  Interference results when a quantum particle is free to choose among a few
indistinguishable paths. A canonical example of Bohr's complementarity
principle [1] is a two-path interferometer with an external detector coupled to
one of the paths. Then, interference between the two paths vanishes (i.e.
dephasing) if one is able to detect, even in principle, the path taken by the
particle. This type of which path (WP) experiment was already executed with
photons, cooled atoms, neutrons, solitons, and more recently with electrons in
a mesoscopic system [2]. In the latter experiment path determination was
provided by inducing a change in the current flowing through the detector. In
the present experiment we perform a more intriguing WP determination: only a
phase change is being induced in the detector while the current remains
unaffected. We show that such detector-interferometer interaction can be
understood both, from the WP information obtained by the detector, or
alternatively by understanding the disturbance in the interferometer caused by
the detector. Moreover, we address the subtle role of detector coherency and
point out difficulties in the intuitive interpretation of the experiment.
