Plasmon-mediated Entanglement Generation Accounts for Multipolar and Nonlocal Effects in Nanophotonics

Entanglement, a key resource for emerging quantum technologies, increasingly relies on nanophotonic systems for its creation and manipulation. Luke C. Ugwuoke, Tjaart P. J. Krüger, and Mark S. Tame, researchers from Stellenbosch University and the University of Pretoria, investigate how accurately modelling light-matter interactions impacts the generation of entanglement between quantum dots. Current approaches often simplify these interactions by considering only the dipole behaviour of light and ignoring the size-dependent properties of the materials involved. This team demonstrates that these simplifications become significant limitations when quantum dots are brought very close together, as multipolar effects and nonlocal optical phenomena dramatically alter the dynamics of entanglement. Their findings reveal that these effects can actually lead to a decay in entanglement, and highlight the importance of considering these factors when designing future quantum technologies based on plasmonic coupling.

The study focuses on understanding non-local effects, where a material’s response isn’t solely determined by its immediate surroundings, but by a wider area, and how these effects impact light-matter interactions at the nanoscale. Understanding these interactions is crucial for developing advanced technologies that harness quantum properties. Plasmonics studies the interaction of light with electrons on the surface of metallic nanostructures, creating collective electron oscillations called plasmons.

These plasmons concentrate light into extremely small volumes, offering potential for sensing, imaging, and manipulating light at the nanoscale. Quantum optics, meanwhile, deals with the quantum nature of light and its interaction with matter, becoming increasingly important as systems shrink and quantum effects become dominant. Traditional models often assume a material’s response to light depends only on the field at a specific point. However, in nanoscale systems, electrons are highly delocalized, meaning their behavior extends beyond a single point. This non-locality significantly influences how light interacts with the material and must be accurately accounted for to predict behavior.

The research utilizes concepts like quantum entanglement and Fisher information to measure the sensitivity of these systems. Scientists developed a sophisticated theoretical framework to accurately model electron behavior in the metal, accounting for electron delocalization. They then studied how this non-local response affects the interaction between plasmons in the nanostructure and quantum emitters, a crucial interaction for applications like quantum sensing and information processing. The team analyzed how non-local effects influence entanglement between quantum emitters and how this impacts the system’s sensitivity to external parameters, using Fisher information as a metric.

They demonstrated that dark modes, plasmonic excitations that don’t directly radiate light, can significantly influence interactions with quantum emitters and enhance performance. Ignoring non-local effects leads to inaccurate predictions of optical properties, especially at the nanoscale. By accurately modeling non-locality, scientists showed how to optimize the interaction between plasmons and quantum emitters, leading to stronger coupling and enhanced performance. Exploiting non-local effects and dark modes can create more sensitive quantum sensors, with potential implications for developing new quantum technologies, including sensors, communication devices, and information processing systems.

This research pushes the boundaries of nanotechnology and quantum optics, allowing for the creation of new devices with unprecedented capabilities. Imagine building a tiny antenna to detect faint signals; accurately modeling electron movement enables the creation of a much more sensitive antenna. This is akin to accounting for blurriness to achieve a sharper signal. This could lead to breakthroughs in medical diagnostics, with more sensitive sensors for disease detection, and environmental monitoring, with greater accuracy in detecting pollutants. It also has potential for faster and more efficient data storage and the development of more powerful quantum computers. They employed a cavity electrodynamic approach to model interactions between quantum dots, carefully examining how induced plasmonic effects modify transition rates during entanglement generation. Researchers accounted for multipolar modes, which become significant when coupling distances are smaller than the nanoparticle size, and size-dependent damping, which influences entanglement stability. To accurately model these effects, the work incorporated a generalized non-local optical response theory, extending beyond traditional dipole approximations.

This approach enabled scientists to simulate light interacting with metallic nanostructures, capturing the complex interplay between size, shape, and optical properties. The team specifically addressed how non-local effects, arising from the dielectric response of the nanoparticles, impact entanglement fidelity and limit coupling strength. By combining these theoretical advancements with detailed simulations, the study revealed that both multipolar modes and size-dependent damping contribute to entanglement decay at small coupling distances, ultimately limiting the potential for mediated entanglement with certain particle sizes. The research employs a cavity electrodynamic approach to model the interaction, revealing how these effects modify transition rates in coupled quantum dots and ultimately limit entanglement. The study establishes a detailed theoretical framework, beginning with equations that account for size-dependent corrections to local parameters due to non-local effects. These equations incorporate the magnitude of the nanoparticle’s dipole moment, size-dependent corrections to transition rates, and the influence of the particle’s geometry on damping rates.

Specifically, the team derived expressions for the frequency shift of multipolar modes, showing that higher-order modes are blueshifted relative to the dipole mode, and quantified the coupling strength between dipole and multipolar modes. Further analysis reveals the impact of the metal’s dielectric properties and the surrounding medium on these parameters. The team calculated the local dielectric function, incorporating both the bulk plasmon response and the contribution of free electrons, and determined how these properties influence the overall coupling strength and damping rates. Measurements confirm that the total plasmon damping includes both non-radiative and radiative components, with the radiative component directly related to dipole radiation.

To model the system, scientists developed a Hamiltonian and Liouvillian to describe the dynamics of the quantum dots and the nanoparticle. This framework allows for the investigation of dark modes and non-local effects on plasmon-mediated entanglement. The team then derived effective parameters for the qubits, including modified excitation rates, coupling strengths, and detuning frequencies, by employing a semiclassical approximation and solving for the stationary solution of the nanoparticle dynamics.