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Quantum

Quantum

Quantum information

01 Mar 1998

There is more to information than a string of ones and zeroes – the ability of “quantum bits” to be in two states at the same time could revolutionize information technology

In the mid-1930s two influential but seemingly unrelated papers were published. In 1935 Einstein, Podolsky and Rosen proposed the famous EPR paradox that has come to symbolize the mysteries of quantum mechanics. Two years later, Alan Turing introduced the universal Turing machine in an enigmatically titled paper, On computable numbers, and laid the foundations of the computer industry – one of the biggest industries in the world today.

Although quantum physics is essential to understand the operation of transistors and other solid-state devices in computers, computation itself has remained a resolutely classical process. Indeed it seems only natural that computation and quantum theory should be kept as far apart as possible – surely the uncertainty associated with quantum theory is anathema to the reliability expected from computers?

Wrong. In 1985 David Deutsch introduced the universal quantum computer and showed that quantum theory can actually allow computers to do more rather than less. The ability of particles to be in a superposition of more than one quantum state naturally introduces a form of parallelism that can, in principle, perform some traditional computing tasks faster than is possible with classical computers. Moreover, quantum computers are capable of other tasks that are not conceivable with their classical counterparts. Similar breakthroughs in cryptography and communication followed.

This quantum information revolution is described in this special issue by some of the physicists working at the forefront of the field. Starting with the most fundamental of quantum properties – single-particle quantum interference in two-path experiments – they show how theorists and experimentalists are tackling problems that go to the very foundations of quantum theory and, at the same time, offer the promise of far-reaching applications.

Anton Zeilinger of the University of Innsbruck introduces the fundamentals of quantum information – quantum bits, entangled states, Bell-state measurements and so forth – and outlines what is possible with quantum communication. The most ambitious scheme, quantum teleportation, has recently been demonstrated with photons and looks to be possible with atoms. The first application of teleportation is, however, likely to be in a quantum computer or communication system rather than anything more cinematic.

Cryptography is the most mature area of quantum information and has now been demonstrated over distances of ten of kilometres (and under Lake Geneva!). Once just the concern of special agents and generals, cryptography now plays an important role in transactions over the Internet. On page 41 of the March issue of Physics World Wolfgang Tittel, Grégoire Ribordy and Nicolas Gisin of the University of Geneva explain how the very properties of quantum theory that so puzzled Einstein et al . can be used to send messages with complete security. A common theme in communication and cryptography is that many applications work best when classical and quantum methods are used in tandem – which is why Alice and Bob, the two central characters in quantum information, are using the telephone in the illustration.

Quantum computers are a more distant proposition, but the first logic gates have been demonstrated in the laboratory and progress is being made on three fronts: trapped ions, photons in cavities and nuclear magnetic resonance experiments. Recent years have also seen significant progress in the development of new algorithms for quantum computers. David Deutsch and Artur Ekert of the University of Oxford present a progress report on page 47 and also delve into some of the deeper implications of quantum theories of information.

Of course it isn’t all plain sailing. Quantum states are notoriously delicate and interactions with the environment can cause a pure quantum state to evolve into a mixture of states. This causes the quantum bit to lose two of its key properties: interference and entanglement. This process, known as decoherence, is the biggest obstacle to quantum computation, as David DiVincenzo IBM and Barbara Terhal of the University of Amsterdam explain on page 53. However, theorists have developed schemes to correct the errors introduced by decoherence and also any inaccuracies generated by the quantum logic gates themselves.

Collaboration is a hallmark of the ever-growing quantum information community. The European Union, for example, is funding a network of eight groups working on the Physics of Quantum Information, while the Quantum Information and Computation collaboration in the US has been awarded some $5 million over five years by the Department of Defense.

We live in an information age that was founded on the applications of basic physics and in which computer power continues to grow exponentially as the feature sizes in microelectronic circuits become ever smaller. Quantum effects can be seen as a threat or an opportunity to this growth. The quantum information technologies described in this issue may have a very long way to go before they rival the sophistication found in their classical counterparts but, as Deutsch and Ekert conclude, “there is potential here for truly revolutionary innovation”.

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