Time-reversal of a wave packet emitted by an internal source: the   perfect inverse filter
  The time-reversal mirror (TRM) of acoustic waves is used as a focusing technique that is robust against disorder and chaos. The propagation of an ultrasonic pulse from the focal point is detected and recorded by a surrounding array of transducers that enclose the cavity region. The registered signals are then played-back in the reverse temporal sequence and an imperfect Loschmidt echo is obtained. A similar technique developed in the quantum domain, the perfect inverse filter (PIF), ensures an exact reversion. Applied to the acoustical case, it allows an improvement in the design of a minimally invasive inverse filter. Although that version of PIF accounts for external source condition, now we propose a generalization where the initial excitation is created inside the cavity. We present the theoretical principles for this form of PIF and perform a numerical test in a multichannel system that improves the focalization quality compared to the TRM version. 
Hernan L. Calvo, Horacio M. Pastawski