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Physical Chemistry Chemical Physics

Delocalized excitons and interaction effects in extremely dilute thermal ensembles

Lukas Bruder, Alexander Eisfeld, Ulrich Bangert, Marcel Binz, Max Jakob, Daniel Uhl, Markus Schulz-Weiling, Edward R. Grant, Frank Stienkemeier

Published Nov 16, 2018• 63 citations
Publisher

Abstract

Long-range interparticle interactions are revealed in extremely dilute thermal atomic ensembles using highly sensitive nonlinear femtosecond spectroscopy. Delocalized excitons are detected in the atomic systems at particle densities where the mean interatomic distance (>10 μm) is much greater than the laser wavelength and multi-particle coherences should destructively interfere over the ensemble average. With a combined experimental and theoretical analysis, we identify an effective interaction mechanism, presumably of dipolar nature, as the origin of the excitonic signals. Our study implies that even in highly-dilute thermal atom ensembles, significant transition dipole-dipole interaction networks may form that require advanced modeling beyond the nearest neighbor approximation to quantitatively capture the details of their many-body properties.

DOI
10.1039/c8cp05851b

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Long-range interparticle interactions are revealed in extremely dilute thermal atomic ensembles using highly sensitive nonlinear femtosecond spectroscopy. Delocalized excitons are detected in the atomic systems at particle densities where the mean interatomic distance (>10 μm) is much greater than the laser wavelength and multi-particle coherences should destructively interfere over the ensemble average. With a combined experimental and theoretical analysis, we identify an effective interaction mechanism, presumably of dipolar nature, as the origin of the excitonic signals. Our study implies that even in highly-dilute thermal atom ensembles, significant transition dipole-dipole interaction networks may form that require advanced modeling beyond the nearest neighbor approximation to quantitatively capture the details of their many-body properties.
Abstract Manifestations of dipole–dipole interactions in dilute thermal gases are difficult to sense because of strong inhomogeneous broadening. Recent experiments reported signatures of such interactions in fluorescence detection-based measurements of multiple quantum coherence (MQC) signals, with many characteristic features hitherto unexplained. We develop an original open quantum systems theory of MQC in dilute thermal gases, which allows us to resolve this conundrum. Our theory accounts for the vector character of the atomic dipoles as well as for driving laser pulses of arbitrary strength, includes the far-field coupling between the dipoles, which prevails in dilute ensembles, and effectively incorporates atomic motion via a disorder average. We show that collective decay processes—which were ignored in previous treatments employing the electrostatic form of dipolar interactions—play a key role in the emergence of MQC signals.
The spectral structure underlying excitonic energy transfer in ultracold Rydberg gases is studied numerically, in the framework of random matrix theory, and via self-consistent diagrammatic techniques. Rydberg gases are made up of randomly distributed, highly polarizable atoms that interact via strong dipolar forces. Dynamics in such a system is fundamentally different from cases in which the interactions are of short range, and is ultimately determined by the spectral and eigenvector structure. In the energy levels' spacing statistics, we find evidence for a critical energy that separates delocalized eigenstates from states that are localized at pairs or clusters of atoms separated by less than the typical nearest-neighbor distance. We argue that the dipole blockade effect in Rydberg gases can be leveraged to manipulate this transition across a wide range: As the blockade radius increases, the relative weight of localized states is reduced. At the same time, the spectral statistics, in particular, the density of states and the nearest-neighbor level-spacing statistics, exhibits a transition from approximately a 1-stable L\'evy to a Gaussian orthogonal ensemble. Deviations from random matrix statistics are shown to stem from correlations between interatomic interaction strengths that lead to an asymmetry of the spectral density and profoundly affect localization properties. We discuss approximations to the self-consistent Matsubara-Toyozawa locator expansion that incorporate these effects.
We investigate the effect of degenerate atomic states on the exciton delocalization of dipole-dipole interacting Rydberg assemblies. Using a frozen gas and regular one-, two-, and three-dimensional lattice arrangements as examples, we see that degeneracies can enhance the delocalization compared to the situation when there is no degeneracy. This enhancement is particularly large in the case of the three-dimensional (3D) random gas, but is absent for 1D arrangements. Using the Zeeman splitting provided by a magnetic field, we controllably lift the degeneracy to study in detail the transition between degenerate and nondegenerate regimes. These observations, although specific to the experimentally clean Rydberg gas, have generic implications for various dipole-interacting systems.
Quantum spectroscopy in solids directly detects nonlinear changes created exclusively by quantum fluctuations of light. So far, it has been realized only by projecting a large set of measurements with a coherent-state laser to a specific quantum-light response. We present two complementary experimental approaches to realize intense and ultrafast thermal-state sources. We investigate the effects of continuous excitation from a superluminescent diode (SLD) as well as an ensemble-averaging technique using phase-modulated pulses. By measuring excitonic nonlinearities in gallium arsenide, we demonstrate that the experimentally realized thermal-state source produces significantly reduced many-body nonlinearities compared to a coherent-state excitation. We also review experimental approaches toward future realization of quantum spectroscopy with thermal states.
Summary form only given. Approaching temperatures near the absolute zero, i.e. the lowest temperature in the whole universe, the atoms develop extreme behaviors, which challenge our understanding. In this extreme regime, the atoms assume exceptional behaviors and form a new type of matter, which is now governed by the laws of quantum mechanics.In this regime, the interactions between many particles dictate the quantum behavior. Highly magnetic atoms, such a s Erbium, feature a novel type of interaction, which i s of long-range nature and isotropic character. This magnetic dipole-dipole interaction (DDI) imprints genuinely novel properties to quantum manyand few-body ultracold systems. The behavior of these systems i s fundamental to understand nature and quantum physics, and reserves u s every day fascinating and counterintuitive surprises. This talk will provide an overview of the fascinating dipolar phenomena from the Innsbruck prospective, ranging from the first-produced quantum gas of Er, to the observation of long-range nearest-neighbor interaction in lattice confined system, and the formation of a quasi self-bound droplet state in analogy with the He case.