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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 quantumelectrodynamics. We define photon "duration" as the standard deviation of thephoton arrival time; we find its asymptotics for long distances and then obtainthe main result of the paper: the linear dependence of photon duration on thedistance. This effect puts the limit to joint increasing of the photon flux andthe distance from the source and it has important consequences both for quantuminformation technologies and quantum foundations. Once quantum communicationdevelops into a real technology, it would be appealing to the engineers toincrease both the photon flux and the distance. And here our "photonflux/distance effect" has to be taken into account (at least if successivelyemitted photons are considered as independent). This effect also has to betaken into account in a loophole free test of Bell's type -- to close jointlythe detection and locality loopholes.

Photon flux and distance from the source: consequences for quantum foundations and technologies