  Exploiting the time-reversal invariance and reciprocal properties of the
lossless wave equation enables elegantly simple solutions to complex
wave-scattering problems, and is embodied in the time-reversal mirror. A
time-reversal mirror operates by recording the wave disturbance as a function
of time at one or more points in a medium, broadcasting a time-reversed version
of the signal(s) in the opposite direction, and allowing the waves to
reverse-propagate back to their origin. Here we demonstrate the implementation
of an electromagnetic time-reversal mirror in a wave chaotic system containing
a discrete nonlinearity, and, as an example of its utility, we have used it to
demonstrate a new form of secure communication.
