  Photons in a nonlinear medium can repel or attract each other, resulting in a
strongly correlated quantum many-body system. Typically, such strongly
correlated states of light arise from the extreme nonlinearity granted by
quantum emitters that are strongly coupled to a photonic mode. However, in
these approaches, unavoidable dissipation, like photon loss, blurs nonlinear
quantum effects. Here, we generate strongly correlated photon states using only
weak coupling and taking advantage of dissipation. We launch light through an
ensemble of non-interacting waveguide-coupled atoms, which induce correlations
between simultaneously arriving photons through collectively enhanced nonlinear
interactions. These correlated photons then experience less dissipation than
the uncorrelated ones. Depending on the number of atoms, we experimentally
observe strong photon bunching or anti-bunching of the transmitted light. This
realization of a collectively enhanced nonlinearity may turn out
transformational for quantum information science and opens new avenues for
generating nonclassical light, covering frequencies from the microwave to the
X-ray regime.
