Fast reprogrammable photonic experiments for quantum foundational studies

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My PhD project will study quantum foundations using a hybrid approach of both experiment and theory. The aim is to work on foundational studies, whilst considering applications to quantum technologies, thus working on the overlap between these fields. I will focus on fast reprogrammable integrated photonics for use in quantum foundations experiments. I will also do theoretical work, some of which will be closely linked to experimental but also some stand-alone theory projects. One of the areas in quantum foundations that I will be looking at is quantum causality. In quantum mechanics, the causal relations between events can have an uncertainty in a similar way to other physical properties of a quantum system, resulting in an indefinite casual order [1]. This could have a significant impact on our understanding of the notion of time, but it could also be utilised in quantum technology. For example, a potential computational advantage from using a quantum superposition of qubit gate orders has been demonstrated [2]. Possible applications to quantum metrology [3] and communications [4] have also been investigated. Processes with indefinite causal order have been demonstrated experimentally [5], but superpositions of more complex processes will need to be created for technological applications. The first project I will work on is a direct extension of my project B, which is a chip experiment on higher order quantum causal structures, i.e., more parties and more permutations of order than any experimentally achieved so far. I will finish off characterising chip components, design the PCB, and then carry out the experiment. This will likely take up my first year. Another project we have planned is an experiment on reversing unknown unitaries. The evolution of a quantum system described by a unitary operation is in principle reversible, but this is only possible if the unitary is known. However, we often do not have a complete description of a physical system in order to determine this unitary. Being able to reverse a unitary operation without the need to fully understand the system would be useful for studying irreversibility in quantum mechanics, and for practical applications such as removing noise by reversing unwanted evolutions of a system. Protocols for reversing unitaries have been investigated, I will work on how such protocols can be implemented experimentally. The project will involve designing a chip, having it manufactured, and then carrying out the experiment. Finally, I also plan to do a theoretical project on the energy consumption of quantum computing. This area has gained a lot of interest recently, with the need to consider the potential negative impacts and resource costs of quantum computing [6]. It requires understanding the quantum thermodynamics of quantum computation operations, and how this relates to the energy consumption of macroscopic components. This could also include studying possible fundamental limits on energy consumption at the quantum level.