  Electromagnetically induced transparency in an optically thick, cold medium
creates a unique system where pulse-propagation velocities may be orders of
magnitude less than $c$ and optical nonlinearities become exceedingly large. As
a result, nonlinear processes may be efficient at low-light levels. Using an
atomic system with three, independent channels, we demonstrate a quantum
interference switch where a laser pulse with an energy density of $\sim23$
photons per $\lambda^2/(2\pi)$ causes a 1/e absorption of a second pulse.
