Communications of the ACM
Satellite Quantum-Communication Circles Closer
Which way is up?
Orbital Sciences Corporation
Communications protected by quantum encryption systems offer unconditional security—if you know which way is up. A new quantum protocol is the first that promises to work independently of orientation, which will prove vital if quantum communications are ever to be sent via satellites.
Many quantum encryption protocols work by measuring the "up" or "down" spins on pairs of entangled photons shared between a sender, conventionally called Alice, and a receiver called Bob. The two members of an entangled pair of photons always have an opposite spin from one another. If an eavesdropper were to intercept one, the very act of reading it would affect the entangled pair in a detectable way.
The distance record for quantum encrypted communications between two sites on Earth is 144 kilometres. If quantum encryption is to go global the data must be sent via satellite links, and here the conventional method hits a snag: a spinning satellite's sense of up and down changes over time, making it harder to interpret a photon's spin and establish a key.
From New Scientist
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