A frequency-modulated beam-splitter between 3D quantum memories

Achieving fast, tunable, high-fidelity bilinear interactions between 3D bosonic memories is a compelling goal in circuit-QED. To facilitate this, these memories are often coupled to a non-linear element that activates specific interactions dependent on the frequency of drive tones applied to it. Strong drives usually result in faster interactions, but a linear coupling to the drive results in a displacement of the non-linear mode and triggers other undesirable processes, reducing the fidelity of the operation. To mitigate this, we use a symmetric DC squid with the common mode acting as the non-linear mode used in the interaction, and the differential mode being pumped to turn the interaction on and off. This effectively gives us a frequency modulation of the common mode without displacing it. We are able to selectively pump the differential mode through a carefully engineered AC flux drive, achieving a parametrically driven high-fidelity beam-splitting process.