All-Optical Single Shot Readout of a Superconducting Qubit*

The rapid development of superconducting quantum hardware is expected to run into significant I/O restrictions due to the need for large-scale error-correction in a cryogenic environment. Classical data centers rely on fiber-optic interconnects to remove similar networking bottlenecks. In the same spirit, here we realize a radio-over-fiber qubit readout that does not require any active or passive cryogenic microwave equipment. We demonstrate all-optical single-shot readout by means of the Jaynes-Cummings nonlinearity in a circulator-free readout scheme. Specifically, we replace the complete cryogenic microwave input and output by an optical fiber and a single electro-optic transducer operated at mK temperatures. The latter is used simultaneously for demodulation of the input probing the qubit state as well as for modulation of the telecom light by means of the reflected microwave tone carrying information of the qubit state. We discuss the performance of this minimalistic readout, its implications on the qubit performance and its limitations. Even though the active optical heat load in the electro-optic link prevents substantial scaling-up of this first-generation implementation - importantly – we do not observe any direct impact of the laser light on the qubit performance, as verified with high-fidelity quantum-non-demolition measurements. This compatibility between superconducting circuits and light is not only a prerequisite to establish modular quantum networks, it is also relevant for multiplexed readout of superconducting photon detector or classical superconducting logic. Our experiment furthermore showcases the potential of electro-optic radiometry in harsh environments.