Bose-Einstein condensation and quantum information have been two of the most exciting areas of physics over the past five years. Now Peter Zoller of the University of Innsbruck in Austria, and colleagues at Innsbruck and the University of Aarhus in Denmark, have shown that these two fields of research can be related. The results could have implications for computation, communication, atomic clocks and frequency standards.
In a Bose-Einstein condensate a gas of atoms is cooled until the de Broglie wavelength of the atoms exceeds the inter-atom spacing. If the atoms are bosons – that is, if they have a “spin” of 0, h/2p, 2(h/2p), 3(h/2p) and so on, where h is the Planck constant – they all collapse into the same quantum ground state. This gives the condensate many unusual quantum properties.
In quantum information the ability of a quantum particle – such as an atom or a photon – to be in two or more quantum states at the same time can be used to perform certain computational tasks much faster than would be possible with a conventional or classical computer. As the number of quantum particles increases, the quantum computer outperforms the classical computer by larger and larger factors. A key requirement for quantum computation is that the particles must be in an entangled state: in such a state the correlations between the particles are much stronger than any classical correlations. However, entangled states are difficult to prepare and maintain.
Zoller and co-workers show theoretically how a specially prepared laser pulse could be used to entangle all the atoms in a condensate. The pulse has the property that its area is p/2. The largest number of particles that has been entangled so far is four. However, the Innsbruck-Aarhus team claim that their technique could eventually be used to entangle any number of atoms.
