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Rydberg Gas


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.



F. Robicheaux, M.M. Goforth, and M.A. Phillips, "Simulation of prompt many-body ionization in a frozen Rydberg gas," Phys. Rev. A 90, 022712 (2014). PDF (254 kB)

An experiment from Tom Gallagher's group strongly suggested that a gas of Rydberg atoms would ionize faster than could be explained by pairs of atoms at the appropriate separation. We performed classical calculations to see whether the number of atoms in a simulation was important or mainly the average density. We performed calculations for 1D, 2D, and 3D arrays of atoms.



This image shows the prompt ionization probability as a function of the scale length for atoms in a cubic array (solid line is 8 atoms and dotted line is 27 atoms). This clearly shows that there is more ionization when more atoms are present even when the distance between the atoms is the same. However, when converted to density, the effect is not large. For example, 1/4 ionization results for~3.1 for 8 atoms and ~3.4 for 27 atoms.


The main question is what happens when the atoms are randomly distributed as in a gas.

This image shows the prompt ionization probability as a function of the scale length for atoms randomly distributed inside a cube (solid line is 8 atoms and dotted line is 27 atoms). In this case, there is very little difference for different number of atoms. Note that the scale size is much larger than above which indicates ionization occurs at much smaller density than for a regular array. This is because the random placement of atoms allows for close pairs which quickly ionize.




F. Robicheaux and N.M. Gill, "Effect of random positions for coherent dipole transport," Phys. Rev. A 89, 053429 (2014). PDF (831 kB)

In this calculation, we studied how the random positions for a gas of Rydberg atoms would affect the hopping of an exciton due to the dipole-dipole interaction. This system should show effects like Anderson localization.



This image shows the long time probability for finding an exciton a distance r from its original atom for 5 different amounts of randomness (from 0.1 to 0.5); the atoms are placed in a line with random displacements along the line. The + are a fit of the numerical results to a stretch exponential. In the more usual Anderson localization, the probability would be an exponential. As expected, larger randomness leads to stronger localization.



This image shows the long time probability for finding an exciton a distance r from its original atom for 5 different amounts of randomness (from 0.1 to 0.5); the atoms are placed in a square array with random displacements along in the plane. Due to computational limitations, none of the calculations are converged. However, the cases with larger randomness have a larger probability for the exciton to be close to the original placement.



Five Recent Publications

Hui Yu and F. Robicheaux, "Coherent dipole transport in a small grid of Rydberg atoms," Phys. Rev. A 93, 023618 (2016). PDF (387 kB)

F. Robicheaux, M.M. Goforth, and M.A. Phillips, "Simulation of prompt many-body ionization in a frozen Rydberg gas," Phys. Rev. A 90, 022712 (2014). PDF (254 kB)

F. Robicheaux and N.M. Gill, "Effect of random positions for coherent dipole transport," Phys. Rev. A 89, 053429 (2014). PDF (831 kB)

S. Zhang, F. Robicheaux, and M. Saffman, "Magic-wavelength optical traps for Rydberg atoms," Phys. Rev. A 84, 043408 (2011). PDF (820 kB)

F. Robicheaux, "Transfer of a wavepacket between atoms," J. Phys. B 43, 215004 (2010). PDF (249 kB) B. Sun and F. Robicheaux, "

Francis Image

robichf[at]purdue.edu
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