Observation of a power transfer controlled by the phase of a quantum superposition

Can the direction of energy flow between two propagating electromagnetic modes be controlled by a purely quantum parameter—specifically, the phase of a coherent superposition of a qubit's ground and excited states—rather than by classical control fields?

The authors utilize a superconducting transmon qubit coupled to two transmission lines. By driving the qubit with two spatially separated modes that have opposite amplitudes, the net classical action on the qubit is canceled. However, the qubit can still facilitate power transfer between these lines through stimulated emission. The researchers measure the output power in one of the lines using a near quantum-limited amplification chain, allowing them to observe how the qubit's quantum state influences the direction of energy transfer.

The study confirms that the direction of power flow is directly governed by the phase of the qubit's superposition. When the qubit is in a coherent state, the stimulated emission interferes with the input drives, creating a directional bias. The researchers observed that the power transfer varies as the cosine of the qubit's phase, allowing for a switchable directionality between phase values of 0 and π. For a maximally coherent state, they achieved a routing efficiency of 63%, which is consistent with the theoretical limit of 69% derived from the qubit's measured coherence times. This demonstrates that quantum coherence acts as a non-classical knob for energy routing.

This experiment provides a clear demonstration of a quantum thermodynamic machine where the energy flow is dictated by quantum coherence. It bridges the gap between fundamental quantum optics and practical quantum energetics, showing how artificial atoms can be used to manipulate energy currents in microwave circuits. The ability to route power using the phase of a superposition offers a new mechanism for controlling energy transport in quantum networks.