Fig. 3

Photon correlations. a Equal-time photon-photon correlations g⁽²⁾(0) of the transmitted light as a function of the diameter, d, of the illumination area for different Rydberg state linewidths, \(\bar \gamma {{\rm /}}2\pi\), and resonant excitation of the 10p-exciton at K⁺. b Critical photon-blockade radius \(R_{{\rm b}}^{({{\rm ph}})}\) for which g⁽²⁾(0) crosses 0.5 (green) as a function of \(\bar \gamma\) at fixed \(\frac{{{{\rm \Omega }}^2}}{{\left| {{\rm \Gamma }} \right|}} = 40\) GHz. The blue line shows the corresponding size \(R_{{\rm b}}^{({{\rm X}})}\) at which the probability to excite two Rydberg excitons within the illumination area is suppressed by a factor of 2. This exciton blockade radius scales as \(\sim \bar \gamma ^{ - 1/6}\)