Efficient qubit measurement with a nonreciprocal microwave amplifier

TL;DR

This paper demonstrates a highly efficient, nonreciprocal microwave measurement setup for superconducting qubits, significantly improving measurement fidelity and enabling advanced quantum feedback and error correction protocols.

Contribution

It introduces a nonreciprocal parametric amplifier that enhances measurement efficiency and protects qubits from backaction, advancing quantum measurement technology.

Findings

Achieved 72% measurement efficiency.

Protected qubits from excess backaction.

Enabled potential for quantum feedback and error correction.

Abstract

The act of observing a quantum object fundamentally perturbs its state, resulting in a random walk toward an eigenstate of the measurement operator. Ideally, the measurement is responsible for all dephasing of the quantum state. In practice, imperfections in the measurement apparatus limit or corrupt the flow of information required for quantum feedback protocols, an effect quantified by the measurement efficiency. Here we demonstrate the efficient measurement of a superconducting qubit using a nonreciprocal parametric amplifier to directly monitor the microwave field of a readout cavity. By mitigating the losses between the cavity and the amplifier we achieve a measurement efficiency of $72\%$. The directionality of the amplifier protects the readout cavity and qubit from excess backaction caused by amplified vacuum fluctuations. In addition to providing tools for further improving the fidelity of strong projective measurement, this work creates a testbed for the experimental study of ideal weak measurements, and it opens the way towards quantum feedback protocols based on weak measurement such as state stabilization or error correction.