  A number of current approaches to quantum and neuromorphic computing use
superconductors as the basis of their platform or as a measurement component,
and will need to operate at cryogenic temperatures. Semiconductor systems are
typically proposed as a top-level control in these architectures, with
low-temperature passive components and intermediary superconducting electronics
acting as the direct interface to the lowest-temperature stages. The
architectures, therefore, require a low-power superconductor-semiconductor
interface, which is not currently available. Here we report a superconducting
switch that is capable of translating low-voltage superconducting inputs
directly into semiconductor-compatible (above 1,000 mV) outputs at kelvin-scale
temperatures (1 K or 4 K). To illustrate the capabilities in interfacing
superconductors and semiconductors, we use it to drive a light-emitting diode
(LED) in a photonic integrated circuit, generating photons at 1 K from a
low-voltage input and detecting them with an on-chip superconducting
single-photon detector. We also characterize our device's timing response (less
than 300 ps turn-on, 15 ns turn-off), output impedance (greater than 1
M{\Omega}), and energy requirements (0.18 fJ/um^2, 3.24 mV/nW).
