Refraction at the Atomic Level

Light propagation in a cavity can now be controlled through interactions with a collection of fewer than 10 atoms, a new experiment shows.

In general, the speed of light can be lowered from the vacuum value by passing it through a dense medium. Light speed can also be altered if the light pulse consists of a superposition of light waves at different frequencies and if the medium is dispersive (if its index of refraction varies for different frequencies). Using this dispersive approach, light was slowed to a halt in a Bose-Einstein condensate containing a million atoms (Update 521).

Now researchers from the University of Tokyo (Japan) and NIST (US) have managed the feat of altering a light pulse's speed in a microcavity with a medium whose density scarcely differs from vacuum—namely a handful of rubidium atoms.

The secret to the control is a long dwell time. The 70-micron-long cavity is so reflective (its "Q" value is high) that the pulse reflects many times before leaking out. This allows the light to interact with the handful of atoms repeatedly, as if there were many more atoms present.

According to the researchers (Yukiko Shimizu, shimizu-yukiko@aist.go.jp) this radical departure may be useful in quantum computing schemes. The pulses used in the experiment were themselves quite ephemeral, amounting to only four tenths of a photon (on average) in the cavity at any one time. The next goal is entangle a single photon with a single atom. (Shimizu et al., Physical Review Letters, 2 December 2002.)