The way Dr Hau and her team have slowed down light by a factor of 600 million or so is to use a group of atoms called a Bose-Einstein condensate (BEC).
That, at least, was the conclusion of Satyendranath Bose and Albert Einstein, who did the relevant sums in the 1920s, decades before anyone had made a laser, let alone the Bose-Einstein condensate that is named in their honour.