  Delivering on the revolutionary promise of a universal quantum computer will
require processors with millions of quantum bits (qubits). In superconducting
quantum processors, each qubit is individually addressed with microwave signal
lines that connect room temperature electronics to the cryogenic environment of
the quantum circuit. The complexity and heat load associated with the multiple
coaxial lines per qubit limits the possible size of a processor to a few
thousand qubits. Here we introduce a photonic link employing an optical fiber
to guide modulated laser light from room temperature to a cryogenic
photodetector, capable of delivering shot-noise limited microwave signals
directly at millikelvin temperatures. By demonstrating high-fidelity control
and readout of a superconducting qubit, we show that this photonic link can
meet the stringent requirements of superconducting quantum information
processing. Leveraging the low thermal conductivity and large intrinsic
bandwidth of optical fiber enables efficient and massively multiplexed delivery
of coherent microwave control pulses, providing a path towards a million-qubit
universal quantum computer.
