  We propose and demonstrate a remote sensor scheme by applying the quantum
mechanical concept of fidelity loss to classical waves. The sensor makes
explicit use of time-reversal invariance and spatial reciprocity in a wave
chaotic system to sensitively and remotely measure the presence of small
perturbations. The loss of fidelity is measured through a classical wave-analog
of the Loschmidt echo by employing a single-channel time-reversal mirror to
rebroadcast a probe signal into the perturbed system. We also introduce the use
of exponential amplification of the probe signal to partially overcome the
effects of propagation losses and to vary the sensitivity.
