Finding quantum entanglement in a crowd

Intricate quantum connections between microscopic particles almost certainly underlie some phenomena perceivable at human scales. Now, for the first time, physicists have measured these connections, known as quantum entanglement, between pairs of photons within a macroscopic beam of light. It’s a step toward understanding how the rules of quantum mechanics scale up to phenomena such as superconductivity that involve large numbers of particles.

In the experiment, described in a study to appear in Physical Review Letters, researchers filtered a specially prepared light beam to observe individual photons and chart the quantum links between them. “Nobody has looked at light in this manner before,” says Alexander Lvovsky, a quantum physicist at the University of Calgary. The physicists confirmed theoretical predictions that all the photons would exhibit some degree of entanglement and that pairs striking photon detectors at the same time would be most strongly entangled. The study may offer a guide for probing entanglement in future lab experiments that imitate complex large-scale processes.

From flocks of birds to schools of fish, nature is full of examples of complex phenomena that emerge from collective interactions between individuals. Vast herds of particles are not entirely analogous — photons don’t have brains or engage in social interaction — but physicists do face the similar challenge of projecting their knowledge of small-scale quantum effects to macroscopic phenomena.

The small-scale research involves examining quantum entanglement between pairs or small collections of particles. Studies have shown that determining the polarization or some other property of one photon reveals what the value of that property for the particle’s entangled partner would be if measured (SN: 11/20/10, p. 22). But studying entanglement between a couple of particles in the lab does not necessarily apply to larger collections of particles, just as examining two birds on the ground doesn’t provide much insight into a flying flock.

On the other hand, some physicists study the equivalent of flocks without getting to see the individual birds. These researchers investigate exotic phenomena such as superconductivity, the resistance-free transport of electrical current that, at least in some cases, is thought to result from entangled electrons. “Entanglement should be present in pretty much any situation with a lot of particles interacting with each other,” says Morgan Mitchell, a quantum physicist at the Institute of Photonic Sciences in Barcelona.

Ideally, Mitchell says, physicists would bridge the gap between those lines of research and study how, with enough entangled particles, phenomena such as superconductivity emerge. But that’s difficult to do. Superconductors, for example, are so densely packed with electrons that it would be difficult to measure a small subset.

So Mitchell and his team decided to work with a simpler macroscopic quantum system: a beam of squeezed light. This kind of light is not physically squeezed, but it is sent through a crystal or other device in a process that enables physicists to measure a particular property of the light — in this case, polarization — with extreme precision. Theory suggests that squeezing entangles the light’s photons.

After squeezing the light, Mitchell’s team filtered the beam and probed it photon by photon with polarization detectors. A click in a detector indicated the arrival of a photon with a particular polarization. Just as theory predicted, particles traveling together shared a tight quantum connection: Two photons arriving at the same time were most likely to have corresponding polarizations (for example, both horizontally polarized or both vertically polarized). Photon pairs that triggered the detectors tens of nanoseconds apart showed a lesser degree of entanglement — the polarization measurements matched up more often than would be expected for two random photons, but not as often as for the simultaneously arriving photons.

Timothy Ralph, who studies quantum optics at the University of Queensland in Brisbane, Australia, says the experiment is interesting, but he is skeptical that the results are relevant for deciphering phenomena beyond squeezed light.