Using Triplets in Quantum Cryptography

"[0]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 polarization 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 it's a problem in many other applications, such as
ground-to-satellite communications or even in chip-to-chip
communications, because it's 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 [1]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."