Quantum entanglement, the process of linking atoms so that a change to one alters others, has taken a huge leap forward. Even by the standards of quantum's strange laws, it is a feat worthy of a subatomic Helen of Troy for a single photon to entangle almost 3,000 atoms.

Quantum entanglement is a phenomenon that can occur in the puzzling world of the very small, where particles can have their fates locked together. According to theoretical models, if one of these particles is affected in some way, for example by having its spin altered, the others will immediately experience a matching change, even if separated by a great distance from the altered particle.

Einstein famously mocked the idea, but experimental evidence has supported what was once pure theory. Particles have been entangled while separated by large distances, and the phenomenon has even been used to make art.

Professor Vladan Vuletić of MIT is on a quest to entangle large groups of atoms. The previous record was 170, so Vuletić's announcement in Nature that his team have entangled almost 3,000 rubidium atoms marked a big increase. However, the work has two additional features of interest.

Previous attempts to entangle large numbers of atoms have succeeded in linking the spins of a small proportion; the previous record-holders entangled the 170 in a grouping of 2,300 atoms. Vuletić worked with 3,100 atoms and was able to show more than 90% became entangled. “Our results represent the first (to our knowledge) experimental verification of the mutual entanglement shared by virtually all atoms in an ensemble that contains more than a few particles,” the authors write.

More amazingly still, Vuletić achieved the feat with a single photon from a weak laser. Indeed, Vuletić says more light disrupts the system.

To get just one packet of light to affect so many atoms, Vuletić cooled his rubidium cloud to fifty millionths of a degree above absolute zero before trapping it in an optical cavity and pulsing an infrared laser into the atoms. The light was repeatedly reflected through the cloud by mirrors on either side before escaping to be captured by a detector.

Vuletić checked the photons as they left the cloud, looking for those whose electric fields had changed direction compared to prior entry. "When we detect such a photon,” Vuletić says, “We know that must have been caused by the atomic ensemble, and surprisingly enough, that detection generates a very strongly entangled state of the atoms."

Credit: McConnell et al. Vertically polarized light shifts to horizontal when it has entangled almost an entire cloud of atoms.

The fact that a photon had its polarization shifted from vertical all the way to horizontal is indicative that almost the entire cloud had become entangled. The team were able to achieve this on a “quasi-deterministic” basis.

Entangling large numbers of atoms could lead to more accurate clocks, since atomic clocks' precision is proportional to the square root of the number of atoms, whereas for entangled atoms the precision rises proportionally to the number of atoms. It is also necessary for quantum information processing.