Nanophotonics Boost Quantum Emitter Links on a Chip

Scientists are increasingly focused on harnessing interactions between solid-state quantum emitters for advanced photonic technologies. Yinhui Kan from the Center for Nano Optics, University of Southern Denmark and Niels Bohr Institute, University of Copenhagen, alongside Shailesh Kumar, Xujing Liu, and colleagues, have developed an integrated nanophotonic platform to facilitate these interactions, working in collaboration with Antonio I. Fernández-Domínguez from Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, and Sergey I. Bozhevolnyi from the Center for Nano Optics, University of Southern Denmark.

This research presents a forward-designed architecture that overcomes challenges in achieving strong, controllable long-range coupling between quantum emitters on a chip, demonstrating both enhanced energy transfer and suppression through engineered surface plasmon polariton interference.

The team’s findings, which predict high levels of entanglement and efficient energy funnelling in multi-emitter systems, establish a scalable framework for on-chip quantum photonics and represent a significant step towards realising complex quantum networks. The predicted concurrence, a measure of entanglement, peaked at 0.493, approaching the theoretical maximum for the transient regime. Previous approaches employing photonic crystals, waveguides, and metasurfaces often suffered from limitations in spatial coupling range or fabrication complexity. Now, a forward-designed platform utilising surface plasmon polaritons (SPPs, collective oscillations of electrons at a metal surface) offers a compelling alternative. Unlike dielectric structures, SPPs offer greater flexibility for engineering on-chip functionality and circumvent the constraints of conventional photonic systems.

The team’s design enables ultracompact nanophotonic architectures that mediate enhanced long-range interactions between quantum emitters. Specifically, they engineered two distinct configurations: one designed to funnel energy between emitters and another to suppress energy transfer. To achieve both enhancement and suppression of energy transfer with such precision is a notable advancement. Experimental validation using nitrogen vacancy centres in nanodiamonds confirmed substantial changes in energy transfer rates compared to bare substrates. Beyond simply demonstrating control over energy flow, the team modelled the time evolution of entanglement. Predicting high concurrence values for emitters separated by several micrometres. Since entanglement is a fundamental requirement for quantum computation and communication, this level of control is essential for building scalable quantum systems. Also, extending the approach to multiple quantum emitters, the team observed enhanced energy funnelling in a three-emitter configuration. This result establishes a compact and scalable framework for on-chip entanglement engineering, and paving the way for integrated nanophotonic systems with potential applications in advanced quantum technologies. At present, the platform represents a significant step towards realising practical, scalable quantum computers and communication networks. Nanophotonic structures optimise entanglement and control energy transfer rates At a concurrence of 0.493, researchers have demonstrated entanglement between quantum emitters on a chip, nearing the theoretical maximum for transient conditions. Here, this value, representing the degree of correlation between the quantum states of two emitters, signifies a substantial advancement in on-chip quantum networking. Yet, achieving this level of control necessitated a forward-designed nanophotonic platform utilising surface plasmon polaritons (SPPs) to mediate interactions between the quantum emitters. Researchers engineered two distinct nanostructures: an elliptic design to funnel energy between emitters and a hyperbolic design to suppress energy transfer. By manipulating the interference of SPPs, they realised both enhancement and suppression of energy transfer rates, a capability previously difficult to attain. Experimental validation confirmed substantial changes in energy transfer rates compared to bare substrates, demonstrating the effectiveness of the designed nanostructures. For instance, the platform enabled the creation of conditions where the strategy was demonstrably increased or decreased, showcasing precise control over emitter interactions. Beyond simple it, The project extends to predicting and demonstrating transient entanglement by modelling the time evolution of concurrence. Revealing the potential for creating entangled qubit pairs on a single chip. Since the platform relies on SPP interference, it offers greater flexibility and overcomes limitations associated with earlier methods employing photonic crystals or waveguides. Still, at present, this effort establishes a compact and scalable framework for on-chip quantum entanglement engineering, potentially paving the way for practical quantum technologies. FDTD modelling of quantum emitter interactions with holographic nanostructures A three-dimensional finite-difference time-domain (FDTD) method underpinned the numerical simulations performed to characterise interactions between quantum emitters and the designed holographic nanostructures. Quantum emitters, modelled as vertically oriented electric dipoles, operated at a wavelength of 670nm, corresponding to the fluorescence emission peak region of nitrogen-vacancy centres in nanodiamonds at room temperature. By positioning the dipole 30nm above the substrate, researchers constructed a layered system comprising a 20nm-thick silica spacer deposited upon a 150nm-thick silver film. Nanostructures surrounding the dipole were modelled using a dielectric material possessing a refractive index of 1.41 and a height of 175nm. Evaluating interaction strength necessitated a specific measurement protocol. A two-dimensional planar field monitor was placed in the x, y plane at the same vertical position as the dipole, located at the target acceptor position. Electric fields recorded by this monitor then allowed computation of the normalized the method rate and extraction of dyadic Green’s functions for both the nanostructured and bare-substrate environments. Such an approach provided a direct comparison of the technique efficiency with and without the nanophotonic enhancement. Understanding the dynamics of entanglement required a different computational framework. Transient concurrence dynamics between two quantum emitters were modelled using an unpumped two-qubit master-equation formalism, accounting for both coherent and incoherent processes. Collective coupling parameters were extracted from the FDTD-simulated electric field. Accurate representation of the electromagnetic environment experienced by the emitters. Through solving this equation, the time-dependent concurrence, a measure of entanglement, could be calculated for specific initial conditions. To extend to multiple emitters demanded a generalisation of this approach. Steady-state concurrence in two and three QEs was determined by including pumping terms within the master equation, allowing for analysis of energy funneling and entanglement in more complex configurations. On that front, the resulting density matrix, obtained from the stationary condition, provided the necessary information to calculate concurrence using the Bell state basis. At present, at the experimental level, device fabrication relies on a two-step aligned electron-beam lithography process. First, a 150nm-thick silver film was thermally evaporated onto a silicon wafer. Followed by deposition of a 20nm SiO2 layer via magnetron sputtering, creating a low-loss plasmonic substrate. Gold alignment markers were then defined through standard EBL, metal evaporation, and lift-off. Then, nanodiamonds containing nitrogen-vacancy centres were spin-coated onto the substrate, and their positions identified using dark-field optical microscopy. A hydrogen silsesquioxane resist layer was then patterned via a second, marker-aligned EBL step, followed by development and rinsing. Optical characterisation employed a confocal microscopy setup with a radially polarized continuous-wave laser at 532nm to excite the emitters. Collecting fluorescence through a high-numerical-aperture objective. Surface plasmon polaritons enable enhanced and suppressed qubit interactions on a chip Scalable quantum computing inches closer to reality with each advance in controlling interactions between individual qubits.

Scientists have unveiled a nanophotonic platform that addresses a longstanding hurdle: establishing dependable, long-range connections between solid-state quantum emitters on a chip. Unlike previous designs relying on photonic crystals or waveguides, this approach employs surface plasmon polaritons. Offering greater design freedom and circumventing limitations in how far qubits can effectively ‘talk’ to each other. For now, to achieve this level of control is not merely about boosting signal strength. By carefully crafting nanostructures, the team demonstrated not only enhancement of this approach between qubits, but also its suppression, a capability vital for complex quantum operations. While the platform shows promise for multi-qubit configurations, scaling to many interconnected qubits will certainly present further engineering challenges. For years, the field has explored various methods to mediate interactions between quantum emitters — but this nanophotonic approach stands out due to its potential for compact integration. Bringing us closer to a future where quantum technologies are no longer confined to the laboratory. 👉 More information 🗞 Integrated nanophotonic platform for on-chip quantum emitter interactions and entanglement 🧠 ArXiv: https://arxiv.org/abs/2602.24090 Tags: