A Quantum Wedding of Quarks

Two particles separated by large distances can still be linked by the strange tendrils of quantum mechanics. Physicists have seen this ghostly behavior with photons and electrons in the lab, and now they've seen it happen with heftier matter as well. Scientists report in a recent issue of Physics Letters B that particles made of quarks--the building blocks of ordinary matter--can also be entangled.

The theory of quantum mechanics says that when two particles are created at the same point and time, their quantum properties remain linked, even if they flee the scene at the speed of light. If one particle, for example, flips its spin, its partner immediately flips its spin in the opposite direction, even if it is miles away. Einstein hated this "spooky action at a distance," but numerous experiments have shown that it happens (ScienceNOW, 10 December 1997), at least with photons and electrons.

A group of physicists working at the European laboratory CERN, near Geneva, Switzerland, thought they might be able to see the effect with particles called kaons which, like protons and neutrons, are made up of quarks. The team used CERN's accelerator to slam antiprotons into a volume of hydrogen gas, giving birth to pairs of kaons and antikaons (the kaon's antimatter twin), which sped off in different directions inside a large detector. While they traveled, each particle existed as a fuzzy quantum hybrid--part kaon, part antikaon--until a measurement determined its identity.

By identifying the debris produced when the particles plowed into the walls of the detector, the researchers could distinguish kaons from antikaons. After culling a few tens of these events, the team found that when one particle appeared as a kaon, it influenced whether the other was an antikaon. Even though the particles were several centimeters apart, their identities were still entwined. "We can at a given point in time predict with 100 percent probability the state of a particle we are not measuring by measuring the other particle of the pair," says team member Armand Muller of the French Commissariat for Atomic Energy.

"This is a very interesting demonstration [that these] weird quantum mechanical correlations at a distance do in fact appear," says CERN theorist John Ellis. Physicists will be able to study these kinds of entanglements in more detail when the Stanford Linear Accelerator in California begins pumping out pairs of the kaon's heavier siblings---the B-mesons--in the next few years.