Einstein and Quantum Information Theory

Abstract

The Rise of Quantum Information Theory In the first decade of the twenty-first century, quantum information theory is indisputably a 'hot topic'. A great deal of theoretical work is being performed in the main branches of the subject-quantum computation, quantum cryptography and quantum teleportation. Many different experimental techniques are being explored with the eventual aim of producing the first useful quantum computer, though it is recognised that this will almost certainly be decades away. In the other two main branches, though, considerable progress has been made; quantum tele-portation has been demonstrated in the laboratory, while quantum cryptography has reached the stage where it is capable of being applied to ensure the security, say, of the financial district of a large city; this will probably happen quite soon. The subject 1-9 came into prominence quite dramatically with two very important discoveries in 1994 and 1995. One 10 was an unexpected proof by Peter Shor that a quantum computer could perform tasks impossible for a classical computer. The second was a theoretical demonstration by Juan Cirac and Peter Zoller 11 of a method to construct a crucial quantum gate using an ion trap. One might say that the first discovery showed that quantum computation could be very useful, which had been far from clear until that point, and the second gave a boost in confidence that building a quantum computer would just be exceptionally difficult, not absolutely impossible. However there had been much steady if less spectacular development before that date. One might just conceivably go back as far as 1959, when Richard Feynman 12 made a famous speech titled 'There's plenty of room at the bottom' in which he called for the miniaturisation of physics and technology. As well as foretelling the coming of nanotechnology, 13 half a century later another hot topic, and indeed quite a controversial one, Feynman suggested that computers, which at that time still occupied very large rooms, should come down to an atomic scale, with the obvious corollary that they would have to be analysed using quantum not classical methods. We could go back to 5 years later, when Gordon Moore, co-founder of Intel, announced the famous 'Moore's Law', which suggested that computing power, as measured by the number of transistors on a silicon chip, was doubling, 273