  Quantum key distribution (QKD) is a concept of secret key exchange supported
by fundamentals of quantum physics. Its perfect realization offers
unconditional key security, however, known practical schemes are potentially
vulnerable if the quantum channel loss exceeds a certain realization-specific
bound. This discrepancy is caused by the fact that any practical photon source
has a non-zero probability of emitting two or more photons at a time, while
theory needs exactly one. We report an essentially different QKD scheme based
on both quantum physics and theory of relativity. It works flawlessly with
practical photon sources at arbitrary large channel loss. Our scheme is
naturally tailored for free-space optical channels, and may be used in
ground-to-satellite communications, where losses are prohibitively large and
unpredictable for conventional QKD.
