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.

Circular argument

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 , 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.

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 at the University of Toronto in Canada.

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The Casimir effect tends to force together two parallel conducting plates when they are a few micrometres apart or less. It arises because of the quantum electromagnetic fluctuations that always occur in a vacuum. The close proximity of the two plates constrains the fluctuations in the gap between them. This makes the fluctuation between the plates weaker than those in the surrounding space, so the plates are pushed together.

The effect was named after Dutch physicist , who predicted its existence in 1948. Nanotechnologists are keen to tame it, because it gums up their nanoscopic machines.

A European research team has now done just that, using a material already used in rewritable CDs and Blu-ray discs. AIST, an alloy of silver, indium, antimony and tellurium, reversibly switches from a crystalline to an amorphous state when heated by a laser, allowing data to be written and rewritten onto a disc.

Golden ball

The team deposited AIST on an aluminium-coated silicon wafer and held it between 40 and 120 nanometres from a gold sphere in an ultra-high vacuum. When AIST was in an amorphous form, the Casimir force measured about 100 piconewtons, but it increased by 20 to 25 per cent when the AIST was in its crystalline form.

This is because the crystalline phase is more reflective, so it confines the electromagnetic fluctuations more effectively and so increases the Casimir force.

Group member at the University of Groningen in the Netherlands says this phenomenon could be used to build a new type of low-power nanoswitch. The switch would be physically moved by altering the state of the AIST, and so changing the strength of the Casimir force.

“The state remains stable even when power is turned off, which is a unique feature,” Palasantzas says – unlike existing nanoswitches, such as those used to switch the transmission frequency in mobile phones.

of the Free University in Amsterdam, the Netherlands, who was not involved in the study, says it is “an important contribution”, though he warns that in real applications, the build-up of electric charge between moving parts may have more of an effect on the nanoswitch than the Casimir force. “If one finds an easy way to control the [electric charge], then the Casimir force becomes indeed interesting – but that is quite a challenge,” he says.

Palasantzas, however, predicts that ways will be found to minimise the electrostatic build-up, so that the Casimir force exceeds the electrostatic force for distances below about 100 nanometres.

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