A priori which-way information in quantum interference with unstable particles

If an unstable particle used in a two-path interference experiment decays before reaching a detector, which-way information becomes available that reduces the detected interference fringe visibility V. Here we argue that even when an unstable particle does not decay while in the interferometer, a priori which-way information is still available in the form of path predictability P which depends on the particle's decay rate Gamma. We further demonstrate that in a matter-wave Mach-Zehnder interferometer using an excited atom with an appropriately tuned cavity, P is related to V through the duality relation P-2 + V-2 = 1. (C) 2014 Elsevier B.V. All rights reserved.