Energy-efficient quantum non-demolition measurement with a spin-photon interface

Spin-photon interfaces (SPIs) are key devices of quantum technologies, aimed at coherently transferring quantum information between spin qubits and propagating pulses of polarized light. We study the potential of a SPI for quantum non demolition (QND) measurements of a spin state. After being initialized and scattered by the SPI, the state of a light pulse depends on the spin state. It thus plays the role of a pointer state, in-formation being encoded in the light's temporal and polarization degrees of freedom. Building on the fully Hamiltonian resolution of the spin-light dynamics, we show that quantum superpositions of zero and single photon states outperform co-herent pulses of light, producing pointer states which are more distinguishable with the same photon budget. The energetic advantage pro-vided by quantum pulses over coherent ones is maintained when information on the spin state is extracted at the classical level by perform-ing projective measurements on the light pulses. The proposed schemes are robust against imper-fections in state of the art semi-conducting de-vices.