Correlations in Circular Quantum Cascades

We introduce a one-way, one-quantum cascade, whereby a single excitation proceeds one-directionwise in a ladder of energy levels. This makes a variation from more famous two-way cascades where the excitation can go up and down following its excitation or relaxation in the ladder. We provide closed-form solutions for two-photon correlation functions between any transitions in such circular cascades. We discuss how the rich correlations that result from what appears to be an extremely simple implementation, are essentially those which have been obtained from complex architectures relying on strongly-correlated, many-body physics or cavity QED effects, and might constitute the liquid phase of the new family of time materials.