Four ions mingle in quantum chorus.

A remarkable new quartet makes its debut.

A Colorado research team has coaxed an unprecedented number of microscopic particles--specifically, four beryllium ions--to share in a strange harmony known as quantum entanglement. Moreover, the scientists did it in a way that promises to make entanglements of larger numbers of particles not just possible but attainable on demand, they say.

"By this method, we believe we will be able to [entangle] essentially any number, maybe up to hundreds, of atoms," says Cass Sackett of the National Institute of Standards and Technology in Boulder, Colo. He and his colleagues describe their experiments in the March 16 NATURE.

Among entangled particles, certain properties such as momentum become subtly linked. Changing the property in one particle then instantly affects the same property in its companions, even when far apart (SN: 6/14/97, p. 367; 11/20/99, p. 334).

With the new method, called push-button entanglement and based on the work of Danish theorists, scientists order up entanglements at will. Previous methods have required them to wait for particles to occasionally, randomly entangle in response to lab manipulations, Sackett says.

To strike their four-particle chord, the researchers chilled the ions to about 200 microkelvins and trapped them with a spinning electric field. Then, they zapped the ions once with pulses from crisscrossed laser beams tuned to carefully chosen frequencies. The pulses correlated electron spins within the ions, creating the desired entangled state.

Scientists say the experiments bring closer to reality a powerful type of computer that would exploit the bizarre rules of quantum mechanics (SN: 4/3/99, p. 220). In an accompanying commentary, Rainer Blatt of the University of Innsbruck in Austria calls the new method "an important step for the emerging field of quantum information processing."