The problem with messages is that you can't be sure who might pick them up at the other end. But a new scheme for a quantum communications ensures only a recipient in exactly the right location can read their message.


If banks were to use the new scheme they could send messages that could not be read outside a certain room, confident that only an eavesdropper who had smuggled a quantum receiver inside that room could intercept it, says its inventor Robert Malaney at the University of New South Wales in Sydney, Australia.

Malaney's system builds on existing quantum communications technology, like that used in a Swiss election in 2007, where a secure link is established by sharing a secret key encoded using entangled photons.


One of each photon pair is sent to the recipient through an optical fibre, and any attempt to intercept it would alter the behaviour of its quantum twin in a detectable way.

A conventional wireless transmission is then used to send the "key" needed to make sense of the quantum data and establish a secure communications channel.

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Location, location, location

Malaney's new approach becomes different after that wireless key is sent. This time the originator of the link, Bank A, sends the key in three separate chunks, each via one of three radio masts in known locations surrounding the recipient, Bank B.


Bank A coordinates the transmission of each chunk so that they all reach Bank B at precisely the same time. Bank B then uses the key to decode the photons and instantly bounce back confirmation via the same three masts to establish the secure link.

Bank A can then use triangulation to verify Bank B's location by measuring the time it took for them to send the confirmation, because signals travel at a known speed through the air.


Crime-proof

"It should be possible to know where Bank B is to within a few metres," says Malaney, even ignoring the few hundred nanoseconds that photon-detecting equipment takes to decode qubits – bits of quantum data.


Any attempt to masquerade as Bank B while not at the right location is doomed, both because of simple geometry and the more complex intricacies of quantum entanglement.

Even if a lone eavesdropper has hijacked the original entangled photons, it is physically impossible for the miscreant to be closer to all three masts simultaneously than Bank B is. So there will be a slight delay in receiving at least one chunk of the wireless key and a detectable hold-up in sending the confirmation back to Bank A.


Although a team of three attackers could take up positions between Bank B and each mast to intercept the signals, they still wouldn't respond promptly enough to fool Bank A. That's because the entangled photons cannot be copied. At least two of the attackers would have to divert their chunk of the wireless key to a third location, containing the photons, to make sense of them, again adding a detectable delay.

Ready to go

Vadim Makarov at the Norwegian University of Science and Technology in Trondheim says it is exciting to hear about the discovery of a new facet to quantum communications, because "this does not happen very often".


The new idea should be feasible using existing techniques, says Makorov, but the limitations of photon detectors and optical fibre might lead to errors in confirming a recipient's location, he adds.

Malaney says such problems are not serious. "It would only make the error a little bigger: instead of knowing where Bank B is to within 2 metres, it might be within 20 metres."


Journal reference: Physical Review A, in press


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Image Credit Phil Strahl