Nearly Quantum-Limited Josephson-Junction Frequency-Comb Synthesizer

While coherently driven Kerr microcavities have rapidly matured as a platform for frequency-comb formation, such microresonators generally possess weak coefficients; consequently, triggering comb generation requires millions of photons to be circulating inside the cavity. This suppresses role quantum fluctuations in dynamics comb. In this paper, we realize minimal version Kerr-mediated microwave-frequency combs circuit electrodynamics (cQED) architecture, where vacuum are primary limitation on coherence. We achieve phase coherence up 35 $\ensuremath{\mu}\mathrm{s}$, approaching theoretical device limit 55 $\ensuremath{\mu}\mathrm{s}$ and vastly longer than inherent lifetimes modes, 13 ns. The ability within cQED engineer stronger nonlinearities optical microresonators, together with operation at cryogenic temperatures, excellent agreement theory indicates promising study complex nonlinear systems.