‘Photonic Molecule’ System Developed

A team from the Laboratory for Photonics and Nanostructures (LPN) of the Centre National de la Recherche Scientifique (CNRS, National Center for Scientific Research) has developed a source of entangled photons 20 times brighter than all existing systems. The device, dubbed the “photonic molecule” system, is capable of considerably boosting the rate of quantum communications and constitutes a key component in future quantum logic processes.

Entanglement (when the properties of one photon depend on those of the other) is considered to be at the very heart of applications such as quantum cryptography, computation and teleportation.

This image represents, in the top right-hand corner, the new component produced in this experiment: Two pillars of micrometric size are coupled to form the “photonic molecule.” The semiconductor quantum dot is inserted into one of the pillars (visible as the bright spot in the right-hand pillar). The lower part of the image shows the radiation pattern of the entangled photons emitted by the component. (Image: Jean-Louis Le Hir)

Normally, researchers use sources of entangled photon pairs that are easy to put into place (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, considerably restricting the rate of any quantum communication. In addition, the size of such sources means that they cannot be easily integrated into microsystems.

In the new photonic molecule system, a semiconductor quantum dot emits a pair of entangled photons per excitation pulse. 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 eight pulses (compared with less than one pair every 100 pulses so far).

According to the researchers, they 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 1 GHz (~1 billion Hz). Moreover, the LPN scientists have shown that the use of this photonic molecule concept allows the quality of the entanglement of the emitted photon pairs to be improved.

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