Coherent “atoms” in (fairly) warm solids

In 2001, Eric Cornell, Wolfgang Ketterle and Carl Wieman won the Nobel prize for physics for demonstrating the phenomenon of Bose-Einstein condensation in a system of trapped ultra-cold atoms. Bose-Einstein condensation is a remarkable quantum phenomenon in which a system of particles all occupy the same quantum state. In this condition they are identical and indistinguishable – in effect the individual atoms have lost their identities and coalesced into a single coherent quantum blob. Now researchers have demonstrated the same phenomenon in a different type of particle, polaritons, confined in a semiconductor nanostructure, at a temperature of 4.2 K. This is not exactly ambient, but it is much more convenient than the temperature of 20 nanoKelvin needed for the atom experiments.

The experiments, reported in this article in Science (abstract, subscription required for full article), were done by grad students Ryan Balili and Vincent Hartwell in David Snoke’s group at the University of Pittsburgh, in collaboration with Loren Pfeiffer and Kenneth West from Bell Labs. The basic structure consisted of a semiconductor quantum well trapped between a pair of reflectors, each made up of alternating dielectric layers, rather like the one shown in the picture in this earlier post. If a laser is shone into the structure, pairs of electrons and holes are generated; these pairs of charge are bound together by the electrostatic interaction and behave like particles called excitons. Meanwhile, light bounces back between the two mirrors, forming standing wave modes. Energy bounces back and forward between these standing wave photons and excitons, and the combination forms a quasi-particle called a polariton.

How on earth can one compare an entity that is composed of a complicated set of interactions between light and matter with something simple and elementary like an atom? The answer to this is rather interesting, and relies on a principle of solid state physics that is fundamental to the subject, but little known outside the field. Simple theory tells us how to understand systems composed out of entities that don’t interact with each other very much; the first theory of electrons in solids one gets taught simply assumes that the electrons don’t interact with each other at all, which on the face of it is absurd because they are charged objects which strongly repel each other. It turns out that you can often lump together the basic entity together with all its associated interactions as a “quasi-particle”, which behaves just like a simple, quantum mechanical particle. The particle is characterised by an “effective mass” which, in the case of these polaritons, is very much smaller than a real atom. It is this very small mass which allows them to form a Bose-Einstein condensate at (relatively) high temperatures.

This is another great example of how being able to make precisely specified semiconductor nanostructures allows one to tune the interaction between light and matter to produce remarkable new effects. What use could this have in the future? Peter Littlewood, from the Cavendish Laboratory in Cambridge, writes in a commentary in Science (subscription required):

“These objects are, on the one hand, a new kind of low-threshold laser, but the fact that they consist of coherent quantum objects (unlike a regular laser) puts them potentially in the class of quantum devices. A rash speculation is that a small polariton condensate could become the basis for an elementary quantum computer, but the easy coupling to light might simplify the wiring issues that many quantum information technologies find challenging.”