From non-Markovian dissipation to spatiotemporal control of quantum nanodevices

Funding: TL, AWC and BWL thank the Defence Science and Technology Laboratory (Dstl) and Direction Générale de l’Armement (DGA) for support through the Anglo-French PhD scheme. BWL acknowledges support from EPSRC grant EP/T014032/1.Nanodevices exploiting quantum effects are critically important elements of future quantum technologies (QT), but their real-world performance is strongly limited by decoherence arising from local `environmental' interactions. Compounding this, as devices become more complex, i.e. contain multiple functional units, the `local' environments begin to overlap, creating the possibility of environmentally mediated decoherence phenomena on new time-and-length scales. Such complex and inherently non-Markovian dynamics could present a challenge for scaling up QT, but – on the other hand – the ability of environments to transfer `signals' and energy might also enable sophisticated spatiotemporal coordination of inter-component processes, as is suggested to happen in biological nanomachines, like enzymes and photosynthetic proteins. Exploiting numerically exact many body methods (tensor networks) we study a fully quantum model that allows us to explore how propagating environmental dynamics can instigate and direct the evolution of spatially remote, non-interacting quantum systems. We demonstrate how energy dissipated into the environment can be remotely harvested to create transient excited/reactive states, and also identify how reorganisation triggered by system excitation can qualitatively and reversibly alter the `downstream' kinetics of a `functional' quantum system. With access to complete system-environment wave functions, we elucidate the microscopic processes underlying these phenomena, providing new insight into how they could be exploited for energy efficient quantum devices.Peer reviewe

Beyond Markovian dissipation at the nanoscale : towards finding quantum design rules for bio-organic nanodevices:Au delà de la dissipation markovienne à l’échelle nanométrique : vers la découverte de règles quantiques pour la conception de nano-dispositifs bio-organiques

A better understanding of dissipation is crucial for understanding real-world quantum systems. Indeed, all quantum systems experience interactions with an (often) uncontrollable outside environment that can lead to a decay of excited state populations and a loss of quantum coherences. The study of dissipation is timely as the development of next-generation nanoscale quantum technologies is on its way, and the existence of non-trivial quantum effects in biological systems is being seriously investigated. However, descriptions of dissipation in quantum systems are reduced (most of the time) to time-local approaches and (everywhere) to space-local independent environments. These simplifying assumptions do render analytic and numerical calculations possible, yet they get rid of a breadth of physical processes that can alter radically the quantum systems' dynamics. In this thesis, building on a numerically exact tensor networks method, we developed a technique able to handle spatio-temporal correlations between a quantum system and bosonic (i.e. vibrational, electromagnetic, magnons, etc.) environments. With this method we studied the signalling process - a form of information backflow - in quantum systems, and uncovered how it can induce non-trivial dynamics, and be leveraged to populate otherwise inaccessible excited states. We also evidenced the ability of 'non-local' environment reorganisation, induced by system-environment interactions, to radically change the nature of the thermodynamically favoured system ground state. The new phenomenology of physical processes, resulting from considering quantum systems interacting with a common environment, has important consequences for the design of nanodevices as it gives access to new control, sensing and cross-talk mechanisms. In another vein, these results might also give us a new framework to study and interpret (quantum?) effects in the biological realm