Molecular Strong Coupling and Entanglement Formation in Extreme Nanophotonic Environments

Nanoscale photonic environments, supporting light-matter interactions with molecules or cold atoms, are at the heart of nano-optics. This work focuses on developing quantum mechanical models to better understand such systems and the design of novel photonic environments for use in quantum information technologies. The following are brief summaries of my ongoing work.

Quantum networks exploit both local and global entanglement over separated network nodes to provide unbreakable encryption and scalable computation. However, most high-fidelity local nodes are only weakly coupled to the electromagnetic field, limiting the inter-node entanglement rate. Further to this, almost all strongly coupled photonic devices are made of silicon and operate at telecommunication wavelengths, making them unsuitable for entanglement with cold atoms. In this work, we design nanophotonic crystal resonators that overcome these challenges, commanding unprecedented optical confinement and strong field enhancements while operating at 780 nm for entanglement with rubidium. Our designs exist deep into the strong coupling regime, where we demonstrate local multipartite entanglement that is robust to displacement of the trapped atoms. High fidelity entanglement and a scalable photonic architecture make our systems ideal for constructing large quantum networks, where both local and remote entanglement can be realized.

Nonreciprocal devices, possessing direction dependent wave propagation, are a key component in quantum information technologies as they act to protect qubits from reflections and noise. It has recently been shown that quantum nonreciprocity can be generated in passive devices, without an external bias, but instead induced through a combination of system nonlinearity and spatial symmetry breaking. In this work we adapt our cavity design to create a realistic photonic crystal waveguide, coupled to two detuned cold atoms, that exhibits nonreciprocity through excitation of a slowly decaying dark state. By expanding the model to larger numbers of atoms, we aim to overcome the theoretical two-atom efficiency and create a system with superior nonreciprocity.

Plasmonic structures facilitate rapid energy exchange between light and molecules, overcoming large heating losses in the metal through confinement of light well below the diffraction limit. Semi-classical and fully quantum formalisms are employed to describe these phenomena. However, most assume the interaction is between a two-level system and a single plasmonic resonance, neglecting a large collection of higher order modes and the complex molecular structure. In this work, we treat the plasmonic cavities as open systems, supporting a set of quasi-normal modes (QNM's) with complex eigenfrequencies. We reveal the impact of high-order modes on the single molecule dynamics and semi-persistent entanglement generation between two molecules. Numerical simulations are performed within the nanoparticle on mirror cavity, and similar systems with morphological changes to the facet shape, making them ideal for room temperature entanglement generation and quantum technologies.

1. Our primary impact will be by supplying the UK knowledge economy with skilled multidisciplinary researchers, equipped with the technical and transferable skills to establish the UK as pre-eminent in topology-based future technologies. The training they receive will make them proficient in the demands of the translation of academic science (with a broad background in condensed matter physics, materials science and applied electromagnetics) to industry, with direct experience from internship and industry engagement days. With their exposure to both theoretical research (including modelling and big data-driven problems) and experimental practice, our graduates will be ideally equipped to tackle research challenges of the future and communicate to a broad audience, ready to lead teams made up of diverse specialised components. The potential impact of our researchers will be enhanced by a broad programme of transferable skills, focusing on innovation, entrepreneurship and responsible research. Beneficiaries here will include the students themselves as they embark on future careers intertwining academic research and industry, as well as the other sectors listed below.

2. The research undertaken by students in the CDT will have impact on the future direction of topological science. Related disciplines, including physics, materials science, mathematics, and information technology will benefit from the cross-disciplinary fertilisation it will enable. The CDT will not only provide an interface between research in physical sciences and engineering, but also provide a route for academia to interact effectively with industry. This will help organise researchers from different disciplines to collaborate around the needs of future technology to design materials based on topological properties.

3. Our research will enable industries to set the direction of topological research around the needs of commercial research and development, leading to wealth generation for the UK, and to influence the mindset of the next generation of future technologists. Specifically, topological design has the promise to revolutionise devices and materials relevant to communications, microwave and terahertz technologies, optical information processing, manufacturing, and cybersecurity. Through partnership with organisations from the wider knowledge sector, we will deepen the relationship between academic research and disciplines including IP law and scientific software development.

4. Our CDT will also have impact on the wider academic community. New specialist courses and training in transferable skills will be developed utilising cutting-edge multimedia technologies. Our international research collaborators, including prominent global laboratories, will benefit from placements and research visits of the CDT students. Our interdisciplinary research, combining the needs of academia and industry will be an exemplar of the effectiveness of the CDT model on an international stage.

5. The wider community will benefit from our organised public engagement activities. These will include direct interaction activities, such as demonstrating at the Birmingham Thinktank Science Centre, the Royal Society Summer Exhibition, local schools and community centres.