Periodic Reporting for period 2 - PIQUaNT (Photonics for High-Dimensional Quantum Networking)
Berichtszeitraum: 2022-10-01 bis 2024-03-31
Modern quantum communication technologies based on two-dimensional systems or “qubits” promise unprecedented levels of information security vital to the functioning of a modern society. However, they are set to face significant bottlenecks in the future, both in terms of operational distance and data rates. The emerging field of high-dimensional quantum photonics provides a pathway to maximise the information capacity of quantum networks and simultaneously enable them to operate in prohibitive regimes of loss and noise. However, its widespread implementation is currently hindered by the lack of novel devices and technologies for the precise manipulation, measurement, and noise-robust transport of multi-mode quantum states of light. The PIQUaNT project aims to develop ways to gain exquisite control over the structure of such quantum states of light in space and time. This will allow us to manipulate and measure complex forms of quantum entanglement and demonstrate their noise-robust transport over commercially available optical fibres. Achieving these goals will unlock applications of entanglement in quantum communication and networking, enabling ultra-secure and high-capacity communication networks that offer unconditional information security.
The PIQUaNT project has made significant progress in its first half. We have developed methods to manipulate the spatial structure of quantum light using a technology known as multi-plane light-conversion (MPLC). This technique uses multiple reflections on a programmable spatial light modulator to implement complex operations on a structured photon. We have used the MPLC platform to design a mode sorter for overlapping quantum states of light as well as implement high-dimensional transformations that simulate the exchange statistics of quasiparticles known as Anyons. In parallel, we have developed a powerful technique that harnesses complex mode-mixing inside a commercial optical fibre to programme quantum circuits for light using methods of inverse design. Using this technique, we have demonstrated the transport, measurement, and control of high-dimensional entanglement within the transmission channel itself! During the course of this experiment, we also developed a new reference-less technique for characterising complex scattering media using physics-based multi-plane neural networks, and applied it for measuring the transmission matrix of a commercial multi-mode fibre. On the theoretical front, we have developed a complete model for high-dimensional entanglement in the spatial and spectral degrees-of-freedom and developed an experimental technique to rapidly characterise it in the lab. This has allowed us to optimise how we generate and measure entanglement in the lab, enabling us to produce quantum states of two photons with a record entanglement dimensionality. Finally, we have developed a powerful one-sided device-independent entanglement (steering) witness that utilises the above methodology to achieve noise and loss-tolerant entanglement distribution. We have implemented this witness in the lab to steer entanglement through conditions of loss equivalent to 79km of telecom fibre in the presence of noise.
Our work on high-dimensional (HD) programmable circuits advances the research field of HD quantum photonics significantly beyond the state of the art as it presents a method for implementing any high-dimensional transformation using a commercial multi-mode fibre, allowing multi-outcome measurements in any spatial mode basis. This capability is critical for quantum information processing with HD quantum states of light and simultaneously serves as a powerful alternative to conventional photonic integrated circuits that are constructed via a bottom-up approach of phase-shifters and beam-splitters. Our work on noise and loss-tolerant quantum steering is very significant for the development of practical quantum communication technologies. Even the best optical fibres in the world suffer from a certain amount of loss, which puts strict limitations on the distance over which entanglement-based quantum communication can be carried out. Having a loss-tolerant method for steering entanglement opens a pathway towards practical quantum communication networks with the ultimate form of security. The simultaneous ability to withstand noise could also allow such networks to operate over our existing telecommunications links, which would carry noisy classical traffic at the same time. Some of the results of this work were very unexpected – we had never expected it to work with single-detectors, which it did, and moving to high-dimensions also reduced the required measurement time by almost two orders of magnitude, which was very counterintuitive!