Reflective amplification without population inversion from a strongly driven superconducting qubit

摘要

Amplification of optical or microwave fields is often achieved by stronglydriving a medium to induce population inversion such that a weak probe can beamplified through stimulated emission. Here we strongly couple asuperconducting qubit, an artificial atom, to the field in a semi-infinitewaveguide. When driving the qubit strongly on resonance such that a Mollowtriplet appears, we observe a 7\% amplitude gain for a weak probe atfrequencies in-between the triplet. This amplification is not due to populationinversion, neither in the bare qubit basis nor in the dressed-state basis, butinstead results from a four-photon process that converts energy from the strongdrive to the weak probe. We find excellent agreement between the experimentalresults and numerical simulations without any free fitting parameters. Thedevice demonstrated here may have applications in quantum informationprocessing and quantum-limited measurements.