Crawling Faster than Light

The statistical interpretation of atomic gases in the last century has successfully predicted many experimental results and one can easily use such models to translate back to the well known results of classical thermodynamics for systems in the high temperature limit. Perhaps the most fascinating of these phenomena is a Bose-Einstein Condensate (BEC). First predicted as a new “superfluid” state of matter by Fritz London of Duke University in 1938, it was not until 1995 that such a condensate was prepared in a laboratory at a temperature of just one millionth of a Kelvin above absolute zero. Indeed, after so long BEC is still an active area of research with the most interesting development being that a beam of light can be brought to a halt by such a condensate.

So what is a BEC? Well first of all it is worth noting that Bose-Einstein condensation can only occur in boson gases made up of particles with integer spin. Spin itself is not so relevant here but Fermions such as electron have half-integer spin meaning that they must obey Pauli’s exclusion principle which does not allow more than one particle in each atomic orbital while bosons are not subject to such constraints. To understand what these condensates are, suppose we have trapped a gas of bosons in a container such that the number of bosons in the container never changes. If these particles are cooled to a very low temperature they will lose their momentum leading to a large fraction of them sharing the lowest energy state available. This saturated low energy state is what we are referring to when we say Bose-Einstein condensate and statistically it behaves like one “super-particle” with the statistical density of all the individual bosons smeared together into one. For so long researchers struggled to prepare these condensates in a lab since the conditions described above are hard to meet in a real experiment.

Here’s where it gets especially cool. A team led by Professor Lene Hau at Harvard conducted a set of experiments to investigate the propagation of light beams through Bose-Einstein condensates in 2005 only a decade after the initial discovery. It is well known in physics that the speed of light in vacuum stands as the speed limit of all interactions in the universe, but what about the minimum speed of light? Prof. Hau and her team were able to take a beam of light about 1-2 miles long in spatial extent and pass it through a Bose-Einstein condensate in one of the coldest environments ever experimentally prepared. What they found was that they could slow the beam to the speed of a bicycle and even bring it to a complete stop inside the condensate. On the surface this wouldn’t be anything to write home about, after all a beam of light can be brought to a complete stop by pointing it at a wall. The difference in this case is that the beam of light is preserved by the cloud of atoms. It can be paused in its tracks and then shot off again whenever the reseachers want. To have this much control over light seems almost godly. The images below show a depiction of this transmission as the light beam becomes compressed and elongated again while entering and exiting the medium.

Apart from the interaction of light and BECs, one can also create a Bose-Einstein condensate out of photons (light particles) since they too are bosons. This was achieved at laboratories in the University of Bonn and Imperial College London, leading to what has been appropriately named a “super-photon”. For me these experiments as strange as they seem, highlight just how universal the results of thermodynamics are, classical or quantum, particle or wave, the laws do not discriminate, BEC allows us to see radiation and matter following the same statistical rules. #TP.

Images © Harvard University 2005.