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

Recently, Mohrhoff [Am. J. Phys. 64, 1468–1475 (1996)] has analyzed a thought experiment of ours [Nature (London) 351, 111–116 (1991)] where a double-slit interferometer for atoms is supplemented by a pair of which-way detectors. Owing to the quantum nature of these detectors, the experimenter can choose between acquiring which-way knowledge and observing an interference pattern. The latter option makes use of a procedure called “quantum erasure.” Mohrhoff (along with other bright colleagues who have made similar statements) claims erroneously that the experimenter has to make this choice before the atom hits the screen. We readdress this issue here and demonstrate that our original assertion was correct: The experimenter can choose between which-way knowledge and quantum erasure at any time, even after the atom has left its mark on the screen.