Harvard's Lene Hau can slow light to a complete stop in a supercooled cloud.
One laser is shot across the width of a cloud of Bose-Einstein condensate, only a billionth of a degree above absolute zero.
This controls the speed of a second pulsed laser beam shot along the length of the cloud.
The first laser sets up a quantum interference such that the moving light beams of the second laser interfere with each other.
When everything is set up just right, the light can be slowed by a factor of 20 million or more.