Future technology such as quantum cryptography and computation, or perhaps even larger scale teleportation, requires a deeper understanding of the phenomenon known as "entanglement", the quantum non-local connection, an aspect of quantum theory at the heart of the EPR paradox developed by Einstein, Podolsky and Rosen in 1935 which was experimentally verified in 1980 by Alain Aspect.

Two photons are entangled if the properties of one depend on those of the other, whatever the distance separating them. A new source of entangled photons twenty times brighter than all existing systems has been developed by a team from the Laboratoire de Photonique et de Nano-structures (LPN) of CNRS and they say the device is capable of considerably boosting the rate of quantum communications.

Normally, researchers use sources of entangled photon pairs that are easy to put in place, such as a laser transforming a photon into two photons of different color but with very low brightness: less than one pulse out of 100 actually contains a pair of entangled photons, which considerably restricts the rate of any quantum communication. In addition, the size of such sources means they cannot be easily integrated into microsystems.

The team from the Laboratoire de Photonique et de Nano-structures (LPN) of CNRS has developed a light source of entangled photons twenty times brighter than all existing systems. The researchers have invented a novel "photonic molecule" system in which a semiconductor quantum dot emits a pair of entangled photons per excitation pulse.

Quantum dots are nanocrystals of semiconductor material, the dimensions of which are typically 20 nanometers (i.e. 20 billionths of a meter). By virtue of its size, a quantum dot behaves like a potential well, which confines carriers within the three dimensions of a space typically of the order of several tens of nanometers in a semiconductor. This confinement gives the quantum dots properties similar to those of an atom.

This photonic molecule constitutes a trap for each of the photons of the pair and allows them to be collected efficiently. This new source operates at a rate of one pair of photons collected every 8 pulses (compared to less than one pair every 100 pulses so far). In the long run, the researchers should be able to reach a rate close to one pair of photons per pulse. This device could make it possible to manufacture electroluminescent diodes of entangled photon pairs, with rates close to one gigahertz (in other words around on billion hertz). Moreover, the LPN scientists have also shown that the use of this "photonic molecule" concept allows the quality of the entanglement of the emitted photon pairs to be improved.

Citation: 'Ultrabright source of entangled photon pairs', Adrien Dousse, Jan Suffczyński, Alexios Beveratos, Olivier Krebs, Aristide Lemaître, Isabelle Sagnes, Jacqueline Bloch, Paul Voisin&Pascale Senellart, Nature 466, 277 (2010).