  Quantum light sources are characterized by their distinctive statistical
distribution of photons. For example, single photons and correlated photon
pairs exhibit antibunching and reduced variance in the number distribution that
is impossible with classical light. Most common realizations of quantum light
sources have relied on spontaneous parametric processes such as down-conversion
(SPDC) and four-wave mixing (SFWM). These processes are mediated by vacuum
fluctuations of the electromagnetic field. Therefore, by manipulating the
electromagnetic mode structure, for example, using nanophotonic systems, one
can engineer the spectrum of generated photons. However, such manipulations are
susceptible to fabrication disorders which are ubiquitous in nanophotonic
systems and lead to device-to-device variations in the spectrum of generated
photons. Here, we demonstrate topologically robust mode engineering of the
electromagnetic vacuum fluctuations and implement a nanophotonic quantum light
source where the spectrum of generated photons is robust against fabrication
disorders. Specifically, we use the topological edge states to achieve an
enhanced and robust generation of correlated photon pairs using SFWM and show
that they outperform their topologically-trivial counterparts. We demonstrate
the non-classical nature of our source using conditional antibunching of
photons which confirms that we have realized a robust source of heralded single
photons. Such topological effects, which are unique to bosonic systems, could
pave the way for the development of robust quantum photonic devices.
