Probing many-body correlations using quantum-cascade correlation spectroscopy

In quantum optics, the radiative quantum cascade—the consecutive emission of photons from a ladder of energy levels—is of fundamental importance. Two-photon cascaded emission has been instrumental in pioneering experiments to test Bell inequalities and generate entangled photon pairs. More recently, correlated and entangled photon pairs in the visible and microwave domains have been demonstrated using solid-state systems. These experiments rely on the nonlinear nature of the underlying energy ladder, which enables the direct excitation and probing of specific single-photon transitions. Here we use exciton–polaritons to explore the cascaded emission of photons in the regime where individual transitions of the ladder are not resolved. We excite a polariton quantum cascade by off-resonant laser excitation and probe the emitted luminescence using a combination of spectral filtering and correlation spectroscopy. The measured photon–photon correlations exhibit a strong dependence on the polariton energy and therefore on the underlying polaritonic interaction strength, with clear signatures of many-body Feshbach resonances. Our experiment establishes photon cascade correlation spectroscopy as a highly sensitive tool to study the underlying quantum properties of novel semiconductor materials and many-body quantum phenomena.