Satellite quantum communication circles closer
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.
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.
If information is encoded in the circular aspect of photon entanglement, it is possible for Alice and Bob to establish a quantum encryption key even if they lack a shared physical frame of reference.
The relatively simple system still leaves the problem of detecting eavesdroppers, but Anthony Laing, one of the team in Bristol, says that there are ways around that. Although the lack of a shared frame of reference precludes the use of conventional up/down spin-measurements to establish an encryption key, the sender and receiver can still measure them.
And a combination of such measurements on the string of photons used to encrypt the communication channel is enough to detect an eavesdropper. The mathematics is tricky, but “physics provides an inherent way to do it”, Laing claims.
Norbert Lütkenhaus at the University of Waterloo in Ontario, Canada, says that the new protocol is likely to be a very useful tool in developing quantum information technology.
But he adds that the biggest issues in Earth-satellite communication are the fact that photons tend to get lost over long distances, reducing the efficiency of the communication, and that photon detectors sometimes register detections when no photons are present, which can cause errors in the data.
“There are other barriers for Earth-satellite quantum communication that may be more challenging,” agrees Hoi-Kwong Lo at the University of Toronto in Canada.
Journal reference: Physical Review A, DOI: 10.1103/PhysRevA.82.012304