It is well known that in a double-slit experiment with single photon source, one can not obtain the which-way info and interference pattern at the same time, since a detector placed behind one slit would "destroy" the interference. So instead of a detector which annihilate a photon, one could place a cavity in the path, which provide optical gain or amplification, thus by comparing the intensity on the screen where interference patterns lie, one might determine the which-way info without losing interference, thereby circumvent the limitation of complimentary principle.
closed as not a real question by David Zaslavsky♦ Oct 13 '12 at 23:07
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