Photonic graph state production using Cavity QED

Novelty of the research methodology:

The interaction of atoms with light, enhanced via coupling to optical cavities, is both an area of fundamental interest and crucially important for quantum communication and information. Our activities in this area focus on the ultimate control of atom-photon interactions at the single-atom and single-photon level. Photonic quantum bits play an instrumental role in the development of advanced quantum technologies, including quantum networking, boson sampling and measurement-based quantum computing.

A promising framework for the deterministic production of indistinguishable single photons is an atomic emitter strongly coupled to a single mode of a high finesse optical cavity. Polarisation control is an important cornerstone, particularly when the polarisation defines the state of a quantum bit.

Aims and Objectives:

This project aims to produce photon chains, representing graph states, such as photonic GHZ states or cluster states by coupling a generalised atomic emitter to an optical cavity. We exploit a particular choice of the quantisation axis for dual-rail qubit encoding in photon polarisation and/or time bins. The concise tasks within this project are twofold: On the one hand, it entails pursuing extensive simulations using machine learning to find ideal laser-pulse sequences for the successful quantum control of the system.

These calculations make use of the University's computing cluster for optimising several thousand control parameters in parallel, based on the implementation of the Master Equation describing the time evolution of the system on a large-scale array of GPUs in a reinforcement-learning approach. On the other hand, it does encompass the demonstration of these processes in the laboratory.

The experiment involves dealing with laser-cooled trapped atoms, delivered one-by-one by an atomic fountain to the mode volume of a high-finesse cavity, the implementation of adiabatic passage techniques, like STIRAP, to prepare the atom in bespoke quantum states, and to eventually drive the photon-generation using a vacuum-stimulated rapid adiabatic passage protocol in strong atom-cavity coupling. This will be complemented with an active routing of photons into various delay lines and photonic networks, to perform quantum operations in multi-photon interferometric optical circuits.

Eventually, photon detection will be achieved with superconductive nanowires, and extensive data evaluation protocols must be implemented to retrieve the relevant photon-photon coincidences demonstrating the successful operation of the protocols.

Implementing such photonic networks with complex photonic states possibly coupling distant stationary entities is going to be a major step forward in scaling up quantum networks, and it'll be key to realising the quantum internet, which is one of the major goals of present quantum technological endeavours.

This project falls within the EPSRC Quantum Technologies research area.