  This topical review addresses how Rydberg atoms can serve as building blocks
for emerging quantum technologies. Whereas the fabrication of large numbers of
artificial quantum systems with the uniformity required for the most attractive
applications is difficult if not impossible, atoms provide stable quantum
systems which, for the same species and isotope, are all identical. Whilst
atomic ground-states provide scalable quantum objects, their applications are
limited by the range over which their properties can be varied. In contrast,
Rydberg atoms offer strong and controllable atomic interactions that can be
tuned by selecting states with different principal quantum number or orbital
angular momentum. In addition Rydberg atoms are comparatively long-lived, and
the large number of available energy levels and their separations allow
coupling to electromagnetic fields spanning over 6 orders of magnitude in
frequency. These features make Rydberg atoms highly desirable for developing
new quantum technologies. After giving a brief introduction to how the
properties of Rydberg atoms can be tuned, we give several examples of current
areas where the unique advantages of Rydberg atom systems are being exploited
to enable new applications in quantum computing, electromagnetic field sensing,
and quantum optics.
