  We discuss complementarity and uncertainty in a gedanken Which-Way
(Welcher-Weg) experiment in a Mach-Zehnder interferometer. Although a
Welcher-Weg measurement can be performed with only a negligible amount of
momentum change in the detector itself, the change in the interference pattern
must correspond to a change in the momentum distribution of the particle.
Entanglement between the Mach-Zehnder interferometer and the Welcher-Weg
measurement introduces orthogonality that disables the interference of the
particle, but simultaneously, it also generates a momentum transfer between the
particle and other parts of the whole entangled system (including macroscopic
parts). This analysis can be related to the Delayed Choice interpretation. We
show where the momentum change takes place and we conclude that entanglement is
the key to understand how complementarity (i.e. Which Way versus interference)
and the position-momentum uncertainty relation are interwoven.
