Directional superradiance in a driven ultracold atomic gas in free-space

Abstract

Ultra-cold atomic systems are among the most promising platforms that have the potential to shed light on the complex behavior of many-body quantum systems. One prominent example is the case of a dense ensemble illuminated by a strong coherent drive while interacting via dipole-dipole interactions. Despite being subjected to intense investigations, this system retains many open questions. A recent experiment carried out in a pencil-shaped geometry reported measurements that seemed consistent with the emergence of strong collective effects in the form of a ``superradiant'' phase transition in free space, when looking at the light emission properties in the forward direction. Motivated by the experimental observations, we carry out a systematic theoretical analysis of the system's steady-state properties as a function of the driving strength and atom number, $N$. We observe signatures of collective effects in the weak drive regime, which disappear with increasing drive strength as the system evolves into a single-particle-like mixed state comprised of randomly aligned dipoles. Although the steady-state features some similarities to the reported superradiant to normal non-equilibrium transition, also known as cooperative resonance fluorescence, we observe significant qualitative and quantitative differences, including a different scaling of the critical drive parameter (from $N$ to $\sqrt{N}$). We validate the applicability of a mean-field treatment to capture the steady-state dynamics under currently accessible conditions. Furthermore, we develop a simple theoretical model that explains the scaling properties by accounting for interaction-induced inhomogeneous effects and spontaneous emission, which are intrinsic features of interacting disordered arrays in free space.

25 pages, 19 figures

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• Emergence of an Epsilon-Near-Zero Medium from Microscopic Atomic Principles
• Thermodynamics of coupled time crystals with an application to energy storage
• Phase-Space Methods for Many-Body Quantum Optics
• Quantum correlated steady states under competing collective and individual decay
• Photon-resolved Floquet theory approach to spectroscopic quantum sensing
• Parameter estimation with one- and two-time measurements on the emission field of the boundary time crystal
• Initiation of Superradiance from Different Collective-Spin States