Entanglement allows particles to have a much closer relationship than is possible in classical physics, and means that we can know the state of one photon by measuring the state of the other. For example, if one photon is horizontally polarized, then its entangled counterpart must have a vertically polarized spin, even if it is many kilometres away. In the source made by Shields and colleagues, the correlated photons that the quantum dot produces have the same polarisation and correlation is seen for not only horizontal and vertical polarizations but also for all possible directions of polarization.

The team produced entangled photons from a crystal just 12 nm in diameter made from indium arsenide embedded within a gallium arsenide and aluminium arsenide cavity. When excited by a laser pulse, the quantum dot captures two electrons and two holes to form a "biexciton" state in the dot. One of the electrons recombines with a hole to create a photon, leaving behind an intermediate "exciton" state in the dot of one electron and one hole. The other electron-hole pair then combines to create a second photon.

According to the team, the polarizations of the two emitted photons are governed by the spins of the electron and hole in the intermediate exciton state, which has two possible spin configurations. Recombination via one of these two states leads to the emission of a random mixture of two vertically polarized or two horizontally polarized photons. The researchers found that entangled photons were only produced by certain dots that have a symmetric shape.

Earlier work by the UK team was only able to produce entangled photons with an efficiency of 49%. The researchers have now improved on this result and have increased the efficiency to 70%, which approaches that required for useful applications. They did this by suppressing the amount of background light emitted by layers other than the quantum dot itself.

A unique feature of the new source is that it generates a pair of entangled photons "on demand", that is, in response to an external trigger. "Such a source is essential for many applications, like quantum communications or computing, where gate operations are triggered by an external clock," says Shields. Most entangled photons are currently produced through the technique of "parametric down-conversion" by shining a laser onto certain crystals.