  The paper explores the fundamental physical principles of quantum mechanics
(in fact, quantum field theory) which limit the bit rate for long distances.
Propagation of photons in optical fibers is modeled using methods of quantum
electrodynamics. We define photon "duration" as the standard deviation of the
photon arrival time; we find its asymptotics for long distances and then obtain
the main result of the paper: the linear dependence of photon duration on the
distance. This effect puts the limit to joint increasing of the photon flux and
the distance from the source and it has important consequences both for quantum
information technologies and quantum foundations. Once quantum communication
develops into a real technology, it would be appealing to the engineers to
increase both the photon flux and the distance. And here our "photon
flux/distance effect" has to be taken into account (at least if successively
emitted photons are considered as independent). This effect also has to be
taken into account in a loophole free test of Bell's type -- to close jointly
the detection and locality loopholes.
