Quantum public key distribution reinvented

A recent article [1] in this journal rediscovers and gives a good elementary exposition of quantum public key distribution (QPKD), a techniqu e which we introduced [2] and analyzed in detail [3,4] a few years ago . Publi c key distribution is the distribution of secret information, between users wh o share no secret information initially, over a channel susceptible to eavesdrop ping; and QPKD achieves it by using a channel of a particular physical sort , subject to fundamental limitations imposed by the uncertainty principle , rather than by usual cryptographic means, which depend on mathematical assumptions such as the intractability of factoring large numbers . More specifically, QPKD begins with the transmission of physical token s (e .g . photons) prepared in a random sequence of quantum states belongin g to two mutually conjugate bases (e .g . one basis comprising the two rectilinear polarizations 0 and 90 degrees, the other the two diagonal polarization s 45 and 135 degrees . Circular polarizations would serve as well) . The notion of conjugate bases, introduced by Wiesner [5] is central to quantu m cryptography . Roughly, two bases are conjugate if they represent propertie s of a quantum system that are incompatible in the sense that any measurement of one property randomizes the value of the other . For example, an y measurement of a photon's rectilinear polarization randomizes its diagona l polarization, and vice versa . Figure 1, taken from our paper [3] shows a protocol essentially the sam e as that rediscovered by Wiedemann, allowing two legitimate users, by a pub lic discussion overheard by the eavesdropper, to test their quantum trans -