  Quantum key distribution (QKD) promises information theoretic secure key as
long as the device performs as assumed in the theoretical model. One of the
assumptions is an absence of information leakage about individual photon
detection outcomes of the receiver unit. Here we investigate the information
leakage from a QKD receiver due to photon emission caused by detection events
in single-photon detectors (backflash). We test commercial silicon avalanche
photodiodes and a photomultiplier tube, and find that the former emit
backflashes. We study the spectral, timing and polarization characteristics of
these backflash photons. We experimentally demonstrate on a free-space QKD
receiver that an eavesdropper can distinguish which detector has clicked inside
it, and thus acquire secret information. A set of countermeasures both in
theory and on the physical devices are discussed.
