A single-qubit engine amplifies microwaves using only quantum measurement
Rémy Dassonneville, Cyril Elouard, Benjamin Huard and colleagues built an engine from a single superconducting transmon qubit, fuelled entirely by the energy that quantum-measurement backaction injects into the system.
By repeatedly measuring an observable that does not commute with the qubit Hamiltonian and applying feedback, the device amplifies a propagating microwave signal, with the measured amplification gain directly reading out the engine's work output. They emphasise that the engine acts as a quantum Maxwell demon operating without any hot thermal reservoir, drawing power from measurement rather than a temperature gradient.
The experiment demonstrates measurement as a genuine thermodynamic resource and probes the energetic bookkeeping of quantum information.