All light is a collection of particles called photons. Quantum technology aims to control light at the level of individual photons. This requires us to be able to create single photons, perform measurements that can detect a single photon, and use photons to store and carry quantum information.

Our activies in this area include:

## Quantum light on a chip

Photons have a number of properties which make them particularly appealing for future quantum technological applications, including long coherence times and the ability to transfer information at the speed of light. A photonic “flying qubit” can be used both as a basis for quantum computation and for coherent transfer of quantum information between stationary matter qubits. The latter is dependent on strong light-matter interactions, which can be achieved by confining optical fields to the order of the wavelength of light or smaller. In our work, we interface matter qubits in the form of quantum dots with nano-photonic structures such as waveguides and cavities, enabling efficient light-matter coupling. Our system leverages highly-developed semiconductor technology and is inherently scalable, a key requirement for quantum technologies.

Highlights:*A chiral light-matter interface*: We demonstrated high fidelity, unidirectional photon emission from quantum dot spin states for in-plane transfer of matter-qubit information. (Nature Communications 7, 11183 (2016))*Strong photon-photon interactions on-chip*: The deterministic operation of photonic quantum gates is dependent on strong photon-photon interactions. We have experimentally demonstrated the generation and electrical control of such interactions on-chip, mediated by a single quantum dot. (Optica 5(5) 644-650 (2018))*Coherent to quantum light conversion on-chip*: Sources of quantum light are a fundamental requirement for quantum photonic technologies. In this work, we used a waveguide QED system to convert classical coherent laser light into bunched, or antibunched non-classical light. We showed the photon statistics of the device output could be manipulated simply by controlling a bias applied to the device. (Phys. Rev. Lett. 122, 173603 (2019))

**Lead researchers:**Luke Wilson, Mark Fox, Andrew Foster.

## Quantum optics of solid state emitters

The field of quantum optics describes the interaction between light and matter at the single particle level, giving rise to important concepts for optical quantum technologies such as single photon sources for quantum communications. Our III-V semiconductor platform allows us to integrate bright, stationary quantum dot emitters in nano-photonic structures, facilitating efficient light-matter interactions at the single photon level. Exploiting these attributes allows us to develop quantum devices such as single photon sources and switches. Through close collaboration with theorists at Sheffield and beyond, we are also exploring how these solid-state emitters differ from simple two-level system models of quantum optics, allowing us to establish the optimal parameter regimes for future solid-state quantum devices.

Highlights:*A gigahertz source of single photons*: An optical nano-cavity greatly enhances the emission rate of a quantum dot, enabling the deterministic generation of single photons with near-ideal coherence at a repetition rate that can reach as high as 10 GHz. (Nature Nanotechnology 13, 835–840 (2018))*Light scattering from solid-state quantum emitters*: Through a joint experimental and theoretical investigation, we reveal that the electron-phonon coupling present in solid-state emitters causes major deviations from the conventional “two-level system” picture of quantum optics. (Phys. Rev. Lett. 123, 167403 (2019))*Phonon-assisted population inversion of a quantum dot*: We demonstrate a novel scheme to excite a quantum dot to population inversion using incoherent excitation, a mechanism that is only possible owing to the intrinsic coupling of the quantum dot to acoustic phonons. (Phys. Rev. Lett. 114, 137401 (2015))

**Lead researchers:**Mark Fox, Luke Wilson, Alistair Brash.

## Topological light

Concepts of topology from condensed matter physics promise exciting new capabilities in the realm of photonics; in particular, the robust unidirectional propagation of light at the micrometre scale, opening up possibilities in the design and implementation of integrated photonic circuits with negligible scattering losses. Realizing such states relies on the controlled engineering of the frequency bands of light confined in photonic structures. Our work focuses on nanoscale III-V semiconductor devices including micropillar arrays, photonic crystals and slab waveguides.

Highlights:*Topological photonic crystal waveguides*: We have developed novel waveguides formed at the interface between topologically distinct photonic crystals, where embedded quantum emitters couple chirally to unidirectional edge states. (Appl. Phys. Lett. 116, 061102 (2020), Optica 7 (12), 1690-1696 (2020))*Photonic 1D topological array*: We experimentally implemented a paradigmatic 1D topological system in a photonic setting, studying how the polarization degree of freedom affects the edge states in the system, providing new theoretical insight. (Phys. Rev. B. 99 081402(R) (2019))*Nonlinear photonic simulators*: We designed and developed a 2D micropillar lattice for photons hybridized with excited electrons, featuring flat energy bands arising due to topology. The lattice, which is analogous to some real world materials with interesting and unconventional properties, allows the emulation of such materials in a controllable system with effective photon-photon interactions. (Phys. Rev. Lett. 120 097401 (2018))

**Lead researchers:**Dmitry Krizhanovskii, Luke Wilson, Andrew Foster, Charles Whittaker.