  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 and to allow
for reconfigurable, software-defined infrastructures. In the same spirit,
ultra-cold electro-optic links have been proposed and used to generate qubit
control signals, or to replace cryogenic readout electronics. So far, the
latter suffered from either low efficiency, low bandwidth and the need for
additional microwave drives, or breaking of Cooper pairs and qubit states. In
this work we realize electro-optic microwave photonics at millikelvin
temperatures to implement 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. Importantly, we do not observe any direct
radiation impact on the qubit state as verified with high-fidelity
quantum-non-demolition measurements despite the absence of shielding elements.
This compatibility between superconducting circuits and telecom wavelength
light is not only a prerequisite to establish modular quantum networks, it is
also relevant for multiplexed readout of superconducting photon detectors and
classical superconducting logic. Moreover, this experiment showcases the
potential of electro-optic radiometry in harsh environments - an
electronics-free sensing principle that extends into the THz regime with
applications in radio astronomy, planetary missions and earth observation.
