Reflective amplification without population inversion from a strongly driven superconducting qubit

TL;DR

This paper demonstrates a novel form of microwave field amplification using a strongly driven superconducting qubit, achieving gain through a four-photon process without population inversion, with potential quantum technology applications.

Contribution

It introduces a new amplification mechanism in superconducting qubits driven strongly, bypassing the need for population inversion, supported by experimental and numerical evidence.

Findings

7% amplitude gain observed for a weak probe

Amplification arises from a four-photon process, not population inversion

Excellent agreement between experiment and simulations

Abstract

Amplification of optical or microwave fields is often achieved by strongly driving a medium to induce population inversion such that a weak probe can be amplified through stimulated emission. Here we strongly couple a superconducting qubit, an artificial atom, to the field in a semi-infinite waveguide. When driving the qubit strongly on resonance such that a Mollow triplet appears, we observe a 7\% amplitude gain for a weak probe at frequencies in-between the triplet. This amplification is not due to population inversion, neither in the bare qubit basis nor in the dressed-state basis, but instead results from a four-photon process that converts energy from the strong drive to the weak probe. We find excellent agreement between the experimental results and numerical simulations without any free fitting parameters. The device demonstrated here may have applications in quantum information processing and quantum-limited measurements.