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Entangling microwave-frequency superconducting quantum processors through optical light at ambient temperature would enable means of secure communication and distributed quantum information processing. However, transducing quantum signals between these disparate regimes of the electro-magnetic spectrum remains an outstanding goal^{, , , , , , –}, and interfacing superconducting qubits, which are constrained to operate at millikelvin temperatures, with electro-optic transducers presents considerable challenges owing to the deleterious effects of optical photons on superconductors^,. Moreover, many remote entanglement protocols^{, , –} require multiple qubit gates both preceding and following the upconversion of the quantum state, and thus an ideal transducer should impart minimal backaction on the qubit. Here we demonstrate readout of a superconducting transmon qubit through a low-backaction electro …