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Physicists Test Quantum Cryptography For Handheld Mobile Devices

Quantum cryptography has only ever been possible between places equipped like quantum optics laboratories. Now physicists have worked out how to do it with handheld mobile devices

Quantum cryptography uses the laws of physics to guarantee the secrecy of messages sent from one location to another. It is one of the few quantum technologies that is become mature enough to make the leap from the laboratory to the commercial world.


So governments, the military and commercial organisations such as banks are all interested having this kind of perfect secrecy. And indeed a number of companies have cropped up in the last 10 years to sell the service.

One problem is that quantum cryptography is only possible between places that have the kind of gear usually only found in quantum optics laboratories. It generally requires that both the transmitter and receiver have a source of single photons, a way of controlling and modifying individual photons and superconducting photon detectors.

What’s more, the equipment at each end has to be carefully aligned so that both parties are able to detect the polarisation of the photons they send. And if there is any noise that changes the polarisation of the photons, the cryptography simply doesn’t work.

That scuppers any possibility of using quantum cryptography with handheld devices which would obviously be difficult to align.

Today, Jeremy O’Brien at the University of Bristol and a few pals reveal a way to solve this problem which they say could make quantum cryptography available in handheld machines.

In the new technique, only one of the parties, Alice say, needs to have the quantum optics gear such as a source of photons and so on. Alice creates the photons and then sends them down an ordinary optical fibre to Bob, the other party.

Bob, merely modifies the photons to encode them with information before sending them back to Alice. This dramatically simplifies the equipment Bob requires, allowing it to fit in a handheld device.

O’Brien and co also use a robust form of quantum key distribution that does not require Alice and Bob to align their equipment before making a measurement.

Instead Alice and Bob make measurements in random directions and then publish the list of directions for anyone to see. Only those measurements that happened to be aligned contribute to the code.

As long as the alignment between Alice and Bob’s devices changes slowly compared to the rate at which photons pass back and forth between them, this mechanism works pretty well. O’Brien and co call the new technique “reference frame independent quantum key distribution” or rfiQKD.

O’Brien and co compare the new technique to a conventional quantum cryptography protocol known as BB84 and it comes off well. When the team deliberately add noise to simulate a change in alignment, the BB84 protocol immediately stops working.

By contrast, rfiQKD is much more robust. It works when noise levels are high and even when it becomes overwhelmed, it begins running again as soon as noise levels drops, unlike the BB84 protocol. “We demonstrated the automatic, passive recovery capability of our system after periods of rapid and substantial noise that force a protocol failure,” say the team.

The end result is a system that has the potential to bring quantum cryptography to a much wider range of applications than has been possible before now, say O’Brien and co. “The results significantly broaden the operating potential for QKD outside of the laboratory and pave the way for quantum enhanced security for the general public with handheld mobile devices.”

Ref: arxiv.org/abs/1308.3436 : Reference Frame Independent Quantum Key Distribution Server With Telecom Tether For On-Chip Client

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