Phys. Rev. Lett. (in the press); preprint at http://arxiv.org/abs/1111.2110 (2011)

Enhancing the efficiency of optical switching or other nonlinear optical processes usually requires high-intensity light. However, it is also possible to maximize efficiency with very-low-intensity light if the interaction time is long. Stopping light pulses and making them interact in a medium is a way to get more for less.

Yi-Hsin Chen and co-workers made two light pulses motionless by slowing them down and trapping them in an atomic vapour. The frozen light pulses could then interact for almost seven microseconds. This long interaction allowed the very-low-intensity light pulse to trigger another light pulse — thus realizing an all-optical switch. The mechanism behind this scheme is a quantum interference effect known as electromagnetically induced transparency, which makes the atomic medium transparent to resonant light.

The results predict that switching could be activated for even-lower-intensity light if the optical density of the atomic medium is increased by cooling the atoms — a promising perspective for low-light-level nonlinear optics and quantum information processing.