Generating Correlated Photons for Laser-Cooled Atoms
C. F. McCormick, V. Boyer, P. D. Lett
Laser Cooling and Trapping Group, Atomic Physics Division
While correlated-photon interference experiments have already demonstrated dramatic or counter-intuitive aspects of quantum mechanics, it would be interesting and potentially useful to extend these experiments to the realm of massive particles. Highly correlated atomic beams could help realize an atom interferometer operating with enhanced phase resolution, enable massive-particle tests of Bellís Inequalities violations, and provide a resource for quantum cryptography and communication.
One possible method for creating such correlated atomic beams is to let correlated photons interact with cold atoms in a controlled manner. Our group has already demonstrated the creation of a single quasi-CW atom laser beam, using a pulsed Raman transition as an output coupler for atoms in a sodium BEC. If the Raman output coupler were operated with correlated photons, under certain conditions the result would be a pair of correlated atom laser beams. Unfortunately, current sources of correlated photons are generally impractical for interaction with cold atoms, because of broad bandwidths and low spectral brightness.
We are building a correlated-photon source appropriate for laser-cooled atoms that is based on the nonlinear response of atomic rubidium vapor. In our initial experiments with a degenerate four-wave mixing process known as two-beam-excited (TBE) conical emission, we have found that weak beams can experience strong nonlinear gain that can be further enhanced using electromagnetically induced transparency (EIT). While this process does generate correlated photons, they are unlikely to be useable for our purposes because of high levels of scattered background radiation from the frequency-degenerate pump lasers. We are now investigating more complicated nonlinear schemes, including three- and four-level systems in rubidium that exploit EIT effects to enhance nonlinear coefficients and reduce scattered background light.