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arXiv:2407.21764

Energy Transport Among Highly-Polarized Atoms

Catherine D. Opsahl, Jiang, Yuan, Samantha A. Grubb, Alan T. Okinaka, Nicolaus A. Chlanda, Hannah S. Conley, Aidan Kirk, Spielman, Sarah E., Thomas J. Carroll, Michael W. Noel

Published Jul 31, 2024• 0 citations
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Abstract

We measure the transport of energy among the internal states of ultracold rubidium Rydberg atoms coupled by dipole-dipole exchange. In a magneto-optical trap, a static electric field of a few V/cm shifts the energy levels of the atoms. For a particular principal quantum number, $n$, the angular momentum eigenstates $\ell > 4$ are nearly degenerate at zero electric field. At nonzero field, a manifold of equally spaced clusters form a ladder with each rung consisting of a set of closely spaced $m$ energy eigenstates. We excite Rydberg atoms to energy levels near the center of the manifold and allow them to exchange energy via resonant dipole-dipole interactions. We measure the time evolution as energy spreads away from the center of the manifold, which reveals that the system may fail to thermalize for long interaction times. A computational model that includes only a few essential features of the system qualitatively agrees with this result.

DOI
10.48550/arxiv.2407.21764

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We measure the transport of energy among the internal states of ultracold rubidium Rydberg atoms coupled by dipole-dipole exchange. In a magneto-optical trap, a static electric field of a few V/cm shifts the energy levels of the atoms. For a particular principal quantum number, nnn, the angular momentum eigenstates ℓ>4\ell > 4ℓ>4 are nearly degenerate at zero electric field. At nonzero field, a manifold of equally spaced clusters form a ladder with each rung consisting of a set of closely spaced mmm energy eigenstates. We excite Rydberg atoms to energy levels near the center of the manifold and allow them to exchange energy via resonant dipole-dipole interactions. We measure the time evolution as energy spreads away from the center of the manifold, which reveals that the system may fail to thermalize for long interaction times. A computational model that includes only a few essential features of the system qualitatively agrees with this result.
The long-range, anisotropic nature of the interaction among atoms in an ultracold dipolar gas leads to a rich array of possibilities for studying many-body physics. In this work, an ultracold gas of highly excited atoms is used to study energy transport due to the long-range dipole-dipole interaction. A technique is developed to measure both the internal energy states of the interacting Rydberg atoms and their positions in space. This technique is demonstrated by observing energy exchange between two spatially separated groups of Rydberg atoms excited to two different internal states. Simulations confirm the general features of the energy transport in this system and highlight subtleties associated with the homogeneity of the electric field used in this experiment.
An isolated system of ultracold Rydberg atoms can come to equilibrium by exchanging energy through dipole-dipole interactions. In our experiment, a static electric field of a few V/cm shifts the energy levels of the atoms, so that the energy levels are nearly degenerate at zero field and fan out with increasing field to form a manifold. We excite atoms to energy levels near the center of the manifold, where the spacing is nearly harmonic. We allow them to interact for a few microseconds, by which time the population of each energy level has reached a steady state. The population is then measured at various densities and compared to simulation results. We use the notion of dynamical typicality to predict the equilibrium configuration and show that the system fails to thermalize.
The dipole-dipole interaction strongly couples ultracold highly excited atoms, known as Rydberg atoms, and is the dominant mechanism for energy exchange among such atoms on microsecond time scales. I present the results of experiments that investigate this interaction using rubidium atoms excited to Rydberg states with an all-diode laser system. In the first of these, high resolution dipole-dipole interaction spectra for different magnetic sublevels of the 32d5/2 state are collected and analyzed. In the second experiment, ultracold rubidium atoms are implusively excited to coherent superpositions of three closely spaced |mj|sublevels of the 32d5/2 state in a weak electric field. The magnitude of the field sets the energy level spacing between these states, and hence the frequency of the phase dependent oscillations resulting from interference in resonant energy exchange processes. In the third experiment, the dipole-dipole interaction induced resonant energy exchange between columns of ultracold rubidium Rydberg atoms is imaged using a new technique involving selective field ionization with a spatially sensitive ion detector.
Ultracold Rydberg atoms in a static electric field can exchange energy via the dipole-dipole interaction. The Stark effect shifts the energy levels of the atoms which tunes the energy exchange into resonance at specific values of the electric field (F\"orster resonances). We excite rubidium atoms to Rydberg states by focusing either a 480 nm beam from a tunable dye laser or a pair of diode lasers into a magneto-optical trap. The trap lies at the center of a configuration of electrodes. We scan the electric field by controlling the voltage on the electrodes while measuring the fraction of atoms that interact. Dipole-dipole interaction spectra are presented for initially excited rubidium ndndnd states for n=31n=31n=31 to 46 and for four different pairs of initially excited rubidium nsnsns states. We also present the dipole-dipole interaction spectra for individual rubidium 32ddd (j,mj)(j,\phantom{\rule{0.16em}{0ex}}{m}_{j})(j,mj​) fine structure levels that have been selectively excited. The data are compared to calculated spectra.
Systems are known to thermalize, or reach equilibrium, with the passing of time. However, when this does not occur, the quantum states of the system may be localized. Localized states carry information of the system’s initial state that is typically lost during the process of thermalization. We aim to study thermalization and localization in systems of rubidium Rydberg atoms, and to further understand these phenomena via simulation while we are developing a more refined experiment. These atoms exchange energy via tuned resonances while contained within a magneto optical trap. This process can be categorized as either a two, three, or four body interaction; localization is only expected to occur in the latter two. As the atoms transfer energy via dipole-dipole interactions, the population of atoms occupying different states shifts. Some atoms are shifted to a measurable state, and each trial of the experiment lasts for only several microseconds as data is collected. The energy transport within the system is simulated on a supercomputer in order to produce a model for the experimental data, while also allowing us to closely analyze parameters that are not accessible through experiment. From the model, we hope to learn whether the system thermalizes or localizes. We present initial simulation results exploring the energy exchange dynamics in a Rubidium many-body system.
We use dynamical quantum typicality to predict the thermal equilibrium state of ultracold Rydberg atoms exchanging energy via long-range dipole-dipole interactions. We excite atoms to the center of a manifold of nearly-harmonically spaced clusters of Stark energy levels and then allow them to equilibrate. Comparing the equilibrium state to our thermal prediction across a range of Rydberg densities, we find that this system generally fails to thermalize, though it approaches the thermal state at the highest tested density. This is the first direct comparison of a dynamical quantum typicality calculation to experiment.