Can we build quantum devices using the voltage state of Josephson junctions?
In superconducting quantum circuits the Josephson junction is the key element because it is the only strongly nonlinear and dissipationless circuit element we know. Usually it is used in the superconducting state where it acts as a nonlinear inductor, for example in Josephson qubits or Josephson parametric amplifiers. But a Josephson junction can also be nonlinear and dissipationless when a non-zero DC voltage below the gap is applied. In this case a Cooper pair current can flow through the junction when the energy 2eV of a tunneling Cooper pair can be dissipated in the linear circuit surrounding it, in the form of photons emitted into one or several of its modes. In this inelastic Cooper-pair tunneling regime, dubbed “Josephson photonics”, the junction acts as an active nonlinear element, transforming DC power into microwave radiation. We have tailored this physics into quantum microwave sources, such as single photon sources and measurement devices, such as quantum limited amplifiers or photo-multipliers. I will show that while these devices tend to be less coherent than their counter parts using the Josephson junction in the zero-voltage state of the junction, they still allow for quantum-limited performance, require much lower hardware overhead, and more readily accept very open configurations allowing for high bandwidth.
In a second more technical presentation on Thursday, Jan 24 at 11 am, I will focus on quantum limited amplification based on inelastic Cooper pair tunneling and give additional details on noise performance, gain bandwidth product and saturation.
Lecture followed by a buffet at 1 pm