To create an effective strong interaction between probe photons, a ladder-type EIT scheme with single-photon probe light at 780 nm and stronger control light at 480 nm is employed to address Rydberg states in an ultra-cold atomic cloud.
Once excitation to a Rydberg state occurs, strong interaction and non-trivial mixing of other Rydberg states is observed. A strong manifestation of the interaction strength is the Rydberg blockade effect: the pair potential (two excitations) V(r) can be higher than the excitation linewidth even at separations of r > 10 micrometer. As a result, for constant laser frequencies, Rydberg excitations can only occur with a certain mesoscopioc separation.
Rydberg-EIT and the blockade effect give rise to a collective nonlinearity : the probe photon propagates at a reduced group velocity (or is even stored) in the medium as a delocalized spin wave, the excitation is shared by all atoms in the excitation volume and the electrical susceptibility is non-local with a non-zero χ(3) component. Nonlinear transmission of single-mode probe light is the most striking evidence for strong interaction, as can be seen in our transmission spectra at different probe powers: compare transmissions at zero detuning.
Plenty of ideas can be realized in systems of strongly interacting photons. As an example from our research, a single photon transistor can be implemented by making use of the Rydberg excitation blockade between states with different principle quantum numbers.
More information can be found in .
 J.D. Pritchard, D. Maxwell, A. Gauguet, K.J. Weatherhill, M.P.A. Jones, C.S. Adams, PRL 105, 193603 (2010)
 T. Peyronel, O. Firstenberg, Q. Liang, S. Hofferberth, A. Gorshkov, T. Pohl, M. Lukin, V. Vuletic, Nature 488, 57 (2012)
 J. D. Pritchard, K. J. Weatherill, C. S. Adams, Annual Review of Cold Atoms and Molecules, 1, 301 (2013) (arXiv:1205.4890)