| It is well known that nonlinear optical processes can be resonantly enhanced with electromagnetically induced transparency (EIT) by simultaneously creating a destructive interference in absorption and a constructive interference in the nonlinear susceptibility. In this thesis we describe theory and experiments studying EIT-based nonlinear optical processes in cold rubidium atomic ensembles. Our first demonstration of EIT enhanced nonlinearity is an all-optical switch based on quantum interference where using a very weak laser beam we cause absorption of another laser beam. Next, using cw lasers and EIT we show generation of counterpropagating paired photons with coherence times of 50 ns and waveforms that are controllable at a rudimentary level. To generate paired photons, strong counter-propagating coupling and pump lasers are applied and parametric interaction results in generation of pairs of photons at Stokes and anti-Stokes frequencies into single-mode fibers at a rate of 12000 pairs per second. We measure second-order intensity correlation as a function of time difference between Stokes and anti-Stokes photodetections. By varying intensity of the coupling laser, two different regimes can be observed. In the first regime group delay of the atomic medium is the dominant timescale and observed correlation has a single peak whose width is equal to the group delay. In the second regime the dominant timescale is set by Rabi frequency of the coupling laser and we observe multiple oscillations in the correlation function and two-photon waveform has several peaks. The Cauchy-Schwartz inequality is violated by a factor of 400, suggesting strong non-classical correlations of photon pairs. |
