Quantum Amplification and Feedback (4)

Michel Devoret explains why a quantum measurement amplifier must transmit a signal without returning its own noise to the system being measured. He relates directional amplification to the breakdown of reciprocity, introducing Faraday rotation and microwave circulators used between superconducting circuits and measurement amplifiers.

The lecture then examines the limitations of magnetic circulators: stray magnetic fields are undesirable near superconducting quantum circuits, and bulky components constrain the number of measurement channels inside a dilution refrigerator. Devoret develops a theoretical route to non-reciprocal microwave components using non-degenerate three-wave mixing in a frequency-conversion regime. The pump phase supplies an active analogue of the magnetic bias in the Faraday effect, allowing directionality to be engineered through phase differences.