Collective Quantum Thermodynamics

Heat engines are the motors of our industrialised society. By converting thermal energy into mechanical work, they set cars, airplanes and ships in motion and drive the generators that deliver electricity to our computers and smartphones. None of these modern applications would be possible without one fundamental theory that emerged 200 years ago and has ever since enabled engineers to develop more and more powerful and efficient machines: thermodynamics. Equipped with only a few elementary concepts and laws, this theory lays down the basic rules that govern the performance of James Watt's 18th century steam engine and today's car engines alike.

During the past two decades, a new era has begun, in which scientists are exploring miniaturisation as a novel design principle for thermal engines. In a series of landmark experiments, smaller and smaller engines have been built and successfully operated. In 2016, this fascinating development led to the realisation of a functional heat engine with only one atom. Objects this tiny are no longer bound by the mechanical rules of our classical world; they can occupy two places at the same time, tunnel through barriers or influence each other at a distance without direct interaction. These counterintuitive phenomena are manifestations of the quantum laws of motion that govern the world at atomic scales. Heat engines operating in this realm can be equipped with features that no classical engineer could have imagined. The scientific discipline that describes this new type of machine and tries to harness their technological potential is still in its infancy and has been dubbed quantum thermodynamics.
Although likely able to overcome classical performance limits, quantum engines are still far from practical applications, not least due to their minuscule energy output; to move a car, one would need roughly as many single-atom engines as there are molecules in one liter of water. This number is absurdly large, mainly because it compares objects at radically different scales. Still, it is clear that, even to be useful for technologies on their own scale, quantum engines need to grow. But how can their size be increased when smallness is precisely the property that makes them quantum?

Quantum mechanics provides a solution to this dilemma: collective behaviour. Due to a strange interaction without a classical counterpart, objects like atoms can act in a coordinated way, like birds in a flock. This remarkable phenomenon has fascinated scientist for decades. Here, we propose to utilise it for the next generation of quantum machines. Imagine an engine working with a collective quantum gas containing millions of atoms instead of just one. Such a device could benefit from quantum effects while still producing significant power output. Moreover, the pistons of this engine could be perfectly synchronized with all the atoms they move around. Thus, an enormous level of control could be achieved, which would be impossible to realise with an ordinary gas, whose atoms follow unpredictable trajectories. Such unique features make collective quantum machines a fascinating yet unexplored subject of quantum engineering. Laying down the conceptual foundations for the design and implementation of this new type of device is the major goal of this project. The theory we will develop at the University of Nottingham will be the counterpart of thermodynamics in the world of collective quantum phenomena: collective quantum thermodynamics.

Quantum technologies are widely expected to shape our century in a similar way as the industrial revolution changed 19th and 20th century. Collective quantum machines have the potential to become the steam engines of this development. They will not move our future cars, but they might well provide the power for our quantum computers and encryption devices.

This proposal pushes for the development of a new line of basic research at the interface of quantum thermodynamics, quantum-many body and ultracold atom physics, three fields that are of strategical importance for the UK as a global centre for quantum technologies. Our major aim is thereby to lay the theoretical groundwork for novel thermal devices that utilise the collective properties of quantum fluids to enable efficient cooling, waste heat recovery and energy transport at small length scales and low temperatures. This vision has significant potential to create a large impact on economy and society, which will mostly develop over a longer time span, once our new concepts and ideas can be efficiently implemented. Since our research is of fundamental nature, short-term impact will primarily occur within the academic sector. In the following, we outline possible impacts and measures for their realisation.

Impact by knowledge. - To realise our ambitions, we will establish new connections between the pool of researchers working at the University of Nottingham in quantum many-body theory and experimental ultracold atom physics as well as the UK's rapidly growing quantum thermodynamics community, e.g. through contributions to regular seminars or national meetings. We will thus generate impact by fostering cross-disciplinary thinking maximising synergies and creating new communication channels. Moreover, we will further corroborate the international recognition of the scientific community in the UK as a world-leading player in quantum technologies by propagating our research in a targeted manner to key communities in the US, Europe and Asia through the extended networks of our collaborators.

People. - Our research programme offers excellent opportunities for all involved PDRAs and PhD students to acquire advanced knowledge in cutting-edge scientific methods, e.g. from statistical physics and dynamical control theory, hands-on experience in the development of new quantum technologies as well as a variety of transferable skills like project management and coherent writing. Our project will thus impact on the formation of the next generation of researchers and provide the community with highly qualified experts, who will also be strong candidates for specialist and leadership position in the UK's emerging quantum technologies industry.

Society and Economy. - The fundamental research proposed here will pave the way for practical thermal devices based on macroscopic quantum effects thereby underpinning the next revolution of "complex quantum technologies". This vision could lead to radically new commercialisation strategies for solid-state based quantum devices and thus has great potential to create strong and long-lasting economic benefit in the long run. While the time scales, on which marketable devices will become available, might well exceed the lifetime of this project, we will still create direct societal impact by helping to shape the next generation of research priorities and strategic goals.

Outreach. - Capitalising on the fact that thermodynamics is a topic of our everyday life, we will create public awareness for fundamental research and engage with the non-academic community by further strengthening established communication channels of the University of Nottingham and the School of Physics and Astronomy. Additionally, we will set up our own blog to promote our research to non-scientists in a targeted manner. Furthermore, the applicant will continue his strong track record in making cutting-edge research accessible to undergraduate and early graduate students through suitable teaching offers, thereby creating impact by attracting young talents to the rapidly growing area of quantum technologies.