Fast Universal Control of an Oscillator with Weak Dispersive Coupling to a Qubit

TL;DR

This paper demonstrates a noise-resilient protocol enabling universal control of a quantum oscillator with weak dispersive coupling, achieving high-fidelity state preparation and error correction faster than traditional methods.

Contribution

It introduces a novel in-situ enhanced nonlinearity protocol that allows universal quantum control even when the nonlinear interaction rate is comparable to decoherence rates.

Findings

Achieved 98% fidelity in single-photon state preparation

Realized 11.1 dB squeezing in microwave cavity

Demonstrated fast, measurement-free logical state preparation for quantum error correction

Abstract

A controlled evolution generated by nonlinear interactions is required to perform full manipulation of a quantum system, and such control is only coherent when the rate of nonlinearity is large compared to the rate of decoherence. As a result, engineered quantum systems typically rely on a bare nonlinearity much stronger than all decoherence rates, and this hierarchy is usually assumed to be necessary. In this work, we challenge this assumption by demonstrating the universal control of a quantum system where the relevant rate of bare nonlinear interaction is comparable to the fastest rate of decoherence. We do this by introducing a novel noise-resilient protocol for the universal quantum control of a nearly-harmonic oscillator that takes advantage of an in-situ enhanced nonlinearity instead of harnessing a bare nonlinearity. Our experiment consists of a high quality-factor microwave cavity with weak-dispersive coupling to a much lower quality superconducting qubit. By using strong drives to temporarily excite the oscillator, we realize an amplified three-wave-mixing interaction, achieving typical operation speeds over an order of magnitude faster than expected from the bare dispersive coupling. Our demonstrations include preparation of a single-photon state with $98\pm 1(\%)$ fidelity and preparation of squeezed vacuum with a squeezing level of $11.1$ dB, the largest intracavity squeezing reported in the microwave regime. Finally, we also demonstrate fast measurement-free preparation of logical states for the binomial and Gottesman-Kitaev-Preskill (GKP) quantum error-correcting codes.