Measurements of cryogenic mirror thermal noise and its impact on quantum electro-opto-mechanical transducers

A sufficiently efficient and low-noise transducer linking microwave and optical frequencies would enable entanglement between superconducting quantum registers separated by long distances. We have previously shown highly efficient and near-quantum transduction by coupling a mechanical mode of a Si₃N₄ membrane simultaneously to a superconducting LC circuit and a high finesse optical Fabry-Perot cavity. Even with large transduction efficiencies, total losses between an attached qubit and the optical readout port would prohibit direct entanglement of superconducting registers. Single-photon readout can be leveraged to tradeoff success probability for entanglement fidelity when confronted with these losses, albeit at the expense of increased sensitivity to noise that is spectrally broader than the transducer bandwidth. In this talk we will discuss how broadband thermal noise from our cryogenically cooled Fabry-Perot mirrors contributes to noise during single-photon readout. We will present measurements of this noise in a dry cycle dilution refrigerator and discuss attempts to suppress the noise by operating phonically filtered mirrors at cryogenic temperatures.