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

Abstract

A driven qubit exchanges energy with the propagating modes that drive it. When two spatially separated modes drive a single qubit with opposite amplitudes, their net action on the qubit cancels. Yet the qubit can still transfer power from one mode to the other through stimulated emission. The directionality originates from opposite stimulated emission powers into each line. We realize this situation with a superconducting transmon qubit coupled to two transmission lines and show that the direction of the power flow is set by the phase $ϕ$ of the qubit superposition between its ground and excited states, rather than by any classical control parameter. From a time-resolved measurement of the output power in one line, we observe a transfer that varies as $\cosϕ$ and hence changes direction between $ϕ=0$ and $ϕ=π$. The directionality of the total power flow is limited by the phase independent contributions of the reflected drive and of spontaneous emission, which sets a routing efficiency that we measure as a function of the input power. For an equal superposition of ground and excited qubit states (maximal coherence), the efficiency reaches $63\%$, close to the bound of $69\%$ expected from the measured qubit coherence times.