Superradiant Detection of Microscopic Optical Dipolar Interactions

Lingjing Ji, Yizun He, Qingnan Cai, Zhening Fang, Yuzhuo Wang, Liyang Qiu, Lei Zhou, Saijun Wu, Stefano Grava, and Darrick E. Chang

Phys. Rev. Lett. 131, 253602 – Published 20 December 2023

Abstract

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.

Received 18 March 2021
Revised 16 August 2023
Accepted 31 October 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.253602

Physics Subject Headings (PhySH)

Coherent control Long-range interactions Quantum description of light-matter interaction Van der Waals interaction

Atomic ensemble

Dicke model First-principles calculations Time-resolved light scattering spectroscopy

Atomic, Molecular & OpticalQuantum Information, Science & TechnologyNonlinear Dynamics

Authors & Affiliations

Lingjing Ji^*,†, Yizun He ^*, Qingnan Cai, Zhening Fang , Yuzhuo Wang, Liyang Qiu , Lei Zhou^‡, and Saijun Wu ^§

Department of Physics, State Key Laboratory of Surface Physics and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Fudan University, Shanghai 200433, China

Stefano Grava ^* and Darrick E. Chang^∥

ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain and ICREA-Institució Catalana de Recerca i Estudis Avançats, 08015 Barcelona, Spain

^*These authors contributed equally to this work.
^†ljji17@fudan.edu.cn
^‡phzhou@fudan.edu.cn
^§saijunwu@fudan.edu.cn
^∥darrick.chang@icfo.eu

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Vol. 131, Iss. 25 — 22 December 2023

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Images

Figure 1
Probing optical spin wave relaxation in a random gas. (a),(b) illustrate the spin-wave order initiated in a Gaussian distributed random 2-level gas for $| k | = ω_{e g} / c$ and $| k^{'} | = 2.9 ω_{e g} / c$ , respectively. The corresponding electric fields $| Re (ϵ_{k} (r)) |$ , calculated over a two-dimensional cut at the sample center, are simulated with the coupled dipole model (CDM) and plotted in (c),(d). See Ref. [51] for details of the simulation. (e) and (f) outline the general spin-wave control scheme and the timing sequence in this work [51], respectively, to unveil microscopic interaction by a reversible suppression of the collective damping with coherent spin-wave control.
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Figure 2
Superradiant dynamics for phase-matched $S^{+} (k)$ spin-wave excitation. Time-dependent fluorescence counts are histogrammed into intensity data $I_{k} (t)$ in (a) and (b). Blue solid (dashed) curves are predictions by CDM (MBE). Red solid (dashed) curves are the expected nearly exponential decay dynamics of the spin wave survival ratio, $O_{k} (t) \approx e^{- (1 + \bar{OD} (\hat{k}) / 4) Γ_{e} t}$ obtained by CDM (MBE) for $| k | = ω_{e g} / c$ . The gray dashed lines indicate the $e^{- Γ_{e} t}$ spontaneous emission of an isolated atom. The insets are absorption images of the type A, C samples [51]. Red arrows highlight the spin-wave $k$ direction along which $I_{k} (t)$ is collected.
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Figure 3
Decay of the $| k^{'} | \neq ω_{e g} / c$ spin wave. (a)–(c): Superradiance $I_{k} (t)$ during the full control sequence [Fig. 1] with $T_{i} = [1.2, 2.5, 25.8, 27.0] ns$ (red, purple, blue, and yellow curves), for three typical, fairly dense samples. Exponential fits to the recalled $I_{k} (t)$ curves leads to $Γ_{k} \approx {1.9, 2.8, 3.2} Γ_{e}$ for (a)–(c) respectively. A black dashed line with $- Γ_{e}$ slope is added as a reference to compare with the peak ${\bar{I}}_{k} (T_{i})$ decay. Substantial deviation of $I_{k} (T_{i})$ from the MBE-predicted line is highlighted with an arrow in (c). The atomic distribution is inferred from strong-exposure absorption images [51], as in the insets (white scale $bar = 20 μ m$ ). (d) The $Γ_{k^{'}}$ estimated from the four ${\bar{I}}_{k} (T_{i})$ peaks [51] is plotted vs the estimated dimensionless peak density parameter $η_{0} = ρ_{0} λ_{e g}^{3}$ in different colors. The $y$ -error bars reflect the statistical and systemmatic uncertainties. The $x$ -error bars $Δ η_{0} = \pm 25 % η_{0}$ are associated with uncertainties in the sample preparation and characterization. The solid black line gives the prediction from the dipolar dephasing theory of Eq. (3), with $\bar{η} = η_{0} / 2 \sqrt{2}$ as the mean density of a Gaussian distribution. The horizontal dashed line, $Γ_{k^{'}} = Γ_{e}$ , based on MBE, ignores microscopic effects associated with atomic granularity.
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