Scientists in the UK have been able to generate pairs of entangled photons from a nanoscale crystal of semiconductor known as a "quantum dot" far more efficiently than was possible before. The breakthrough was made by Andrew Shields at Toshiba and colleagues at the University of Cambridge, who produced entangled photons with an efficiency of 70% - compared to a previous best figure of 49%. The improved performance approaches that required for useful applications, which means that devices emitting entangled light could one day be as common as lasers and light-emitting diodes (New J. Physics 8 29).
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, 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.