  We demonstrate real-time detection of self-interfering electrons in a double
quantum dot embedded in an Aharonov-Bohm interferometer, with visibility
approaching unity. We use a quantum point contact as a charge detector to
perform time-resolved measurements of single-electron tunneling. With increased
bias voltage, the quantum point contact exerts a back-action on the
interferometer leading to decoherence. We attribute this to emission of
radiation from the quantum point contact, which drives non-coherent electronic
transitions in the quantum dots.
