Most methods for generating entangled states rely on selecting the entangled pairs, for instance, from a large number of other non-entangled particles. However, last year Klaus Mølmer and Anders Sørenson from the University of Aarhus in Denmark proposed a method for entangling ions confined in an ion trap "to order" with a single laser pulse. Now Chris Monroe of the US National Institute of Standards and Technology in Boulder, Colorado, and co-workers have used this technique to entangle four beryllium ions.

When the ions are placed in a magnetic field, their ground states split into two "hyperfine" levels that can be considered as "spin-up" and "spin-down". By applying a laser pulse of the correct frequency and duration, it is possible to create an N-particle entangled state in which the particles are all spin-up or all spin-down. Monroe and colleagues created two- and four-particle entangled states and it should be possible to extend the technique to higher values of N, where N is an even integer. A reliable method for generating and controlling entangled states is essential for the construction of a "quantum computer" that could, in principle, outperform a classical computer by many orders of magnitude. Quantum computers will rely on fundamental quantum properties such as entanglement and superposition - the ability of quantum particles to be in two or more quantum states at the same time - for their operation.