Prof. Francis Robicheaux
Dr. Robicheaux is a professor at Auburn University since 1993 and a Fellow of the American Physical Society since 2002. His research area is Theoretical Atomic Physics, mainly focusing on time dependent atomic phenomena, highly excited (Rydberg) atoms, electron scattering, strong fields and ultracold plasmas. He is a member of the ALPHA collaboration which was the first group to trap the antimatter version of the Hydrogen atom and the first group to quantitatively measure some of its properties.
In 1998, two groups simultaneously performed experiments where the electronic interaction between the atoms in the gas was not the smallest energy in the system. In these experiments, the atoms were cooled to 1/1,000,000 of room temperature. A large fraction of the atoms were excited into Rydberg states. Because the atoms were cold, they were essentially stationary for the duration of the experiment. The large dipole moments of the atoms gave large interaction between the atoms and the lack of atomic motion gave time for interesting state of matter to evolve.
The frozen Rydberg gas is an interesting new example of a correlated system. We have performed several studies of this system with our focus on a full quantum solution of the developing correlation. Some examples of studies we've performed: 1) a spin-echo like effect when a Rydberg gas is created by two or three laser pulses, 2) the hopping of an excitation through the gas, 3) calculations of the dipole blockade that occurs when the atoms are excited by a very narrow-band laser, and 4) solution of a model problem to understand whether the correlations in the gas are quantum mechanical or classical. Below is a brief description of results in two recent publications.
B. Sun and F. Robicheaux, “Spectral linewidth broadening from pair fluctuations in a frozen Rydberg gas,” Phys. Rev. A 78, 040701 (2008). PDF (115 kB)
In the early experiments, the linewidth of transitions to excited states in a Rydberg gas were much larger than expected from the strength of the interaction between Rydberg atoms.
This image shows the width from W.R. Anderson et al, Phys. Rev. Lett 80, 249 (1998) Fig. 2 as a function of the density of the gas. The width is about 80X larger than might be expected from the interaction strength. This was interpreted as due to many body effects or to diffusion.
In our calculations, we could directly compute the diffusion of an excitation through a Rydberg gas. We found that diffusion, while important, is not a dominant effect.
This image shows the probability that an excitation remains on the same atom and the inset shows the energy width of the excitation in a Rydberg gas. The hopping rate is too small to explain the experimental results.
This image shows the line width with (red lines) and without (blue lines) hopping included in the calculation. The diffusion/many body effects only account for a factor of 2. The main effect is the fluctuation in interaction strength that comes from the random placement of atoms in the gas.
C. S. E. van Ditzhuijzen, A. F. Koenderink, J. V. Hernandez, F. Robicheaux, L. D. Noordam, and H. B. van Linden van den Heuvell, “Spatially Resolved Observation of Dipole-Dipole Interaction between Rydberg Atoms,” Phys. Rev. Lett. 100, 243201 (2008). PDF (330 kB)
In a collaboration with the experimental group of Ben van Linden van den Heuvell, we investigated how transitions occur between spatially separated atoms.
This image shows a schematic of the arrangement. One laser beam excites a group of atoms to the 41d state of Rb and another laser beam excites the group to the 49s state. By putting a small electric field on, there is no energy cost for the 41d to transition to 42p if simultaneously a 49s atom transitions to 49p.
In this figure we computed (lines) and measured (symbols) the fraction of atoms that transitioned to the 49p state as a function of time for 6 different separations of the beam. red is overlapping beams, yellow separated by 20 microns, green 30 microns, dark green 40 microns, blue 50 microns, purple infinity. As the beams are separated, the interaction decreases which means the time for making transitions increase and the fraction of atoms that transition decrease.
Five Recent Publications
J.V. Hernandez and F. Robicheaux, “Simulations using echo sequences to observe coherence in a cold Rydberg gas,” J. Phys. B 41, 195301 (2008). PDF (148 kB)
B. Sun and F. Robicheaux, “Numerical study of two-body correlation in a 1D lattice with perfect blockade,” New J. Phys. 10, 045032 (2008). PDF (728 kB)
J.V. Hernandez and F. Robicheaux, “Simulations of a strong van der Waals blockade in a dense ultracold gas,” J. Phys. B 41, 045301 (2008). PDF (336 kB)
M.L. Wall, F. Robicheaux, and R.R. Jones, “Controlling atom motion through the dipole-dipole force,” J. Phys. B 40, 3693 (2007). PDF (410 kB)
J.V. Hernandez and F. Robicheaux, "Coherence conditions for groups of Rydberg atoms," J. Phys. B 39, 4883 (2006). PDF (281 kB)