  An imaging interferometer was created in a two-dimensional electron gas by
reflecting electron waves emitted from a quantum point contact (QPC) with a
circular mirror. Images of electron flow obtained with a scanning probe
microscope at liquid He temperatures show interference fringes when the mirror
is energized. A quantum phase shifter was created by moving the mirror via its
gate voltage, and an interferometric spectrometer can be formed by sweeping the
tip over many wavelengths. Experiments and theory demonstrate that the
interference signal is robust against thermal averaging.
