Progress made in secure quantum communications

Ars Technica : Quantum key distribution (QKD) uses the polarization of light and a reference frame mutually agreed on by a sender and receiver to encrypt communications and to reveal if messages have been intercepted. One difficulty in using QKD is that the polarization is easily disrupted, particularly by the fiber-optic cables used for data transfer. A new technique, which uses a simple device the size of a cell phone, appears to overcome that. A server sends a laser pulse, which the client device reduces to a single photon. The client device then randomly chooses one of the six possible polarization states for that photon, as well as a reference frame, and returns it to the server. That creates the key and allows communication to proceed as normal. If the photon’s polarization is in one of four states, then when the photon passes through the fiber, the fiber distorts the polarization in a way that is directly correlated with one of the remaining three states. When the photon reaches the server, the strength of the correlation is measurable, indicating the initial polarization. Anyone eavesdropping would further distort the polarization, which would reduce the correlation and reveal the tampering. The technique is still limited by range and by point-to-point communication with the server, but it does appear to be a step forward in ensuring secure quantum communications.