oscillation frequency up to 20 GHz

Decay times of oscillations are similar for both methods

Microwave-free manipulation achieves faster operations

Abstract

We report on two different manipulation procedures of a tunable rf SQUID. First, we operate this system as a flux qubit, where the coherent evolution between the two flux states is induced by a rapid change of the energy potential, turning it from a double well into a single well. The measured coherent Larmor-like oscillation of the retrapping probability in one of the wells has a frequency ranging from 6 to 20 GHz, with a theoretically expected upper limit of 40 GHz. Furthermore, here we also report a manipulation of the same device as a phase qubit. In the phase regime, the manipulation of the energy states is realized by applying a resonant microwave drive. In spite of the conceptual difference between these two manipulation procedures, the measured decay times of Larmor oscillation and microwave-driven Rabi oscillation are rather similar. Due to the higher frequency of the Larmor oscillations, the microwave-free qubit manipulation allows for much faster coherent operations.