Dr. Hau's team found that, at a microkelvin, the pulse of light traveled through the cold, laser-bathed sodium atoms at an astonishingly slow speed of about 50 meters per second. At this incredibly low temperature, the sodium atoms entered a new exotic state of matter called a Bose-Einstein condensate . Constructed for the first time in the laboratory only a few years ago in 1995, Bose-Einstein condensates have opened up a new exciting field in cryogenics. Besides generating an enormously rapidly varying index of refraction, the laser light had another important consequence for the cloud of sodium atoms. Normally, the cloud is opaque, meaning that light cannot travel through it.
Light is absorbed as it enters an opaque medium.
This effect is known as electromagnetically induced transparency because laser light is used to render the cloud transparent. To achieve the electromagnetically induced transparency, it is important for the cloud to be very cold so as to allow the laser light to interact efficiently with the sodium atoms. Like many scientists who achieve an amazing result for the first time, Dr. Hau is more excited about future prospects than what she has already accomplished. The system gives us a completely new scientific tool that can probe Bose-Einstein condensates. Future practical applications include nonlinear optics at the single photon 4 level.