Entangled photon pairs are useful because the quantum state of one photon is revealed automatically by measuring the state of the other. This property could someday be exploited in quantum computers which, in principle, could outperform classical computers for certain tasks.

The most common way to produce entangled photons involves passing light through a non-linear crystal, which very occasionally converts a single photon into an entangled pair of lower-energy photons. However, this parametric down conversion process is inefficient and it is difficult to control precisely the number of pairs produced in this way. Therefore, some researchers are looking towards electrically-controlled semiconductor devices as a way of gaining better control over photon-pair production.

Alex Hayat and Meir Orenstein of the Israel Institute of Technology (Technion) in Haifa have observed two-photon emission from layered semiconductor devices based on gallium indium phosphide quantum wells – structures that allow electron energy levels to be modified by confining electrons to nearly two dimensions.

The emission process is stimulated by applying an electrical signal, which pumps electrons to higher energy levels. Although the majority of excited electrons return to a lower energy level by emitting a single photon, some follow a different route via intermediate energy levels and produce a photon pair. While this process has been observed in atomic systems, the researchers say that this is the first time it has been observed in a semiconductor.

According to Hayat, two-photon emission in semiconductors could be used to create compact sources that produce lots of entangled photons at room-temperature. However, the researchers have yet to confirm that the photons are entangled, which is the team's next planned experiment.

Hayat and Orenstein are not the first to propose a semiconductor-based source of entangled photons. Last year Robert Young and colleagues at Cambridge University in the UK unveiled a semiconductor quantum dot that produced entangled photon pairs. Unlike the Technion source, in which light is emitted in a process involving one electron, the Cambridge source involved a two-electron “biexciton” process. While the Cambridge source is perhaps less practical -- it operates at very low temperatures (10 K) and is optically, rather than electrically pumped -- it realized very precise control over photon-pair production.