  Photonic molecules are composed of two or more optical resonators, arranged
such that some of the modes of each resonator are coupled to those of the
other. Such structures have been used for emulating the behaviour of two-level
systems, lasing, and on-demand optical storage and retrieval. Coupled
resonators have also been used for dispersion engineering of integrated
devices, enhancing their performance for nonlinear optical applications.
Delicate engineering of such integrated nonlinear structures is required for
developing scalable sources of non-classical light to be deployed in quantum
information processing systems. In this work, we demonstrate a photonic
molecule composed of two coupled microring resonators on an integrated
nanophotonic chip, designed to generate strongly squeezed light uncontaminated
by noise from unwanted parasitic nonlinear processes. By tuning the photonic
molecule to selectively couple and thus hybridize only the modes involved in
the unwanted processes, suppression of parasitic parametric fluorescence is
accomplished. This strategy enables the use of microring resonators for the
efficient generation of degenerate squeezed light: without it, simple
single-resonator structures cannot avoid contamination from nonlinear noise
without significantly compromising pump power efficiency, and are thus limited
to generating only weak degenerate squeezing. We use this device to generate
8(1) dB of broadband degenerate squeezed light on-chip, with 1.65(1) dB
directly measured, which is the largest amount of squeezing yet reported from
any nanophotonic source.
