Reflective amplification without population inversion from a strongly driven superconducting qubit

Understand

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.