Under certain circumstances, an ultraviolet photon can spontaneously split into two lower-energy infrared photons - this is known as down-conversion. The polarizations of these two photons are intimately related: a measurement of the polarization of one photon would reveal the polarization of the other, even if they were widely separated. This is an example of 'entanglement' - a correlation that can exist between quantum particles that is much stronger than those allowed in classical physics.

But entangled photon pairs of this kind arise rarely in ultraviolet beams. In order to create more pairs, Lamas-Linares and colleagues shone a pulsed ultraviolet laser through a crystal of barium borate. As expected, one of the millions of photons split into two infrared photons via the down-conversion process. These photons left the crystal at an angle to the direction of travel of the laser pulse, and mirrors then reflected them back into the crystal. Meanwhile, the laser pulse that passed through the crystal was also reflected back towards it. The mirrors were arranged so that the reflected laser pulse reached the crystal at exactly the same time as the entangled photons.

The quantum interaction of the entangled photons and the reflected pulse sparked the production of another pair of entangled photons. Classically, this would lead to two photon pairs, but because this is a quantum interference process, it can produce a maximum of four pairs of photons or a minimum of zero. This is analogous to the reinforcement or cancellation of light waves in a diffraction pattern, and can result in a four-fold increase in the number of entangled photons produced. The phenomenon can also multiply the number of entangled photons by sixteen if it is applied to an even rarer system composed of four entangled photons.

"Currently available sources of entangled photons are extremely weak, but the laser action for entangled photons can produce very bright sources of entangled photon pairs", Lamas-Linares told PhysicsWeb. "Laser action will also create far more complicated entangled states that involve many photons and are likely to play an important role in the realization of several recent theoretical developments in quantum information."

The amplification demonstrated by Lamas-Linares and co-workers is analogous to the light that bounces between the mirrors at the ends of a laser cavity. In practice, however, the light in a conventional laser cavity undergoes many reflections, whereas the initial entangled pair in the Oxford experiment is reflected only once.

"We are now working on a system in which the laser passes through the crystal many times", says Lamas-Linares. The refined set-up could lead to fluxes containing up to 100 entangled photons.