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New Journal of Physics

Optimal subradiant spin wave exchange in dipole-coupled atomic ring arrays

Yuxiang Han, Hao Chen, Wenxiao Liu, Jiao-Jiao Xue, Hongrong Li

Published Sep 26, 2023• 3 citations

Abstract

Abstract The subwavelength array of quantum emitters provides an ideal platform for exploring rich many-body dynamics, such as super- and subradiance. In this paper, we explore the dynamics of spin wave exchange between two dipole-coupled atomic ring arrays. Subradiant spin waves lead to low-loss and high efficiency of ring-to-ring transfer. The optimal subradiant spin wave exchange occurs at appropriate separations between coplanar rings, despite the fact that the energy transfer efficiency is monotonically enhanced (in the regime ⩽ λ 0 / 2 ) as the rings’ separation decreases. However, the spin wave will scatter due to the dephasing mechanism of close-by atom pairs, as the separation of two rings is too small. With the increase in the number of atoms on the ring, the subradiant shielding effect also strengthens, leading to a shorter distance for the transfer of spin waves. We investigate the rotation of one of the rings and find that the optimal spin wave exchange corresponds to the scenario where the line connecting the two nearest atoms of the two rings aligns with the center of the circle. Moreover, we study the influence of transition dipole moment orientations on the effective interaction between two atomic rings. We observe that there is a critical point where the effective interaction strength changes dramatically owing to the cooperative effect of the subwavelength atomic array. We believe that our results could be important for quantum information processing based on atomic arrays.

DOI: 10.1088/1367-2630/acfd55

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Abstract The subwavelength array of quantum emitters provides an ideal platform for exploring rich many-body dynamics, such as super- and subradiance. In this paper, we explore the dynamics of spin wave exchange between two dipole-coupled atomic ring arrays. Subradiant spin waves lead to low-loss and high efficiency of ring-to-ring transfer. The optimal subradiant spin wave exchange occurs at appropriate separations between coplanar rings, despite the fact that the energy transfer efficiency is monotonically enhanced (in the regime ⩽ λ 0 / 2 ) as the rings’ separation decreases. However, the spin wave will scatter due to the dephasing mechanism of close-by atom pairs, as the separation of two rings is too small. With the increase in the number of atoms on the ring, the subradiant shielding effect also strengthens, leading to a shorter distance for the transfer of spin waves. We investigate the rotation of one of the rings and find that the optimal spin wave exchange corresponds to the scenario where the line connecting the two nearest atoms of the two rings aligns with the center of the circle. Moreover, we study the influence of transition dipole moment orientations on the effective interaction between two atomic rings. We observe that there is a critical point where the effective interaction strength changes dramatically owing to the cooperative effect of the subwavelength atomic array. We believe that our results could be important for quantum information processing based on atomic arrays.

Collective excitations in ordered subwavelength atomic arrays can exhibit strongly suppressed radiative decay due to interference between light scattered by neighboring emitters. These so-called subradiant states make these systems a promising platform for storing and manipulating photonic excitations. The external geometry of the array, combined with dynamical control of the atomic dipole orientation, enables localized trapping and coherent transport of these subradiant excitations. Here, we theoretically demonstrate these capabilities in ring-shaped atomic arrays. Specifically, we show adiabatic transport of a localized excitation around a single ring, coherent transfer of a single excitation between two neighboring rings with geometry-controlled selectivity, and interaction-induced conditional phase shifts between two simultaneously trapped excitations in neighboring rings. The latter can be interpreted as effective controlled-phase operations between stored excitations. Together, these results demonstrate the potential of ordered atomic arrays as a platform for coherent photonic quantum information processing with dissipation-protected collective excitations.

Dipole-coupled subwavelength quantum emitter arrays respond cooperatively to external light fields as they may host collective delocalized excitations (a form of excitons) with super- or subradiant character. Deeply subwavelength separations typically occur in molecular ensembles, where in addition to photon-electron interactions, electron-vibron couplings and vibrational relaxation processes play an important role. We provide analytical and numerical results on the modification of super- and subradiance in molecular rings of dipoles including excitations of the vibrational degrees of freedom. While vibrations are typically considered detrimental to coherent dynamics, we show that molecular dimers or rings can be operated as platforms for the preparation of long-lived dark superposition states aided by vibrational relaxation. In closed ring configurations, we extend previous predictions for the generation of coherent light from ideal quantum emitters to molecular emitters, quantifying the role of vibronic coupling onto the output intensity and coherence.

A ring of dipole-coupled quantum emitters positioned at subwavelength distances possesses only a few radiant but many subradiant collective excitations with lifetimes growing exponentially with the atom number. These exhibit a three-dimensionally confined spatial radiation field pattern and form nanoscale high-

Q

optical resonators. Tailoring size, orientation, and distance between two rings allows for increasing the ratio of coherent ring-to-ring energy transfer versus free-space emission by orders of magnitude. In particular, the more dark and delocalized over several sites are excitons, the higher is their fidelity for transport to a second ring.

When quantum emitters couple indistinguishably to light, they can synchronize into a collective light matter system with radiative properties profoundly different from those of independent particles. To date, the resulting collective effects have largely been confined to point like or homogeneous ensembles. Here, we open access to a qualitatively new collective regime by realizing geometrically ordered, spatially extended atom arrays with subwavelength spacing. This establishes a fundamentally new platform in which collective emission is no longer confined to a single Dicke mode but instead emerges from an ordered network of photon mediated interactions. We find that 2D atom arrays undergo strong super and subradiant emission. Despite subwavelength spacing, we achieve site resolved imaging and directly observe the buildup of spatial correlations, demonstrating the transformation of cooperative decay into a strongly correlated many-body process. We observe extensive scaling of superradiance, uncover superradiant revivals, and reveal the ferromagnetic nature of superradiance and the antiferromagnetic nature of subradiance. Our results realize a novel programmable platform for exploring and utilizing dissipative many-body quantum physics, opening new possibilities for photon capture, storage, and atom photon entanglement.

In recent years, there has been an increasing interest in subradiance in ordered atomic arrays, both for its rich phenomenology and its potential importance in realizing robust quantum optical applications. However, the weak interaction of subradiant states with light makes it challenging to observe and measure these states. Recently, an experimental technique was demonstrated, which allows for efficient and coherent manipulation of the wavevector associated with atomic spin- wave excitations. We propose that this can be a useful tool to unlock subradiant phenomena. In particular, we theoretically analyze how this technique can be used to map the subradiant dynamics of a highly excited atomic array onto the properties of the emitted light.

Subradiance is a phenomenon where coupled emitters radiate light at a slower rate than independent ones. While its observation was first reported in disordered cold atom clouds, ordered subwavelength arrays of emitters have emerged as promising platforms to design highly cooperative optical properties based on dipolar interactions. In this work we characterize the eigenmodes of 2D and 3D regular arrays, using a method which can be used for both infinite and very large systems. In particular, we show how finite-size effects impact the lifetimes of these large arrays. Our results may have interesting applications for quantum memories and topological effects in ordered atomic arrays.