  The ``Which Path?'' interferometer consists of an Aharonov-Bohm ring with a
quantum dot (QD) built in one of its arms, and an additional quantum point
contact (QPC) located close to the QD. The transmission coefficient of the QPC
depends on the charge state of the QD. Hence the point contact causes
controllable dephasing of transport through the QD, and acts as a measurement
device for which path an electron takes through the ring. We calculate the
suppression of the Aharonov-Bohm oscillations which is caused both by dephasing
and by the orthogonality catastrophe, i.e., respectively, by real and virtual
electron-hole pair creation at the QPC.
