Universal photon blockades via parametric amplification in second-order nonlinearly coupled cavities

Photon blockade (PB) is generally classified into conventional and unconventional types based on their distinct physical mechanisms. Recent studies have aimed to bridge the two regimes to realize a universal photon blockade (universal PB) that combines their respective advantages. Here, we propose a scheme for achieving universal PB based on second-order nonlinearity. The second-order nonlinear coupling between two cavity modes mediates the fundamental interaction, while an optical parametric amplifier embedded in one cavity introduces additional quantum interference pathways. We analytically derive the optimal conditions for universal PB, and the analytical results show excellent agreement with numerical simulations. By comparing universal PB with conventional PB, we highlight its unique performance advantages: the scheme operates effectively in both strong and weak coupling regimes, relaxes experimental constraints, and achieves an ultra-low equal-time second-order correlation function g⁽²⁾(0) down to 10^-7, while maintaining a moderate mean photon number N_a ∼ 10^-2. This work provides a feasible and tunable platform for realizing high-purity single-photon sources and exploring quantum statistical properties in nonlinear cavity systems.