FOR many electronic engineers the quantum world is a nuisance. Make a device too small, for example, and electrons within it can simply vanish from one place and re-appear in another because their true location is quantumly indeterminate. Currents thus leak away, and signals are degraded.
Other people, though, see the quantum world as an electronic opportunity. Some of the weird things that go on down there afford the possibility of doing computing in a new and better way, and of sending messages that cannot be intercepted by an eavesdropper. A group of the latter persuasion gave a progress report to the annual meeting of the American Association for the Advancement of Science (AAAS) in Vancouver.
Some of their efforts, such as building a quantum computer capable of solving problems which stump today’s machines (for example, finding prime factors of numbers with hundreds of digits or trawling through large databases) are still years away—though, as we report in this week’s print edition, progress is being made. Others, however, especially in the field of cryptography, are already yielding practical results.
Quantum cryptography rests on the phenomenon of entanglement. When two particles interact quantum mechanically they become joined at the hip. A photon of light, for instance, has different types of polarisation, each taking one of two possible values, which can be labelled 1 and 0. If two security-conscious interlocutors, known conventionally as Alice and Bob, measure the same type of polarisation, the value they find is always the same. But if Alice measures one type of polarisation and Bob another, quantum mechanics dictates that the results will only match part of the time.
This can be used to detect Eve, the eavesdropper. All Alice and Bob need do is for each to pick a type of polarisation for successive photons at random and then compare which type they used. Since they are not comparing values, but only how they were measured, this can be done over an unsecured channel. They keep only those where they measured the same thing. If they then compare a subset of these and get a perfect match, they can use the remaining values to encrypt their future chats. Had Eve intercepted one of the entangled photons, she would have had to send another unentangled one on. As a result, Alice and Bob’s readings would not tally. They would throw away the key and ask for another.
Ever since Artur Ekert harnessed this phenomenon to distribute cryptographic keys in 1991, the field has bourgeoned. Dr Ekert, who splits his time between Oxford university and the Centre for Quantum Technologies (CQT) in Singapore, but who found the time to drop by the AAAS meeting, used pairs of light particles created by firing a laser at a material called a non-linear crystal.
In 2007 the Swiss authorities used a secure fibre-optic quantum network constructed along similar principles to Dr Ekert’s to transmit voting data during a general election. The same year a team led by Anton Zeilinger, from the University of Vienna, successfully dispatched an entangled photon 144km through the air from the Canary island of Tenerife to another, La Palma. Though in 2010 two teams showed that some early versions of theoretically impregnable quantum-cryptographic systems were not foolproof and could be hacked by attacking the weakest links, this only spurred the makers of these systems to eliminate the loopholes.
Should they succeed, there is another bizarre consequence of entanglement that makes such systems desirable. In theory, Alice and Bob can get a secure key even if the photons were sent by Eve herself. That is because Eve does not know which polarisation they will measure. It means that you can buy a quantum encoder from an adversary and use it without fear of being snooped on. Such “device-independent” cryptography is currently all the rage and Dr Ekert is exploring how to test if a given device, irrespective of provenance, generates secure keys.
Following the success of the Tenerife-La Palma link-up Dr Zeilinger, too, has grander ambitions. He is working together with the European Space Agency and the Chinese Academy of Sciences to take entanglement ballistic. The hope is to put the quantum bird in orbit by 2016 and to use it to beam secure keys down to seven ground stations sprinkled across Europe.
A rival proposal for intercontinental quantum-encrypted communication comes from Thomas Jennewein, from the Institute for Quantum Computing in Waterloo. He and his colleagues have been busy with the Quantum Encryption and Science Satellite (QEYSSAT), having enlisted Canada’s space agency to help get it off the ground in the same year Dr Zeilinger is aiming for.
Alex Ling, Dr Ekert’s colleague at CQT, has similar ideas. Later this year his team is planning to test the space-worthiness of their design by subjecting it to various trials, including the vibrations of a simulated launch, and, they hope, lofting it high into the atmosphere on a weather balloon to ensure it can operate remotely on the edge of space. If all goes to plan, the experiment will launch in 2013 on a small satellite designed by Dr Ling’s collaborators at Singapore’s Nanyang Technological University.
Satellites, of course, even in low-Earth orbit, are several hundred kilometres away. But this is less of a problem than might appear at first blush. When a beam of photons travels parallel to the Earth’s surface, as in the Canaries, it must pass through the densest part of the atmosphere, increasing the likelihood of bumping into a molecule of air, which destroys the entanglement. But point a beam up to a satellite (as in Dr Jennewein’s project) or down from one (as in Dr Zeilinger’s) and most of the distance is filled with a vacuum. (Dr Ling’s device will test entanglement in orbit, without exchanging entangled photons with ground stations.)
Besides offering the world’s paranoid a way to foil eavesdroppers, the quantum-equipped satellites will serve a more rarefied purpose. Comparing one of a pair of entangled photons moving through space at around 25,000kph with another on the ground will let physicists probe what happens when quantum entanglement meets the theory of relativity, which kicks in at such speeds, upending ordinary notions of space and time. Quantum meets Einstein. It should be quite a match.
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