Photonic triplets hint at quantum comms three-way

An international team of scientists have succeeded in splitting a single photon into three, beating some seriously long odd in the process. The work could pave the way for three-way quantum communications and according to lead researcher, Associate Professor Thomas Jennewein will "open a new frontier of quantum optics and allow a new class of experiments in quantum computing using photons."

Scientists have been able to split a single photon in two for some time by sending a single photon through a special class of optical crystal. This produces an entangled pair of photons; each retaining many of the characteristics of the other.

An entangled pair is useful in many experiments: they allow instant and secure communication between two parties. Entangled photons are also useful in optical lithography, in theory making it possible to etch at as little as on eighth of the wavelength of the light carving the chip.

It was thought likely that a similar technique could then split one of those daughter photons in two again. But the odds of such an thing happening in nature are something like one in a billion billion. Not something the average physicist is prepared to sit around and wait for, and something that seemed for many years to be experimentally out of reach.

But thanks to advances in waveguide design, the team says it can now generate photonic triplets at a rate of almost five per hour - against a background rate of one every two hours or so.

From Nature.com: The team used periodically poled potassium titanyl phosphate (PPKTP) and periodically poled lithium niobate (PPLN) — non-linear crystals that are more efficient than those generally used in previous experiments — and inserted a waveguide into one. The waveguide helps to confine the optics along the crystal's length to increase efficiency further, so that the probability of a photon splitting into three is one thousand times greater — a more experimentally friendly one in a million billion.

The next step is to see if all three of the photons are entangled, which could lead to interesting experiments in three-way quantum communication.

The work is published in the journal Nature.