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Exchanging keys in the Canaries

26 October 2005

"What had all this to do with quantum computation, I wondered?" Cherry Lewis explores the unfamiliar world of quantum computation.

Rarity's computer, and many others like it, represent the culmination of years of technological advancement that began with the ideas of Charles Babbage (1791-1871), early pioneer of the computer. Surprisingly, however, although computers have become more compact and considerably faster in performing their task, they are fundamentally no different from their ancestors because the task remains the same: to manipulate and interpret an encoding of binary bits into a useful computational result.

A 'bit' is a fundamental unit of information, classically represented as 0 or 1 in your computer. A file stored on your hard drive, for example, is described by a string of zeros and ones. But whereas your computer obeys the well-understood laws of classical physics, a quantum computer adheres to the laws of quantum mechanics, which differ radically from the laws of classical physics. In a quantum computer the fundamental unit of information is called a 'quantum bit' or qubit.

A qubit can exist not only in a state corresponding to the logical state 0 or 1 as in a classical bit, but also in states corresponding to a blend or superposition of these classical states. In other words, a qubit can exist as a zero, a one, or simultaneously as both 0 and 1. At Bristol, Rarity's group uses single photons (particles of light) as qubits - other groups use single electrons or single atoms. Interactions between those single quantum particles can be used to construct the fundamental 'gates' needed to build a quantum computer. Each gate operation 'entangles' two particles (qubits) and by repeating this operation one can build a multi-particle entangled state. Entanglement is a state of two or more quantum particles like photons, in which many of their physical properties are strongly correlated.

Other than mathematicians, who would be interested in very big numbers?

The entangled particles share information in a form which cannot be accessed in any experiment performed on either of the particles alone. This happens no matter how far apart the particles may be at the time. As the number of qubits increases, the number of superpositions increases dramatically, providing a vast number of testable solutions. Using suitable gate arrangements (ie a quantum computer), the correct solutions can then be extracted from the tests.

Richard Feynman was among the first to show how a quantum system could be used to do computations and eventually it was realised that a quantum computer would have capabilities far beyond those of any traditional computer. In particular, it could be used to factorise huge numbers extremely rapidly. It could do in seconds what it would take a classical computer many years to complete. With this breakthrough, quantum computing became transformed from an academic curiosity into something of world interest. But why? Other than mathematicians, who would be interested in very big numbers? And what has all this got to do with a map of the Canaries?

What we were trying to achieve on the Canaries was prove that you can exchange keys over 150 kilometres, which will be a world record," Rarity explained. "The previous world record, held by our group, stands at 23 kilometres in free space (as opposed to down a fibre). We were doing a feasibility study trying to prove that in principle a ground to satellite key exchange is possible. We were doing it in the Canaries because Tenerife is home to the European Space Agency's Optical Ground Station where they do space-to-ground optical communications experiments."

"Exchange keys?" I ask. "What kind of keys?" "If a financial institution, for example, wants to send a secret to someone else, they could lock the secret in a 'box' before sending it". replied Rarity. "If the person receiving the box has the right key he can open it and read the secret. With digital communications the key is a string of 'bits' with which the secret is encoded and an identical bit-string (key) is needed to decode the message. To distribute keys we could send couriers on motorbikes with discs of information (keys) in briefcases handcuffed to their wrists. However, this method requires that we trust the courier. In our work we distribute the keys securely, using quantum means. We do this by using a very simple quantum computer which generates the same random bit-string in two places at once. Which brings us back to entanglement."

Entanglement is crucial for long-distance quantum key distribution, which uses entangled pairs of photons to encode the qubits. It relies on the fact that the information defining the key only 'comes into being' after measurements performed by 'Alice', the sender, and 'Bob', the receiver. The photons are distributed so that Alice and Bob each end up with one photon from each entangled pair and consequently each has a copy of the key. In Rarity's experiment in the Canaries, Alice and Bob are in fact two observatories located on two different islands, separated by 150 kilometres. The success of this experiment will demonstrate that key exchange to a satellite could be possible in future - their ultimate objective being to create a global key distribution system.

Of course, Rarity cannot send a whole message yet. All he is trying to do at this stage is establish an identical bit-string between the sender and the receiver over a relatively long distance. But the long-term implications for this technology are enormous.

A brand-new Nanoscience and Quantum Information Centre is due to open in spring 2007

Other work at Bristol is focused on low-cost, secure, key-exchange systems built from off-the-shelf components for consumer applications. For example, it could protect every transaction made at an ATM, and be a candidate to replace chip and pin - which is one of the reasons why the University is heavily investing in quantum information and nanoscience.

A brand-new Nanoscience and Quantum Information Centre is due to open in spring 2007. The basement will house 12 exceptionally 'low-noise' laboratories - acoustically, vibrationally, electrically and electro-magnetically - and will probably be the quietest experimental space in the world. The architectural design of the building also incorporates features and spaces to stimulate interdisciplinary interactions and innovation, from which new research directions will emerge.

A number of senior appointments have recently been made in the areas of bionano-technology, nanobiophysics, chemistry and biochemistry and many world leaders in those fields are already queuing up to spend time in this novel, exciting space that will be buzzing with excitement about the unknown and unexpected.

John Rarity/Department of Electrical and Electronic Engineering

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