Atom pairs in the micromaser

Although in standard micromaser experiments the collective events, in which two or more atoms are inside the resonator at one time, are very rare, they may cause dramatic effects nevertheless. We find in particular that the so-called “trapped vacuum state” (in which single atoms would undergo a complete Rabi cycle while traversing the cavity and emerge without having emitted a photon) is destroyed even when more than 99% of the atoms participate in one-atom events.