⚛️ quantum physics

Excitation of subradiant states in atom chains by surface plasmon waves

This paper proposes a novel method to selectively excite specific subradiant states in a finite atomic chain by coupling them to surface plasmon waves on a metallic film, thereby enabling controllable photon loading, storage, and release for future quantum memory applications.

Original authors: Giorgio Vimercati, Nicola Piovella

Published 2026-10-07

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CC BY 4.0

Original authors: Giorgio Vimercati, Nicola Piovella

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quiet world of quantum physics, light and matter engage in a delicate conversation that can be harnessed for powerful technologies. Imagine a group of atoms acting not as individuals, but as a single, coordinated team. When these atoms are spaced very closely together—closer than the width of the light wave they emit—they can interfere with one another in a way that creates a special state of stillness. In this state, known as a subradiant state, the atoms effectively hide their energy from the surrounding space. Instead of releasing their light quickly and fading away, they hold onto it for a remarkably long time. This ability to store light without losing it makes these atomic groups a promising candidate for the memory banks of future quantum computers, where information must be kept safe and stable. However, there is a significant hurdle: while these states are excellent at hiding, they are incredibly difficult to find and enter in the first place. Because they are so well-hidden, standard methods of shining light on them usually fail to trigger the specific pattern of cooperation required to wake them up.

Researchers Giorgio Vimercati and Nicola Piovella from the University of Milan have proposed a new way to solve this problem by using a trick involving the surface of a metal. In their study, they describe a setup where a line of atoms is placed just above a thin metallic film. Instead of trying to hit the atoms directly with a laser beam from open space, they use a special kind of light wave that travels along the surface of the metal, known as a surface plasmon. This wave is unique because it carries more momentum than light traveling through empty air, allowing it to match the specific, hidden rhythm of the subradiant atoms. The researchers simulated a chain of one hundred atoms, spaced closely together, and showed that when this surface wave passes by, it can selectively excite just one of these hidden states. The atoms absorb the energy from the wave and settle into a long-lived, cooperative state where the excitation remains trapped within the group.

The beauty of this method lies in its control. The researchers found that by simply adjusting the distance between the atoms and the metal surface, they could tune how strongly the atoms interact with the surface wave. When the atoms are held at a specific distance, the system loads the energy efficiently. Once the atoms are in this stored state, the laser driving the surface wave can be turned off. In a standard setup, the stored energy would either leak away quickly or remain stuck forever. Here, the distance to the surface acts as a switch. If the atoms are kept close to the metal, the stored energy can be released on demand, effectively turning the atomic chain into a controllable memory device. The study suggests that this approach works best when the atoms are oriented in a specific way relative to the surface, allowing them to couple more efficiently to the invisible waves traveling along the metal.

The team's simulations indicate that this method successfully bypasses the usual difficulties of preparing these dark states. By using the surface plasmon to provide the extra momentum needed, they can target a specific subradiant mode that would otherwise be inaccessible. The results show that after the driving laser is switched off, the excitation does not fade away rapidly as it would in normal conditions; instead, it persists for a long time, confirming that the atoms have entered a subradiant state. Furthermore, the researchers demonstrated that if the atoms are moved closer to the surface after the laser is turned off, the stored energy begins to leak out more quickly, proving that the distance acts as a precise control knob for releasing the information. This work suggests a realistic path forward for building quantum memories, transforming these elusive, dark states from theoretical curiosities into practical tools that can be loaded, stored, and released with simple adjustments to the experimental setup.

Technical Summary: Excitation of Subradiant States in Atom Chains by Surface Plasmon Waves

Problem Statement
Subradiant states in finite subwavelength atomic chains offer significant potential for photon storage and quantum sensing due to their markedly reduced decay rates and long coherence times, arising from destructive interference among emitter radiation fields. However, the preparation and selective control of these states remain experimentally challenging. In free space, subradiant states with wave vectors $k > k_0$ (where $k_0$ is the free-space wave vector) lie outside the light cone, rendering them inaccessible to conventional optical excitation. Existing strategies, such as spatially tailored driving fields or adiabatic population transfer, face increasing difficulty as system size grows due to the high degeneracy and near-continuum nature of the subradiant manifold. Furthermore, while waveguide-coupled chains offer a solution, they require specific integration. The authors note that previous work has shown cooperative effects can be suppressed when atoms are near surfaces with polariton resonances, leading to rapid decay into surface modes rather than the preservation of collective states.

Methodology
The authors propose a theoretical model where a finite chain of $N$ two-level atoms is positioned in vacuum parallel to and at a distance $h$ from a metallic film. The system is driven by a Surface Plasmon (SP) wave propagating along the metal-vacuum interface.

Excitation Mechanism: The SP wave is excited via a laser incident in total internal reflection conditions from a dielectric substrate (Kretschmann configuration). The SP wave possesses a wave vector $k_x > k_0$ , matching the momentum requirements of subradiant modes that are otherwise dark in free space.
Theoretical Framework: The dynamics are analyzed using a coupled-dipole model within the single-excitation limit. The system is described by an effective Hamiltonian incorporating field-mediated dipole-dipole couplings via the dyadic Green's tensor.
Surface Effects: The model accounts for the surface by adding a scattering term to the free-space Green's tensor. This includes both radiative interference effects and non-radiative decay channels into surface modes. The authors distinguish between the interaction with polariton resonances (which they argue suppresses cooperativity) and the interaction with a propagating SP wave (which enables selective coupling).
Simulation Parameters: Simulations consider a chain of $N=100$ atoms with a transition wavelength $\lambda_0 = 780$ nm. The metal film has a dielectric constant $\epsilon_1 = -2 + 0.1i$ and thickness $d=100$ nm. The atoms are driven by a weak, far-detuned SP field ( $\Omega_0 = 0.1\Gamma_0$ , $\Delta = 10\Gamma_0$ ) at a distance $h = \lambda_0/\pi$ . Both parallel and perpendicular dipole orientations relative to the surface are investigated.

Key Results

Selective Excitation: The SP wave successfully drives the atomic chain into a steady state where the excitation is selectively populated in a "quasi-Bloch" state within the subradiant region of the spectrum ( $k \approx k_x$ ). The momentum distribution of the excitation confirms that the system accesses modes with $k > k_0$ , which are inaccessible via direct laser excitation in free space.
Long-Lived Storage: Upon switching off the driving laser, the mean excitation probability remains nearly constant over long timescales, lacking the characteristic superradiant decay observed in free-space laser driving. This indicates that the excited state is composed almost entirely of subradiant modes. Transversely oriented dipoles show particularly efficient coupling to the evanescent SP field.
Control via Distance: The atom-surface distance ( $h$ $h$ ) acts as a critical control parameter.
- Loading/Storage: At a specific distance, the SP wave loads the subradiant state.
- Release Mechanism: If the surface is kept near the chain after the laser is turned off, the surface acts as an additional decay channel. As the distance decreases, the cooperative behavior is partially suppressed, and the single-atom decay rate increases, allowing the stored excitation to be released.
Comparison with Free Space: Direct laser excitation in free space results in negligible population of subradiant modes and rapid decay. In contrast, while the SP-driven system achieves a steady-state excitation probability that is orders of magnitude smaller than that obtained under direct laser excitation (due to the exponential decay of the SP field amplitude with atom-surface distance), the key distinction is the nature of the populated state. The SP-driven system selectively populates long-lived subradiant modes, whereas direct laser excitation primarily populates bright modes that decay rapidly.

Significance and Claims
The paper claims to provide an efficient method for addressing selectively subradiant modes with $k > k_0$ by exploiting the evanescent field of a Surface Plasmon wave. The primary significance lies in:

Momentum Matching: SPs supply the necessary momentum to directly couple to highly subradiant collective modes of ordered atomic arrays, bypassing the light-cone limitation of free-space optics.
Tunability: The atom-surface distance serves as a simple experimental "knob" to tune the coupling strength and selectively prepare long-lived collective excitations.
Quantum Memory Potential: The ability to load, store, and release photons in the atoms by controlling the chain/surface distance positions this system as a promising tool for future quantum memories.
Experimental Feasibility: The authors suggest that while most dielectric materials do not support SP resonances at optical frequencies, nanophotonic metasurfaces can be engineered to match atomic transitions. Furthermore, precise control of the atom-surface distance can be achieved via tunable trapping potentials (e.g., external electric fields), and the excitation process can be rapidly interrupted by switching off the laser or rotating its polarization.

The work transforms subradiant states from intrinsically dark modes into experimentally controllable resources for collective quantum optics, offering a realistic strategy for accessing these states without the complexity of free-space phase engineering or waveguide integration.

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