Achieving Robust Single-Photon Blockade with a Single Nanotip.

Backscattering losses (BSL), arising from intrinsic imperfections or unavoidable external perturbations in optical resonators, can severely impact photonic devices. In single-photon systems, robust quantum correlations against BSL remain largely unexplored despite their significance for various applications. Here, we demonstrate that single-photon blockade (SPB), a purely quantum effect, can be preserved against BSL by introducing a nanotip near a Kerr nonlinear resonator with intrinsic defects. Without the tip, BSL disrupts SPB, but tuning the tip's position restores robustness even under strong BSL. Notably, quantum correlations emerge while the classical mean photon number remains suppressed due to the interplay between resonator nonlinearity and tip-induced optical coupling. Our findings highlight nanoscale engineering as a powerful tool to protect and harness fragile quantum correlations, paving the way for robust single-photon sources and backscattering-immune quantum devices.