Entanglement allows particles to have a much closer relationship than is possible in classical physics: if two particles are entangled, we can know the state of one particle by measuring the state of the other. For example, two particles can be entangled such that the spin of one particle is always "up" when the spin of the other is "down", and vice versa. An additional feature of quantum mechanics is that the particle can exist in a superposition of both these states at the same time. By taking advantage of such quantum phenomena, a quantum computer could, in principle, outperform a classical computer for certain tasks.

Using lasers and ultra-cold electromagnetic traps, the NIST scientists entangled six beryllium ions so that all their nuclei were collectively spinning in both clockwise and anticlockwise directions at the same time (Nature 438 639). These states are also known as "cat" states after Erwin Schrödinger's famous thought experiment in which a cat was somehow both alive and dead at the same time. Using similar techniques, the Austrian scientists entangled eight calcium ions that were more robust because they were stable even if some particles were removed (Nature 438 643).

The new results break the previous record of five entangled photons achieved last year. Moreover, the entangled states can be produced "on demand" and made available for further tasks without being destroyed -- something that has never been done before. The number of particles entangled could be increased even further, leading the way to large-scale quantum computers.

Cat states could be used to correct errors in quantum computation and so make fault-tolerant quantum computers. These entangled states are also more sensitive to decoherence -- the transition from quantum to classical behaviour that occurs when the particles interact with their environment -- than other types of superpositions. They could therefore be useful in applications such as precision spectroscopy and quantum cryptography, which allows data to be transmitted with complete security.