Two particles that have never interacted with each other can be forced to become "entangled"--a peculiar quantum condition that inextricably weds particles so that nothing can be said about either particle individually. A report in the 4 May issue of Physical Review Letters provides the first experimental evidence of "entanglement swapping," where two entangled photons transfer their entanglement to a completely unrelated, and unentangled, pair.
Entangled particles are so intimately connected that any measurement on one particle immediately determines what happens to the other, even if they are kilometers apart (ScienceNOW, 10 December, 1997 ). Entanglement usually results when two particles are created simultaneously or are made to interact with each other in some way. But quantum theory allows two particles to become entangled even without any direct interaction. That's a bizarre claim even by the standards of quantum mechanics, so Anton Zeilinger and his group at the University of Innsbruck, Austria, wanted proof.
The team sent two pulses of ultraviolet light through a crystal, which spawned first one and then another independent set of entangled photons, labeled 1,2 and 3,4. Then they funneled one photon from each pair--2 and 3--into a device called a beam splitter. The interference inside the splitter entangled photons 2 and 3 and randomly "projected" them into one of four equally likely quantum states. Meanwhile, photons 1 and 4 had parted from their mates.
If entanglement swapping works, photons 1 and 4 should also become entangled--even though they have never met--and project into the same quantum state as photons 2 and 3. By measuring the polarization--the direction of electric field oscillation--of the photons involved, the team confirmed that photons 1 and 4 had indeed become entangled.
"It is definitively a beautiful experiment," says Nicolas Gisin, a physicist at the University of Geneva in Switzerland, who last year used telephone wires in Geneva to show that photons remain entangled over distances of tens of kilometers. The experiment is yet another confirmation that quantum mechanics is robust, he says, even though it gives us a "very strange world-view."