The Josephson mixer, a Swiss army knife for microwave quantum optics
Emmanuel Flurin (LPA)

Abstract

This thesis work explores unique features offered by the Josephson mixer in the
rising field of microwave quantum optics. We have demonstrated three major roles
the Josephson mixer could play in emerging quantum information architectures.
First, we have designed and fabricated a state-of-the-art practical quantum limited
amplifier with the best quantum efficiency achieved to date. This tool is crucial for
probing mesoscopic systems with microwaves, and in particular superconducting
circuits. Hence, it has enabled us to realize successfully the stabilization of quantum
trajectories of a superconducting qubit by measurement-based feedback.
Second, we have shown how this circuit can generate and distribute entangled
microwave radiations on separated transmission lines at different frequencies. Using
two Josephson mixers, we have provided the first demonstration of entanglement
between spatially separated propagating fields in the microwave domain, the so-
called Einstein-Podolsky-Rosen states.
Finally, we have used the Josephson mixer as a frequency converter. Acting as a
switch, it is able to dynamically turn on and off the coupling to a low loss cavity. This
feature allowed us to realize a quantum memory for microwaves. In combination with
the ability to generate entanglement, we have measured the time-controlled
generation, storage and on-demand release of an entangled state, which is a
prerequisite for nodes of a quantum network.