  Many important phenomena in quantum devices are dynamic, meaning that they
cannot be studied using time-averaged measurements alone. Experiments that
measure such transient effects are collectively known as fast readout. One of
the most useful techniques in fast electrical readout is radio-frequency
reflectometry, which can measure changes in impedance (both resistive and
reactive) even when their duration is extremely short, down to a microsecond or
less. Examples of reflectometry experiments, some of which have been realised
and others so far only proposed, include projective measurements of qubits and
Majorana devices for quantum computing, real-time measurements of mechanical
motion and detection of non-equilibrium temperature fluctuations. However, all
of these experiments must overcome the central challenge of fast readout: the
large mismatch between the typical impedance of quantum devices (set by the
resistance quantum) and of transmission lines (set by the impedance of free
space). Here, we review the physical principles of radio-frequency
reflectometry and its close cousins, measurements of radio-frequency
transmission and emission. We explain how to optimise the speed and sensitivity
of a radio-frequency measurement, and how to incorporate new tools such as
superconducting circuit elements and quantum-limited amplifiers into advanced
radio-frequency experiments. Our aim is three-fold: to introduce the readers to
the technique, to review the advances to date and to motivate new experiments
in fast quantum device dynamics. Our intended audience includes
experimentalists in the field of quantum electronics who want to implement
radio-frequency experiments or improve them, together with physicists in
related fields who want to understand how the most important radio-frequency
measurements work.
