Refraction at the atomic level

In general, the speed of light in a dense medium is determined by the medium’s refractive index, which can vary significantly from that in vacuum. A highly dispersive medium—one in which the index of refraction varies rapidly with frequency—can allow greatly slowed or even speeded up group velocities for light. Now, researchers from the University of Tokyo (Japan) and NIST (Maryland) have altered a light pulse’s speed in a microcavity with a medium—less than 10 rubidium atoms—whose density scarcely differs from vacuum. The secret to the effect is a long dwell time. The 70-µm-long cavity was so reflective (its “finesse” was high) that the pulse reflected many times before leaking out. Thus the light interacted repeatedly with the handful of atoms, which makes the macroscopic concept of refractive index meaningful. The pulses used in the experiment were themselves quite ephemeral, amounting to only an average of four-tenths of a photon...