One of the strangest claims of quantum mechanics is that two particles can be "entangled"--inextricably linked at birth, even though the pair may have traveled to opposite sides of the cosmos. In last week's Physical Review Letters, physicists report having created the same eerie link among a trio of photons. The finding could provide a simple true-or-false test for quantum mechanics.
Physicists create entangled duos by firing a photon into a crystal, which splits it into two daughters. Their common parentage means the photons' properties are linked, and if the first one is horizontally polarized, the other has to be vertical, and so on. Such properties remain indeterminate until they are actually measured, according to quantum mechanics. So if a measurement on one photon finds that it has vertical polarization, its sibling somehow instantly "knows" that its own polarization is horizontal.
Physicists at the University of Innsbruck in Austria now have transformed two pairs of entangled photons into an entangled trio. First, they directed a high-frequency laser onto the photon-splitting crystal. Occasionally two photons cleave simultaneously, giving two entangled pairs. Each time this happens, three of the four photons pass through a system of polarization-sensitive beam splitters and other optical elements, which serve to confuse their parentage.
Next, each entangled trio heads toward three single-photon counters. In front of each one is a polarization filter and a shutter that is triggered by the fourth, unentangled photon. The orientation of the polarization filters is set so that, if the photons are entangled, all three detectors will fire simultaneously, says senior team member Harald Weinfurter.
Daniel Greenberger of the City University of New York, City College, says a "race" was on to create such three-photon states, and the Innsbruck team "did a phenomenal job." Greenberger says the findings will give physicists an alternative way to test the validity of quantum theory. By studying pairs of entangled photons, they have already tested the quantum prediction that a measurement on one of the pair is correlated with the outcome of a measurement on the other. But these tests have to be run over and over to be sure these "nonlocal" effects aren't due to chance, and purists find such statistical evidence dissatisfying. Three-photon entanglement means that the experiment in effect registers a photon in one detector if nonlocality is operating, but in a different one if it isn't. "This could be like a single shot test of quantum mechanics," says Vlatko Vedral of Britain's University of Oxford. Weinfurter says his team has already made a first stab at the measurements. The early news: "Quantum mechanics is correct."