  A switch capable of routing microwave signals at cryogenic temperatures is a
desirable component for state-of-the-art experiments in many fields of applied
physics, including but not limited to quantum information processing,
communication and basic research in engineered quantum systems. Conventional
mechanical switches provide low insertion loss but disturb operation of
dilution cryostats and the associated experiments by heat dissipation. Switches
based on semiconductors or microelectromechanical systems have a lower thermal
budget but are not readily integrated with current superconducting circuits.
Here we design and test an on-chip switch built by combining tunable
transmission-line resonators with microwave beam-splitters. The device is
superconducting and as such dissipates a negligible amount of heat. It is
compatible with current superconducting circuit fabrication techniques,
operates with a bandwidth exceeding $100\,\mathrm{MHz}$, is capable of handling
photon fluxes on the order of $10^{5}\,\mu\mathrm{s}^{-1}$, equivalent to
powers exceeding $-90\,\mathrm{dBm}$, and can be switched within approximately
$6-8\,\mathrm{ns}$. We successfully demonstrate operation of the device in the
quantum regime by integrating it on a chip with a single-photon source and
using it to route non-classical itinerant microwave fields at the single-photon
level.
