Spontaneous four-wave mixing can generate highly correlated photon pairs from atomic vapors. We show that multiphoton pumping of dipole-forbidden transitions in a recoil-free geometry can result in ultrabright pair emission in the full $4\pi$ solid angle, while strongly suppressing background Rayleigh scattering and associated atomic heating, Such a system can produce photon pairs at rates of $~10$${}^{12}$ per second, given only moderate optical depths of $10-100$, or alternatively, the system can generate paired photons with subnatural bandwidths at lower production rates. We derive a rate-equation based theory of the collective atomic population and coherence dynamics and present numerical simulations for a toy model, as well as realistic model systems based on ${}^{133}\mathrm{Cs}$ and ${}^{171}\mathrm{Yb}$ level structures. Last, we demonstrate that dark-state adiabatic following electromagnetically induced transparency and/or time-scale hierarchy protects the paired photons from reabsorption as they propagate through the optically thick sample.

• Received 19 November 2009

DOI:https://doi.org/10.1103/PhysRevA.81.033807