High-efficiency measurement of an artificial atom embedded in a parametric amplifier

TL;DR

This paper presents a novel superconducting device embedding a transmon qubit within a flux-pumped Josephson parametric amplifier, achieving high measurement efficiency by reducing losses and backaction.

Contribution

The work introduces a new integrated design that enhances measurement efficiency without increasing qubit backaction, enabling more precise quantum measurements.

Findings

Achieved up to 80% measurement efficiency.

Enabled high-power operation reducing information loss.

Accurately modeled the interplay of gain and backaction.

Abstract

A crucial limit to measurement efficiencies of superconducting circuits comes from losses involved when coupling to an external quantum amplifier. Here, we realize a device circumventing this problem by directly embedding a two-level artificial atom, comprised of a transmon qubit, within a flux-pumped Josephson parametric amplifier. Surprisingly, this configuration is able to enhance dispersive measurement without exposing the qubit to appreciable excess backaction. This is accomplished by engineering the circuit to permit high-power operation that reduces information loss to unmonitored channels associated with the amplification and squeezing of quantum noise. By mitigating the effects of off-chip losses downstream, the on-chip gain of this device produces end-to-end measurement efficiencies of up to 80 percent. Our theoretical model accurately describes the observed interplay of gain and measurement backaction, and delineates the parameter space for future improvement. The device is compatible with standard fabrication and measurement techniques, and thus provides a route for definitive investigations of fundamental quantum effects and quantum control protocols.