  We review the recent progress in direct active cooling of the
quantum-electric degrees freedom in engineered circuits, or quantum-circuit
refrigeration. In 2017, the invention of a quantum-circuit refrigerator (QCR)
based on photon-assisted tunneling of quasiparticles through a
normal-metal--insulator--superconductor junction inspired a series of
experimental studies demonstrating the following main properties: (i) the
direct-current (dc) bias voltage of the junction can change the QCR-induced
damping rate of a superconducting microwave resonator by orders of magnitude
and give rise to non-trivial Lamb shifts, (ii) the damping rate can be
controlled in nanosecond time scales, and (iii) the dc bias can be replaced by
a microwave excitation, the amplitude of which controls the induced damping
rate. Theoretically, it is predicted that state-of-the-art superconducting
resonators and qubits can be reset with an infidelity lower than $10^{-4}$ in
tens of nanoseconds using experimentally feasible parameters. A QCR-equipped
resonator has also been demonstrated as an incoherent photon source with an
output temperature above one kelvin yet operating at millikelvin. This source
has been used to calibrate cryogenic amplification chains. In the future, the
QCR may be experimentally used to quickly reset superconducting qubits, and
hence assist in the great challenge of building a practical quantum computer.
