Quantum doughnuts slow and freeze light at will

The study was carried out by the Universities of Warwick, Exeter and the Universidade Federal de Sao Carlos-UFSCar, Brazil and is now published in the journal Physical Review Letters.

The key to this new research is the ‘exciton’ This describes the pairing of an electron that has been kicked into a higher energy state by a photon, with a hole or gap it (or another electron) leaves within the shell or orbit around the nucleus of an atom. Despite its new high energy state the electron remains paired with one of the holes or positions that has been vacated by electrons moving to a higher energy state. When an electron’s high energy state decays again it is drawn back to the hole it is linked to and a photon is once again emitted.

That cycle usually happens very quickly but if one could find a way to freeze or hold an exciton in place for any length of time one could delay the reemitting of a photon and effectively slow or even freeze light.

The researchers looked at the possibilities presented by some tiny rings of matter accidentally made during the manufacture quantum dots. When creating these very small quantum dots of a few 10-100nm in size physicists some times cause the material to splash when depositing it onto a surface leaving, not a useful dot, but a doughnut shaped ring of material. Though originally created by accident these ‘nano-rings’ are now a source of study in their own right and in this case seemed just the right size for enclosing an exciton. However simply being this useful size does not, in itself, allow them to contain or hold an exciton for any length of time.

Remarkably the research team have found that if a combination of magnetic and electric fields is applied to these nano-rings they can actually then simply tune the electric field to freeze an exciton in place or let it collapse and re-emit a photon.

While other researchers have used varying exotic states of matter to dramatically slow the progress of light this is the first time a technique has been devised to completely freeze and release individual photons at will.

Dr Mikhail Portnoi of the University of Exeter’s School of Physics, a member of the research team, said: “The ability to release trapped light by simply changing an external electric field has important implications for fiber-optic telecommunications and other emerging photonic technologies.“

The technique could also be used to develop a ‘buffer’ of incoming photons which could re-release them in sequence at a later date thus creating an effect not unlike the concept of ‘Slow Glass’ first suggested by science fiction author Bob Shaw several decades ago.

The new research paper is entitled Exciton storage in a nanoscale Aharonov-Bohm ring with electric field tuning