Losing Light in a BEC

Two decades ago theorists predicted that the index of refraction in a gas of bosons would be enhanced compared to its value in a classical gas. The effect occurs because bosonic atoms tend to “bunch” into the same momentum state, which amplifies their scattering of light. Although the effect is small, even at low temperatures, the modification was expected to be observable in a gas near its transition to a Bose-Einstein condensate (BEC). Now, Peter van der Straten and colleagues from the University of Utrecht, Netherlands, have, for the first time, observed the predicted enhancement in an ultracold gas of sodium atoms.