Submission + - Theoretical Breakthrough For Quantum Cryptography (technologyreview.com)
KentuckyFC writes: Quantum cryptography uses the quantum properties of photons to guarantee perfect secrecy. But one of its lesser known limitations is that it only works if Alice and Bob are perfectly aligned so that they can carry out well-defined polarisation measurements on the photons as they arrive. Physicists say that Alice and Bob must share the same reference frame. That's OK if Alice and Bob are in their own ground-based labs but its a problem in many applications such as ground-to-satellite communications and even in chip-to-chip comms because its hard to keep chips still over distances of the order of the wavelength of light. Now a group of UK physicists have developed a way of doing quantum cryptography without sharing a reference frame. The trick is to use entangled triplets of photons, so-called qutrits, rather than entangled pairs. This solves the problem by embedding it in an extra abstract dimension, which is independent of space. So as long as both Alice and Bob know the way in which all these abstract dimensions are related, the third provides a reference against which measurements of the other two can be made. That allows Alice and Bob to make any measurements they need without having to agree ahead of time on a frame of reference. That could be an important advance enabling the widespread use of quantum cryptography.
