Whereas quantum cryptography ensures security by virtue of complete indistinguishability of nonorthogonal quantum states, attenuation in quantum communication channels and unavailability of single-photon sources present major problems. Since the restrictions imposed by non-relativistic quantum mechanics and used to formulate key distribution protocols are largely exhausted, new principle is required. The fundamental relativistic causality principle in quantum cryptography can be used to propose a new approach to ensuring unconditional security of quantum cryptography that eliminates these difficulties. Photons represent truly relativistic massless particles (the massless quantized field states) which travel at a maximum permissible speed. That is why in the development and realization of quantum cryptography in open space it would be unnatural to take no advantage of the additional possibilities offered by nature.
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