Three photons entangled, not just two

A process that works once in every quadrillion attempts has produced trios of entangled photons.

Particles with properties that are entangled share an extraordinary connection: Measuring the properties for one particle immediately determines the properties for the other, no matter the distance between particles (SN: 11/20/10, p. 22).

Physicists routinely create entangled pairs of photons. But confirming the entanglement of more than two photons can be done only after measuring the particles, which destroys their delicate entanglement state.

Now physicist Deny Hamel of the University of Waterloo in Ontario and colleagues have removed this limitation by using crystals that split one photon into two. The photons that emerge are entangled by their polarization, the horizontal or vertical orientation of the light’s vibration.

The researchers sent a beam of blue photons toward a crystal; for every billion or so photons that passed through, one transformed into two entangled red photons. A red photon from each pair then passed through a second crystal. One in a million of those split into two entangled infrared photons.

When this rare sequence of events occurred, three entangled particles emerged: one red and two infrared.

The result, reported September 14 in Nature Photonics, could shape plans for future quantum cryptography and computers.