Satellite quantum-communication circles

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.

Clockwise corkscrew
A team at the University of Bristol in the UK has invented a protocol independent of orientation that exploits the fact that photons can have an entangled circular polarisation as well as entangled spin.

Circularly polarised light can be imagined to corkscrew either clockwise or anticlockwise along its axis of travel. The two forms are readily identified regardless of the receiver's orientation.

Some modern 3D-movie projector systems already polarise light in this way to differentiate the two images used to form the 3D illusion. Doing so ensures that a cinemagoer wearing polarised glasses sees the 3D effect even if they tilt their head.

A 3D system that uses horizontally and vertically polarised light to differentiate the two images only works if the viewer's glasses are orientated in the same up-and-down direction as the theatre projector – in other words, only if the glasses and the projector share the same physical frame of reference.