It is possible to transmit secret keys using individual photons with different phases or polarizations. Due to the uncertainty principle, it is not possible for a third party to intercept and measure the photons without altering the quantum state of the photons, thus revealing the snooping. The scheme originally proposed by C. H. Bennett and Gilles Brassard in 1984 used photons with polarizations of 0°, 45°, 90°, and 135°. The recipient then chooses between measuring horizontal or perpendicular polarization. Practical transmission schemes have been devised which work over commercial fiber optic cables of length up to 23 km.
References
Basdevant, J.-L. and Dalibard, J. "Quantum Cryptography." Ch. 14 in The Quantum Mechanics Solver: How to Apply Quantum Theory to Modern Physics. Berlin: Springer-Verlag, pp. 99-107, 2000.
Peterson, I. "Bits of Uncertainty: Blazing a Quantum Trail to Absolute Secrecy." Sci. News 140, 90-92, Feb. 10, 1996.
