Fig. 1: Comparison of conventional and optical qubit readout set-ups in a dilution refrigerator.

a, Typical microwave in–microwave out set-up consisting of carefully thermalized coaxial cables, attenuators, filters, circulators, a driven parametric amplifier (faded) and a d.c.-biased high-electron-mobility-transistor amplifier, all of which are approximately wavelength sized (centimetres). Note that the components are inserted above the respective temperature stage to make the illustration more compact. b, Reduced microwave in–optics out readout set-up replacing the microwave output components with an optically driven, resonant EO transducer. c, All-optical, optics in–optics out circulator-free qubit readout based on simultaneous microwave downconversion and upconversion of an optical carrier. Here, all cryogenic microwave components are replaced by a single EO transceiver.