Correlation of Photons in Cascade and the Coherence Time of the6P13State of Mercury

We have observed directly the time correlation between successively emitted photons in the $7{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}-6\mathrm{}{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}-6\mathrm{}{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ cascade of atomic mercury, and have deduced from it direct measurements of both the natural lifetime of the $6{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ state and the dependence of the mean correlation time (coherence time) on the density of the mercury vapor surrounding the primary radiation region. The density-dependent coherence time constitutes a direct observation of the imprisonment of resonance radiation and provides an experimental confirmation of the theory describing this phenomenon.