Nearly quantum-limited Josephson-junction Frequency Comb synthesizer

TL;DR

This paper demonstrates a microwave frequency comb generator in circuit QED that approaches quantum-limited coherence, enabling exploration of quantum nonlinear system dynamics with longer coherence times than traditional optical microresonators.

Contribution

It introduces a circuit QED-based microwave frequency comb that operates near the quantum limit, surpassing the coherence times of optical microresonator combs and enabling new quantum nonlinear studies.

Findings

Achieved a comb phase coherence of up to 35 microseconds.

Approached the theoretical device quantum limit of 55 microseconds.

Demonstrated coherence vastly longer than the modes' inherent 13 nanoseconds lifetime.

Abstract

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