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arXiv:2410.04067 · Physical review. B./Physical review. B

Taming plasmonic nanocavities for persistent entanglement

Angus Crookes, Ben Yuen, Angela Demetriadou

Published Jun 27, 2025• 4 citations
arXivHTMLPDF

Abstract

Recent rapid advances in quantum nanoplasmonics offer the potential for accessing quantum phenomena at room temperature. Despite this, entangled states have not yet been realized, and remain an outstanding challenge. In this work, we demonstrate how entanglement emerges in plasmonic nanocavities, which are inherently multimode, and demonstrate the conditions necessary for entanglement to persist. We find that, in general, these conditions are broken due to coupling with multiple plasmonic modes of different parity. We address this challenge with a new nanocavity design that supports high selective coupling to a single mode, enabling the robust generation of persistent entanglement in nanoplasmonics. Our results open exciting prospects for leveraging simple plasmonic setups in ambient conditions for applications in quantum communication, sensing, and rapid quantum memories.

DOI
10.1103/skc6-gnyr

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Recent rapid advances in quantum nanoplasmonics offer the potential for accessing quantum phenomena at room temperature. Despite this, entangled states have not yet been realized, and remain an outstanding challenge. In this work, we demonstrate how entanglement emerges in plasmonic nanocavities, which are inherently multimode, and demonstrate the conditions necessary for entanglement to persist. We find that, in general, these conditions are broken due to coupling with multiple plasmonic modes of different parity. We address this challenge with a new nanocavity design that supports high selective coupling to a single mode, enabling the robust generation of persistent entanglement in nanoplasmonics. Our results open exciting prospects for leveraging simple plasmonic setups in ambient conditions for applications in quantum communication, sensing, and rapid quantum memories.
Plasmonic nanocavities are known for their extreme field enhancement and subwavelength light confinement in gaps of just a few nanometers. Pairing this with the ability to host quantum emitters, they form highly promising platforms to control or engineer quantum states at room temperature. Here, we use the lossy nature of plasmonic nanocavities to form subradiant entangled states between two or more quantum emitters, that persist for ∼ 100 times longer than the plasmonic excitation. We develop a theoretical description that directly links quantum variables to experimentally measurable quantities, such as the extinction cross section, and unlike previous studies includes plasmonic excitations necessary to resonantly form subradiant states. This work paves the way towards engineering quantum entangled states in ambient conditions with plasmonic nanocavities.
Abstract In the light–matter strong coupling regime, the excited state of quantum emitters is inextricably linked to a photonic mode, leading to hybrid states that are part light and part matter. Recently, there has been a huge effort to realize strong coupling with nanoplasmonics, since it provides a versatile environment to study and control molecules in ambient conditions. Among the most promising designs are plasmonic nanocavities that confine light to unprecedentedly small volumes. Such nanocavities, though, support multiple types of modes, with different field profiles and radiative decay rates (bright and dark modes). Here, we show theoretically that the different nature of these modes leads to mode beating within the nanocavity and the Rabi oscillations, which alters the spatiotemporal dynamics of the hybrid system. By specifically designing the illumination setup, we decompose and control the dark and bright plasmon mode excitation and therefore their coupling with quantum emitters. Hence, this work opens new routes for dynamically dressing emitters, to tailor their hybrid states with external radiation.
Quantum technologies are widely expected to bring a revolution in communications and information technologies in the coming decade allowing, for example, un-paralleled levels of secure communications. For quantum communication, photonic quantum effects have played a central role, but most photonic solutions and systems based on light-matter interaction also require cryogenic environments. Strong coupling of light and matter at the single emitter level is a fundamental quantum resource offering deterministic energy exchange between single photons and a two-level system, and the possibility to achieve single-photon nonlinearities via the anharmonicity of the Jaynes-Cummings ladder. Until recently, however, the conditions for achieving strong-coupling were most commonly met at cryogenic temperatures such that de-coherence processes are suppressed. As a major step forward, we have recently demonstrated room-temperature strong coupling of single molecules [1] and single quantum dots [2] to ultra-confined light fields in plasmonic resonators at ambient conditions. The fact that strong-coupling conditions may be reached at room temperature is of immense interest because it represents a clear route to a practical implementation and use of quantum behaviour in nanophotonic systems and its application in bio-sensing [3]. The talk will discuss how nanoplasmonics can be an enabler of ultrafast room-temperature quantum nanophotonic networks via strong coupling and ultrafast quantum dynamics [4]. We will highlight the physics associated with recently demonstrated room-temperature strong coupling of single molecules in a plasmonic nano-cavity [1] and near-field generated strong coupling of single quantum dots [2] and single quantum emitter Dicke enhancement [5] paving the road towards single-photon quantum nonlinearities. The presentation will also explain near-field enhanced single-photon emission in near-zero index materials [6] and ultrafast multi-partite quantum entanglement [7]. This provides the foundation for unprecedented control over photon number, single photon dynamics, and dynamic multi-photon coherence. These properties are all imperative for the development of next-generation, nanoscale building blocks in ambient-temperature photonic quantum communication technologies.
Abstract Quantum nanophotonics places nanoscale structuring in direct dialogue with photonic quantum information—compressing functionality to the few-nanometre, few-femtosecond frontier while retaining energy efficiency, speed and room-temperature operation. We highlight nanoplasmonic cavity-QED architectures in which extreme mode confinement delivers strong coupling under ambient conditions, enabling ultrafast single-photon emission and near-field multipartite entanglement using industry-grade near-field transducers that nanofocus guided light at practical efficiencies. We set these advances against the arc from early plasmonic cQED proposals and single-emitter Purcell studies to modern time-domain measurements that resolve sub-picosecond polariton/Rabi dynamics—establishing a clear route from concept to device. In parallel, we point to low-loss, high-index Mie resonators whose spectrally sharp electric and magnetic modes afford narrow-band coherent control, filtering and radiation-pattern engineering, and which naturally co-integrate in hybrid metal–dielectric stacks that combine ultrastrong local coupling with low-loss routing and storage. Finally, we discuss quantum nanoplasmonic coherent perfect absorption (qnCPA)—a waveguide-fed, non-perturbing protocol that selectively prepares and protects plasmon–emitter dressed states via dissipation-assisted, unidirectional feeding. Together with macroscopic QED and quantum quasinormal-mode quantization, which regularize open, lossy resonators and clarify when loss can be a resource, these developments chart robust, chip-scale routes to room-temperature quantum technologies.
Abstract In recent years, quantum nanophotonics has forged a rich nexus of nanotechnology with photonic quantum information processing, offering remarkable prospects for advancing quantum technologies beyond their current technical limits in terms of physical compactness, energy efficiency, operation speed, temperature robustness and scalability. In this perspective, we highlight a number of recent studies that reveal the especially compelling potential of nanoplasmonic cavity quantum electrodynamics for driving quantum technologies down to nanoscale spatial and ultrafast temporal regimes, whilst elevating them to ambient temperatures. Our perspective encompasses innovative proposals for quantum plasmonic biosensing, driving ultrafast single-photon emission and achieving near-field multipartite entanglement in the strong coupling regime, with a notable emphasis on the use of industry-grade devices. We conclude with an outlook emphasizing how the bespoke characteristics and functionalities of plasmonic devices are shaping contemporary research directives in ultrafast and room-temperature quantum nanotechnologies.