  Photonic quantum technologies promise a revolution of the world of
information processing, from simulation and computing to communication and
sensing, thanks to the many advantages of exploiting single photons as quantum
information carriers. In this scenario, single-photon detectors play a key
role. On the one hand, superconducting nanowire single-photon detectors
(SNSPDs) are able to provide remarkable performance on a broad spectral range,
but their applicability is often limited by the need of cryogenic operating
temperatures. On the other hand, single-photon avalanche diodes (SPADs)
overcome the intrinsic limitations of SNSPDs by providing a valid alternative
at room temperature or slightly below. In this paper, we review the fundamental
principles of the SPAD operation and we provide a thorough discussion of the
recent progress made in this field, comparing the performance of these devices
with the requirements of the quantum photonics applications. In the end, we
conclude with our vision of the future by summarizing prospects and unbeaten
paths that can open new perspectives in the field of photonic quantum
information processing.
