Quantum erasure in double-slit interferometers with which-way detectors

Recently, Mohrhoff [Am. J. Phys. 64, 1468-1475 (1996)] has analyzed a thoug ht experiment of ours [Nature (London) 351, 111-116 (1991)] where a double- slit interferometer for atoms is supplemented by a pair of which-way detect ors. Owing to the quantum nature of these detectors, the experimenter can c hoose between acquiring which-way knowledge and observing an interference p attern. The latter option makes use of a procedure called "quantum erasure. " Mohrhoff (along with other bright colleagues who have made similar statem ents) claims erroneously that the experimenter has to make this choice befo re the atom hits the screen. We readdress this issue here and demonstrate t hat our original assertion was correct: The experimenter can choose between which-way knowledge and quantum erasure at any time, even after the atom h as left its mark on the screen. (C) 1999 American Association of Physics Te achers.