Towards single-photon switching via two-photon absorption in Rb vapour

Abstract

Photonics offers a potential avenue for scalable fault-tolerant quantum computing. However, the inherently probabilistic nature of linear photonic quantum gates has obstructed large-scale photonic quantum computing. High gate success probabilities are essential to the development of scalable devices for quantum optics. The strong nonlinearities mediated by atomic vapours have the potential to achieve optically controlled storage and switching at room temperature, opening the way to scalable, integrated, and miniaturised devices for quantum computing. A strong candidate for engineering such interactions is the combination of optical waveguides and cavities with warm alkali vapours.

A current challenge for quantum photonics is the difficulty of implementing high speed, ultra low loss switching. This motivates the study in this thesis of whether atomic vapours could be used for next generation switching. This work presents a theoretical model to analyse the light-matter interactions of a signal field in a ring-cavity system mediated by an atomic ensemble. In this model, a ring-cavity translates Kerr nonlinearity induced by an optical control field at room temperature into a switch for a signal field. The practical demonstration of this switch is realised using a doubly resonant ring-cavity mediated by a ladder system in 43.4°C 87Rb vapour operating at room temperature. The switching contrast calculated for a 3.62mW signal field at frequency 780nm switched by a 7.51mW control field at 776nm inside this cavity was 1.9dB, serving as the first practical demonstration of all-optical switching in a doubly resonant cavity mediated by a ladder-system in rubidium, paving the way to fast and low-loss synchronisation of operations across photonic quantum processors for temporal multiplexing.

I also investigate losses in these systems and discuss the potential for miniaturisation and increased speed. In particular, the trade off between dissipation and dispersion near resonance which presents a fundamental limit to the fidelity of photonic-based quantum gates. I discuss how realistic cavity losses limit the switching performance and suggest optimal designs in the context of building practical switching devices with applications to quantum logic gate operations at room temperature.

Date of Award	19 Feb 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Josh Nunn (Supervisor), Peter Mosley (Supervisor), Cameron McGarry (Supervisor) & Alex Davis (Supervisor)