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Wideband Quantum Optics with Josephson Junctions

Final Report Summary - WIQOJO (Wideband Quantum Optics with Josephson Junctions)

Within the project WiQOJo (Wideband Quantum optics with Josephson junctions) we have explored quantum devices based on small voltage-biased Josephson junctions. A Cooper pair tunneling through the junction provides the energy to generate one or several microwave photons in the linear circuit in which the junction is embedded, i.e. the Josephson junction acts as an active device transforming DC power to microwave radiation. This is in contrast to usual superconducting quantum devices such as in circuit QED, where the Josephson junction is in its superconducting state and acts as a nonlinear inductor. We have shown theoretically and experimentally that inelastic Cooper pair tunneling allows for the generation of nonclassical microwave radiation, such as anti-bunched photons, but can also be used to amplify microwave signals with minimal noise. By carefully designing the linear circuit in which the Josephson junction is embedded, we have built a first generation of useful devices: a bright on-demand single photon source and a broadband DC-powered microwave amplifier operating very close to the quantum limit. These devices are based on very simple circuits and are easier to use than their circuit QED counter-parts because they are powered by a simple DC voltage. After further optimization we expect these devices to simplify measurement operations in quantum processors in the microwave to mm-wave range.
To build these devices we have implemented a very versatile fabrication process based on NbN-MgO-NbN Josephson tunnel junctions, which allows for very wide ranges of junctions sizes and transmission line impedances.