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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Excitation of Rydberg Atoms in a Bose-Einstein Condensate

R. Löw, R. Heidemann, U. Raitzsch, Vera Bendkowsky, Björn Butscher, Tilman Pfau

Published Jun 1, 2007• 0 citations
Publisher

Abstract

The authors performed experiments on the coherent dynamics of Rydberg excitation in thermal clouds of87Rb for a wide range of atomic densities, single-atom Rabi-frequencies and excitation times in a specialized vacuum chamber. We further observed a clear signature of the phase transition to a Bose-Einstein condensate in the fraction of excited Rydberg atoms when cooling the thermal cloud below Tc. In our experiments the sample size is bigger than the so called blockade radius, which is defined as the interatomic distance where the interaction energy becomes equal to the power broadened linewidth of the excitation.

DOI
10.1109/cleoe-iqec.2007.4386734

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•Jan 1, 2007•2 citations
The authors performed experiments on the coherent dynamics of Rydberg excitation in thermal clouds of87Rb for a wide range of atomic densities, single-atom Rabi-frequencies and excitation times in a specialized vacuum chamber. We further observed a clear signature of the phase transition to a Bose-Einstein condensate in the fraction of excited Rydberg atoms when cooling the thermal cloud below Tc. In our experiments the sample size is bigger than the so called blockade radius, which is defined as the interatomic distance where the interaction energy becomes equal to the power broadened linewidth of the excitation.
Rydberg atoms provide a wide range of possibilities to tailor interactions in a quantum gas. Here, we report on Rydberg excitation of Bose-Einstein condensed 87Rb atoms. The Rydberg fraction was investigated for various excitation times and temperatures above and below the condensation temperature. The excitation is locally blocked by the van der Waals interaction between Rydberg atoms to a density-dependent limit. Therefore, the abrupt change of the thermal atomic density distribution to the characteristic bimodal distribution upon condensation could be observed in the Rydberg fraction. The observed features are reproduced by a simulation based on local collective Rydberg excitations.
The excitation to high-lying Rydberg states is investigated in a sample of 87-Rb atoms undergoing the Bose-Einstein condensation phase transition, in the limit that the spatial dimensions of the condensed cloud are smaller than the (Wan-der-Waals) dipole blockade radius. We measure the number of excitations by single ion counting as a function of the dimensions of the condensed and thermal fraction. We observed a distinct decrease in the number of excitations due to the change of the atomic spatial density distribution, from the thermal cloud, through a bimodal distribution, to the condensed cloud. When only the condensed part is present, the average number of excitations measured levels off at around one which is compatible with having a single collective excitation in the condensed cloud.
We observe phononlike excitations of a Bose-Einstein condensate (BEC) in a dilute atomic gas. 87{}^{87}87Rb atoms are optically trapped and precooled, loaded into a magnetic trap, and then evaporatively cooled through the BEC phase transition to form a condensate. We excite the condensate by applying an inhomogeneous oscillatory force with adjustable frequency and symmetry. We have observed modes with different angular momenta and different energies and have studied how their characteristics depend on interaction energy. We find that the condensate excitations persist longer than their counterparts in uncondensed clouds.
Abstract Recent experiments with Bose–Einstein condensates have entered a regime in which thousands of ground-state condensate atoms fill the Rydberg-electron orbit. After the excitation of a single atom into a highly excited Rydberg state, scattering off the Rydberg electron sets ground-state atoms into motion, such that one can study the quantum-many-body dynamics of atoms moving within the Rydberg atom. Here we study this many-body dynamics using Gross–Pitaevskii and truncated Wigner theory. Our simulations focus in particular on the scenario of multiple sequential Rydberg excitations on the same rubidium condensate which has become the standard tool to observe quantum impurity dynamics in Rydberg experiments. We investigate to what extent such experiments can be sensitive to details in the electron–atom interaction potential, such as the rapid radial modulation of the Rydberg molecular potential, or p-wave shape resonance. We demonstrate that both effects are crucial for the initial condensate response within the Rydberg orbit, but become less relevant for the density waves emerging outside the Rydberg excitation region at later times. Finally we explore the local dynamics of condensate heating. We find that it provides only minor corrections to the mean-field dynamics. Combining all these insights, our results suggest Bose–Einstein condensates as a viable platform for the in situ and real time interrogation of ultra-cold chemistry dynamics involving Rydberg states.
This thesis features fundamental experiments on the coherence properties of Bose-Einstein condensates (BEC) and atom lasers, made possible by a newly developed apparatus combining a Bose-Einstein condensate with an ultrahigh nesse optical cavity.Using the cavity as a single atom detector, atoms extracted from a trapped cloud are detected with unprecedented sensitivity, and the rst time-resolved counting of single atoms from a quantum degenerate source was demonstrated.O-diagonal long-range order is a dening property of a BEC and equivalent to longrange phase coherence.Its formation is a complex process accompanied by spontaneous symmetry breaking.We witnessed this formation in an ultracold cloud of 87 Rb atoms crossing the phase transition of BEC.Two overlapping atomic beams originating from dierent positions inside the condensate propagated downwards into the cavity.They showed a matter wave interference pattern with a visibility determined by the coherence of the probed regions.The modulation of the ux of atoms, recorded with single-atom resolution, yields the visibility, whereas its mean value gives information about the density of the sample.By shock cooling, a cloud of atoms was prepared in a highly nonequilibrium state slightly above the phase transition.While the ensemble subsequently relaxed, it crossed the phase transition into an equilibrium with a small condensate fraction.Employing our minimally invasive measurement method, the growth of both the rst-order coherence and the density of the sample was tracked in real time while the system crossed the phase transition.The growth of the coherence was found to start later, proceed faster and nish earlier than the increase in density.The coherent region increased at one fth of the speed of sound.Employing time-resolved single atom counting with the cavity detector, the full counting statistics of an atom laser beam was recorded.In a Hanbury Brown and Twiss type experiment, the second-order correlation function was found to be constant g (2) (τ ) = 1.00±0.01,proving the atom laser's second-order coherence and the absence of excess intensity uctuations that were observed in a pseudothermal beam.An analysis of the time interval distributions of atoms in the beam conrmed these ndings.The iii