Quantum leap for entanglement
Mar 17, 2000
Entanglement is one of the most mysterious and fundamental properties of quantum mechanics. When two or more particles are "entangled", the wavefunction describing them cannot be factorized into a product of single-particle wavefunctions. This means that a measurement on one particle will immediately influence the state of the other particles in the entangled system. A group of physicists in the US has now "entangled" four particles for the first time (Nature 404 256).
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.