Abstract While typical theories of atom–light interactions treat the atomic medium as being smooth, it is well-known that microscopic optical effects driven by atomic granularity, dipole–dipole interactions, and multiple scattering can lead to important effects. Recently, for example, it was experimentally observed that these ingredients can lead to a fundamental, density-dependent dephasing of optical spin waves in a disordered atomic medium. Here, we go beyond the short-time and dilute limits considered previously, to develop a comprehensive theory of dephasing dynamics for arbitrary times and atomic densities. In particular, we develop a novel, non-perturbative theory based on strong disorder renormalization group (RG), in order to quantitatively predict the dominant role that near-field optical interactions between nearby neighbors has in driving the dephasing process. This theory also enables one to capture the key features of the many-atom dephasing dynamics in terms of an effective single-atom model. These results should shed light on the limits imposed by near-field interactions on quantum optical phenomena in dense atomic media, and illustrate the promise of strong disorder RG as a method of dealing with complex microscopic optical phenomena in such systems.
Abstract While typical theories of atom–light interactions treat the atomic medium as being smooth, it is well-known that microscopic optical effects driven by atomic granularity, dipole–dipole interactions, and multiple scattering can lead to important effects. Recently, for example, it was experimentally observed that these ingredients can lead to a fundamental, density-dependent dephasing of optical spin waves in a disordered atomic medium. Here, we go beyond the short-time and dilute limits considered previously, to develop a comprehensive theory of dephasing dynamics for arbitrary times and atomic densities. In particular, we develop a novel, non-perturbative theory based on strong disorder renormalization group (RG), in order to quantitatively predict the dominant role that near-field optical interactions between nearby neighbors has in driving the dephasing process. This theory also enables one to capture the key features of the many-atom dephasing dynamics in terms of an effective single-atom model. These results should shed light on the limits imposed by near-field interactions on quantum optical phenomena in dense atomic media, and illustrate the promise of strong disorder RG as a method of dealing with complex microscopic optical phenomena in such systems.
(English) The interface between light and cold atomic ensembles is a fundamental platform to unravel the quantum world and develop quantum technological applications. Its success relies on the simple idea that the efficiency of such an interface can be collectively enhanced by the use of many atoms. While the interaction between its building blocks, a single photon, and a single atom, is theoretically and experimentally understood, instead, the interaction between light and a macroscopic ensemble of motionless atoms is generically a complex system featuring multiple scattering and many-body dipole interactions. To avoid the complexity, typical theories of atom-light interactions treat the atomic medium as smooth. However, it is well-known that microscopic optical effects driven by atomic granularity can lead to important effects, especially in dense media. These phenomena and their consequences on the performance of applications are not completely understood. To take them into account exactly, Chapter 1 introduces a ``spin model'' for light-matter interaction. The rest of the thesis is then divided into three chapters, which push forward our understanding of the interaction of light with dense atomic media. In Chapter 2 it is argued that because of the overwhelming collective macroscopic response an ensemble can exhibit (well captured by the standard theory), many microscopically-driven effects that have been predicted, have also been challenging to observe so far. An essential step is thus to suppress the macroscopic light propagation, so as to allow the microscopic correlations to build up and to be analyzed in a background-free fashion. To solve this issue, a technique to suppress the macroscopic optical dynamics in free space, which allows to precisely investigate many-body aspects of light-matter interaction, will be presented and demonstrated. In particular, we unravel and precisely characterize a microscopic, density-dependent dipolar dephasing effect that generally limits the lifetime of the optical spin-wave order in ensemble-based atom-light interfaces. In Chapter 3 we will go beyond the short-time and dilute limits considered previously, to develop a comprehensive theory of dephasing dynamics for arbitrary times and atomic densities. In particular, our non-perturbative approach is based on the strong-disorder renormalization group (RG), in order to quantitatively predict the dominant role that near-field optical interactions between nearby neighbors have in driving the dephasing process. This theory also enables one to capture the key features of the many-atom dephasing dynamics in terms of an effective single-atom model. These results should shed light on the limits imposed by near-field interactions on quantum optical phenomena in dense atomic media, and illustrate the promise of strong disorder RG as a method of dealing with complex microscopic optical phenomena in such systems. Chapter 4 tries to answer the question of why ordinary materials exhibit a refractive index of order unity and if the answer can come from an electro-dynamical argument. While textbook theories predict nonphysical values when extrapolated to densities of solids, here, we will evaluate the exact linear optical response of a three-dimensional lattice of two-level atoms, first from the band structure and then from a direct numerical simulation. Interestingly, when multiple scattering of light is exactly taken into account, as a result of perfect interference, it is found that an ideal unity-filled array of atoms can have a refractive index that grows with the density and is furthermore real. This implies that a saturation mechanism for the index should come from the quantum chemistry interactions that arise in real materials. Whether saturation could be circumvented, could lead to novel optical materials with transformative technological potential. (Español) La interfaz entre la luz y conjuntos de átomos fríos es una plataforma fundamental para explorar el mundo cuántico y para el desarrollo de las tecnologías cuánticas. Sus logros están basados en la sencilla idea de que la eficiencia de esta interfaz puede mejorar al utilizar muchos átomos. A pesar de que la interfaz entre sus constituyentes, un único átomo y un único fotón, está experimentalmente y teóricamente entendida, en cambio, la interacción entre la luz y un conjunto macroscópico de átomos inmóviles es un sistema complejo que incluye dispersiones múltiples e interacciones dipolo-dipolo entre muchos cuerpos. Para reducir esta complejidad, las teorías tradicionales tratan el medio atómico como continuo. Sin embargo, es conocido que los efectos ópticos microscópicos, causados por la granularidad de los átomos, pueden dar lugar a consecuencias importantes, sobre todo en el caso de medios densos. Estos fenómenos y sus consecuencias en la eficiencia de las aplicaciones siguen sin entenderse por completo. Para tenerlos en cuenta exactamente, el Capítulo 1 introduce un "modelo de espines" para la interaccíón luz-materia. El resto de la tesis se divide luego en tres capítulos, que impulsan nuestra comprensión de la interacción de la luz con medios atómicos densos. En el Capítulo 2 se argumenta que debido a la enorme respuesta macroscópica colectiva de un conjunto (captada correctamente por la teoría estándar), muchos de los efectos microscópicos que se han predicho también han sido difíciles de observar hasta ahora. El reto, por lo tanto, es suprimir la propagación de la luz macroscópica, para permitir que las correlaciones microscópicas se acumulen y se analicen sin ese fondo. Para resolver este problema, se presentará y demostrará una técnica para suprimir la dinámica óptica macroscópica en el espacio libre, que permite investigar con precisión los aspectos de muchos cuerpos de la interacción luz-materia. En particular, caracterizamos con precisión un efecto de desfase dipolar microscópico, que depende de la densidad, y limita la vida útil del orden de onda de espín óptico en las interfaces átomo-luz basadas en conjuntos. En el Capítulo 3 iremos más allá de los límites diluidos y de tiempo corto considerados anteriormente, para desarrollar una teoría completa de la dinámica de desfase para tiempos y densidades atómicas arbitrarias. En particular, nuestro método no-perturbativo se basa en el grupo de renormalización (RG) del desorden fuerte, para predecir cuantitativamente el papel dominante que tienen las interacciones ópticas de campo cercano entre átomos vecinos en el proceso de desfase. Estos resultados deberían aclarar los límites impuestos por las interacciones de campo cercano en los fenómenos ópticos cuánticos en medios atómicos densos e ilustrar la promesa del RG como método para tratar los fenómenos ópticos microscópicos complejos en estos sistemas. El Capítulo 4 trata de responder a la pregunta de por qué los materiales ordinarios exhiben un índice de refracción de orden uno y si la respuesta puede proceder de un argumento electrodinámico. Si bien las teorías de los libros de texto predicen valores no físicos cuando se extrapolan a densidades de sólidos, aquí evaluaremos la respuesta óptica lineal exacta de una red tridimensional de átomos de dos niveles, primero a partir de la estructura de bandas y luego a partir de una simulación numérica directa. Curiosamente, cuando se tiene en cuenta de manera exacta la dispersión múltiple de la luz, como resultado de la interferencia perfecta, se encuentra que una matriz de átomos ideal tiene un índice de refracción que crece con la densidad y, además, es real. Esto implica que un mecanismo de saturación para el índice debería provenir de las interacciones de la química cuántica que surgen en los materiales reales. La posibilidad de evitar la saturación podría dar lugar a nuevos materiales ópticos con un potencial tecnológico transformador.
Quantum light-matter interfaces, based upon ensembles of cold atoms or other quantum emitters, are a vital platform for diverse quantum technologies and the exploration of fundamental quantum phenomena. Most of our understanding and modeling of such systems are based upon macroscopic theories, wherein the atoms are treated as a smooth, quantum polarizable medium. Although it is known that such approaches ignore a number of microscopic details, such as the granularity of atoms, dipole-dipole interactions and multiple scattering of light, the consequences of such effects in practical settings are usually mixed with background macroscopic effects and difficult to quantify. In this work we demonstrate a time-domain method to measure microscopically-driven optical effects in a background-free fashion, by transiently suppressing the macroscopic dynamics. With the method, we reveal a microscopic dipolar dephasing mechanism that generally limits the lifetime of the optical spin-wave order in a random gas. Theoretically, we show the dephasing effect emerges from the strong resonant dipole interaction between close-by atomic pairs.
It is interesting to observe that all optical materials with a positive refractive index have a value of index that is of order unity. Surprisingly, though, a deep understanding of the mechanisms that lead to this universal behavior seems to be lacking. Moreover, this observation is difficult to reconcile with the fact that a single isolated atom is known to have a giant optical response, as characterized by a resonant scattering cross section that far exceeds its physical size. Here, we theoretically and numerically investigate the evolution of the optical properties of an ensemble of ideal atoms as a function of density, starting from the dilute gas limit, including the effects of multiple scattering and near-field interactions. Interestingly, despite the giant response of an isolated atom, we find that the maximum index does not indefinitely grow with increasing density but rather reaches a limiting value of n ≈ 1.7. This limit arises purely from electrodynamics, as it occurs at densities far below those where chemical processes become important. We propose an explanation based upon strong-disorder renormalization group theory, in which the near-field interaction combined with random atomic positions results in an inhomogeneous broadening of atomic resonance frequencies. This mechanism ensures that, regardless of the physical atomic density, light at any given frequency only interacts with at most a few near-resonant atoms per cubic wavelength, thus limiting the maximum index attainable. Our work is a promising first step to understand the limits of the refractive index from a bottom-up, atomic physics perspective, and it also introduces the renormalization group as a powerful tool to understand the generally complex problem of multiple scattering of light overall.
It is interesting to observe that all optical materials with a positive refractive index have a value of index that is of order unity.Surprisingly, though, a deep understanding of the mechanisms that lead to this universal behavior seems to be lacking.Moreover, this observation is difficult to reconcile with the fact that a single isolated atom is known to have a giant optical response, as characterized by a resonant scattering cross section that far exceeds its physical size.Here, we theoretically and numerically investigate the evolution of the optical properties of an ensemble of ideal atoms as a function of density, starting from the dilute gas limit, including the effects of multiple scattering and near-field interactions.Interestingly, despite the giant response of an isolated atom, we find that the maximum index does not indefinitely grow with increasing density but rather reaches a limiting value of n ≈ 1.7.This limit arises purely from electrodynamics, as it occurs at densities far below those where chemical processes become important.We propose an explanation based upon strong-disorder renormalization group theory, in which the nearfield interaction combined with random atomic positions results in an inhomogeneous broadening of atomic resonance frequencies.This mechanism ensures that, regardless of the physical atomic density, light at any given frequency only interacts with at most a few near-resonant atoms per cubic wavelength, thus limiting the maximum index attainable.Our work is a promising first step to understand the limits of the refractive index from a bottom-up, atomic physics perspective, and it also introduces the renormalization group as a powerful tool to understand the generally complex problem of multiple scattering of light overall.
The interaction between light and cold atoms is a complex phenomenon potentially featuring many-body resonant dipole interactions. A major obstacle toward exploring these quantum resources of the system is macroscopic light propagation effects, which not only limit the available time for the microscopic correlations to locally build up, but also create a directional, superradiant emission background whose variations can overwhelm the microscopic effects. In this Letter, we demonstrate a method to perform "background-free" detection of the microscopic optical dynamics in a laser-cooled atomic ensemble. This is made possible by transiently suppressing the macroscopic optical propagation over a substantial time, before a recall of superradiance that imprints the effect of the accumulated microscopic dynamics onto an efficiently detectable outgoing field. We apply this technique to unveil and precisely characterize a density-dependent, microscopic dipolar dephasing effect that generally limits the lifetime of optical spin-wave order in ensemble-based atom-light interfaces.
We performed a numerical investigation of the dynamics of the Bloch equations, extended for dense media (media having many atoms within a cubic resonance wavelength), for optical pulses whose duration is much less than an induced-dipole dephasing time. We find a signature of near dipole-dipole interaction which may provide a useful method for validating the first-principles model, for measuring the strength of the interaction, and for coherent pumping. Further, we demonstrate a new and unique optical switching mechanism which may lead to the development of important devices.