  Squeezed light is a critical resource in quantum sensing and information
processing. Due to the inherently weak optical nonlinearity and limited
interaction volume, considerable pump power is typically needed to obtain
efficient interactions to generate squeezed light in bulk crystals. Integrated
photonics offers an elegant way to increase the nonlinearity by confining light
strictly inside the waveguide. For the construction of large-scale quantum
systems performing many-photon operations, it is essential to integrate various
functional modules on a chip. However, fabrication imperfections and
transmission crosstalk may add unwanted diffraction and coupling to other
photonic elements, reducing the quality of squeezing. Here, by introducing the
topological phase, we experimentally demonstrate the topologically protected
nonlinear process of spontaneous four-wave mixing enabling the generation of
squeezed light on a silica chip. We measure the cross-correlations at different
evolution distances for various topological sites and verify the non-classical
features with high fidelity. The squeezing parameters are measured to certify
the protection of cavity-free, strongly squeezed states. The demonstration of
topological protection for squeezed light on a chip brings new opportunities
for quantum integrated photonics, opening novel approaches for the design of
advanced multi-photon circuits.
