Strongly interacting Rydberg exciton-polaritons (SIREP)

Photons are excellent carriers of quantum information. They only weakly interact with their environment, meaning quantum information can be easily transmitted over great distances without being lost. However, the lack of interactions is a double-edged sword as it means the photons do not easily interact with each other. Without photon-photon interactions, it becomes difficult to entangle photons and perform certain quantum gate operations on them. It is possible to create photon-photon interactions by propagating photons through a medium. Inside the medium, hybrid light-matter particles known as polaritons can form, which interact through their matter component. However, finding a medium where the interaction strength is large enough to allow single photons to have a high-probability of interacting and becoming entangled with each other is challenging. One system which has achieved large enough interaction strengths is Rydberg atoms. Rydberg atoms are atoms where one electron is promoted to a highly excited state. The excited electron is far from the core of the atom, giving Rydberg atoms exaggerated properties, including enhanced interactions between atoms. By mapping these interactions onto light, it is possible to achieve a nonlinear response at the single photon level and strong enough effective photon-photon interactions to create entanglement between photons. However, technologies based on Rydberg atoms are hard to combine with other photonic technologies, such as waveguides. Rydberg excitons are highly excited bound states of electrons and holes in a semiconductor and have recently emerged as a solid-state analogue of Rydberg atoms. They occupy a unique position, providing the possibility of combining the interactions and nonlinearities associated with Rydberg atoms, with the scalablity of semiconductor technologies. The current leading material for Rydberg excitons is cuprous oxide, where highly excited Rydberg states and interactions between Rydberg states have been observed. In 2022, Rydberg excitons in cuprous oxide were strongly coupled to light, creating Rydberg exciton-polaritons. Rydberg exciton-polaritons allow the Rydberg interactions to be observed as effective photon-photon interactions. However, the effective photon-photon interaction realised in cuprous oxide are still well below the level required for creating two-photon entangled states. This is because cuprous oxide is an absorptive material, which leads to loss in the material. This project will pioneer the study of Rydberg excitons in a new material, zinc diphosphide, and strongly couple them to light. Zinc diphosphide shows similar Rydberg lines to cuprous oxide but crucially has a 50 times lower absorption. Due to the lower absorption and strong interactions between Rydberg states, it will be possible to create arrays of single Rydberg exciton-polaritons in a microcavity for the first time, which will emit single photons and will allow photon-photon interactions to reach the single photon level. Additionally, the wavelengths of light involved in this material (775-780 nm and 1550 nm) mean that it is potentially compatible with other quantum technologies (e.g. atomic quantum memories) and telecommunication infrastructure (which commonly uses 1550 nm light). The creation of effective photon-photon interactions at the single photon level will have significant impact in photonic quantum information processing, as it will lead to the ability to entangle two single photons with high probability, allowing conditional quantum gate operations to be performed.