Slowing light to a crawl

Light, which normally travels from the Moon to Earth in less than two seconds, has been slowed to 38mph using a state of matter first proposed by Albert Einstein and Satyendra Nath Bose in 1924.

Based on the theory that atoms crowded close enough together in ultra-low temperatures will lose their identity as individual particles and behave like a single super-atom, it is not until now that scientists have been able to actually put the Bose-Einstein condensate to the test.

Led by Harvard physicist Lene Hau, and first reported in the Harvard University Gazette, the experiments required vacuums trillions of times lower than the pressure of air at the Earth’s surface, and temperatures almost a billion times colder than that in interstellar space.

Hau began with a beam of sodium atoms, moving at speeds of more than a thousand mph, injected into a vacuum chamber.

Laser beams moving at the normal speed of light collide with the atoms which absorb particles of light (photons) and slow down. The laser light also orders their random movement so they move in only one direction.

When the atoms slow to around 100mph, they are loaded into a web of more laser beams. Each time an atom collides with a photon it’s knocked back in the direction from which it came, further slowing it down, or cooling it.

Finally, with most of the atoms packed in a cigar-shaped clump, atoms still too hot or energetic are kicked out of the magnetic field in a process described as ‘evaporative cooling’.

The stage is now set for slowing light. A laser is shot across the width of the cloud of condensate, 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 correctly, the light can be slowed by a factor of 20 million.

It is thought that the ability to slow the speed of light will find useful applications in the potential to send data, sound, and pictures in less space and with less power. It is also thoughts that the results of these experiments will help research into new types of laser projection systems and night vision cameras with power requirements a million times less than what is presently possible.