Long-Range Blockade Between Counter-Propagating Photons
Realizing strong interactions between individual photons, mediated by matter, is a cornerstone for advancing photonic quantum computing and quantum nonlinear optics. In such systems, the interaction range is typically limited by the narrow bandwidth of the material excitation, resulting in a tradeoff between interaction strength and pulse duration. Here, we address this limitation by exploring interactions between counter-propagating photons mediated by Rydberg polaritons. We experimentally demonstrate strong photon-photon interactions, achieving a record-long anti-correlation range exceeding $1~\mu s$. This extended range enables photon pulses that are long enough to fit within the polariton bandwidth, yet short enough to remain within the interaction range. Under these conditions, we observe complete photon blockade of entire pulses, tunable via their relative timing. Extending to the three-photon regime, we observe enhanced interactions when a photon encounters a counter-propagating pair. These results, supported by analytical theory and numerical simulations, establish counter-propagating Rydberg polaritons as a powerful platform for engineering interactions in quantum light fields, enabling deterministic photonic operations and opening new directions in few-photon quantum dynamics.