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arXiv:0803.1132 · Physical Review A

Dynamics of low-density ultracold Rydberg gases

J. O. Day, Erik Brekke, Thad G Walker

Published May 23, 2008• 47 citations
arXivHTMLPDF

Abstract

Population dynamics in weakly excited clouds of ultracold $^{87}\text{R}\text{b}$ Rydberg atoms were studied by means of trap loss, fluorescence detection, and state-dependent stimulated emission. Rydberg atoms were excited to various $nl$ Rydberg states via continuous two-photon excitation from a magneto-optical trap. A stimulated emission probe laser was then used to bring the Rydberg atoms down to the $6{P}_{3/2}$ state, allowing state-dependent detection of the Rydberg atoms. Measurements of trap loss and fluorescent emission reveal information about the evolution of the Rydberg populations. In particular, population in the initial Rydberg state quickly transfers to other Rydberg states by a noncollisional mechanism, likely superradiant emission. The trap-loss measurements are consistent with black-body ionization as the dominant loss mechanism.

DOI
10.1103/physreva.77.052712

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Population dynamics in weakly excited clouds of ultracold 87Rb^{87}\text{R}\text{b}87Rb Rydberg atoms were studied by means of trap loss, fluorescence detection, and state-dependent stimulated emission. Rydberg atoms were excited to various nlnlnl Rydberg states via continuous two-photon excitation from a magneto-optical trap. A stimulated emission probe laser was then used to bring the Rydberg atoms down to the 6P3/26{P}_{3/2}6P3/2​ state, allowing state-dependent detection of the Rydberg atoms. Measurements of trap loss and fluorescent emission reveal information about the evolution of the Rydberg populations. In particular, population in the initial Rydberg state quickly transfers to other Rydberg states by a noncollisional mechanism, likely superradiant emission. The trap-loss measurements are consistent with black-body ionization as the dominant loss mechanism.
We have studied the decay of Rydberg excitations in a cold Rb gas. A 10 ns, pulsed-dye-amplified diode laser excites Rb atoms at $70\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in a magneto-optical trap to nsnsns or npnpnp Rydberg states with principal quantum numbers $26\ensuremath{\le}n\ensuremath{\le}40$. Time-delayed state-selective field ionization is used to directly monitor the population in the initial and neighboring Rydberg levels. The measured time dependence of the Rydberg population is well described by numerical simulations which consider only spontaneous emission and population transfer by blackbody radiation. No evidence for collective decay is found at atom densities up to $3\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. This result is in contrast to a previous study [Wang et al., Phys. Rev. A 75, 033802 (2007)], in which superradiant decay was theoretically predicted and experimentally inferred for atom density and laser focal volume conditions very similar to those considered here. Suppression of collective emission is likely due to variations in transition energies within the atom sample, dominated by inhomogeneities in dipole-dipole exchange interactions for initial sss states, or by a combination of dipole-dipole and electric field inhomogeneities for the initial ppp states.
We report the measurement of the Rydberg excitation spectrum by two-photon process in ultracold 40K^{40}\mathrm{K}40K Fermi gases. Two different methods are employed to measure the Rydberg excitation spectrum, depending on the power of the probe laser. One scheme is to reduce atomic losses by means of electromagnetically induced transparency. The other is to enhance the atomic losses by spontaneous avalanche ionization due to the strong Rydberg-Rydberg interactions. We verify the consistency of both of the methods. The highest Rydberg states detectable in our experiment are limited to $n\ensuremath{\le}62$ due to the competition between the long Rydberg blockade effective range and the limited atomic cloud size.
We report on the real-time detection of internal-state dynamics of cold Rubidium atoms being excited from the ground state to the 30D5/230D_{5/2}30D5/2​ Rydberg state via two-photon excitation in a ladder-type system. The atoms are first trapped and then released from a magneto-optic trap (MOT) positioned inside the mode of a low-finesse cavity. Then, Rydberg excitation is performed by a simultaneous pulse from two counter-propagating laser beams transverse to the cavity axis. This excitation changes the number of atoms able to absorb or emit cavity photons; thus, changing the collective atom-cavity coupling. We detect this by weakly driving the cavity and collecting the light transmitted through it with an avalanche photodiode (APD). We are able to observe not only the 2-photon excitation dynamics, but also the transfer of population out of the Rydberg state due to black-body radiation (BBR) and the decay back to the ground state.
Abstract Understanding Coulomb driven many-body dynamics in ultracold atomic systems far from equilibrium remains an open challenge, particularly when ultrafast excitation channels create competing pathways toward Rydberg gases or ultracold plasmas. Here, we investigate the many-body dynamics in a 87 Rb Bose-Einstein condensate after exposure to a single femtosecond laser pulse. By tuning the laser wavelength across the two-photon ionization threshold, we can control the initial state that is either dominated by free electrons and leads to an ultracold plasma or dominated by electrons in excited states which leads to a dense Rydberg gas. The large bandwidth enables overcoming the Rydberg blockade that limits the excitation density for narrow-linewidth lasers. We directly measure the kinetic energy of the released electrons and compare the final distribution of free, bound and plasma electrons to molecular dynamics simulations where the electrons are modeled as individual particles including collisional ionization and recombination processes. We find very good agreement between the simulated electron distribution and the experimental observation. We identify charge imbalance as main driver for the decay of a dense Rydberg gas into an ultracold plasma.
We present an all-optical protocol for detecting population in a selected Rydberg state of alkali-metal atoms. The detection scheme is based on the interaction of an ensemble of ultracold atoms with two laser pulses: one weak probe pulse which is resonant with the transition between the ground state and the first excited state, and a pulse with high intensity which couples the first excited state to the selected Rydberg state. We show that by monitoring the absorption signal of the probe laser over time, one can deduce the initial population of the Rydberg state. Furthermore, it is shown that---for suitable experimental conditions---the dynamical absorption curve contains information on the initial coherence between the ground state and the selected Rydberg state. We present the results of a proof-of-principle measurement performed on a cold gas of 87Rb^{87}\mathrm{Rb}87Rb atoms. The method is expected to find application in quantum computing protocols based on Rydberg atoms.