A method and apparatus for secure distribution of cryptographic key information based on quantum cryptography is described. The apparatus incorporates or is used with a transmitter comprising a source of pairs of dim, depolarized light pulses together with a phase modulator and random number generator that are used to encode the pulse pairs with the binary key information by changing the relative phases of the pulses of some pairs. The apparatus incorporates a receiver comprising a polarization beam splitter, and a pair of interferometers and optical detectors. The invention overcomes problems associated with polarization evolution in quantum cryptography systems that incorporate a non-polarization-preserving optical channel (e.g. standard optical fiber). In addition the invention removes the need for an active random number generator and phase modulator at the receiver, because the polarization beam splitter acts as if it were a random router, thereby achieving significant reduction of the cost and complexity of the apparatus.