Quantum Gas in a Box

This project is in the field of experimental quantum many-body physics, which means that we study the collective behaviour of many identical particles that interact in some way (pushing or pulling each other), while obeying the laws of quantum mechanics.

In our work, we aim to uncover the general principles of many-body physics using ultracold atomic gases cooled to less than a millionth of a degree above absolute zero temperature. These are highly controllable model systems, in which, for example, the strength of inter-particle interactions can be tuned at will. In our novel "quantum gas in a box" setup the atoms are literally held in a tiny box, about the size of a human hair, made out of laser beams. Compared to the more traditional ultracold-atom experiments, in which the atomic clouds are held in parabolic (bowl-like) traps, this new system offers several advantages - it generally allows for easier comparisons of experiments with the theoretical models and in some cases it allows measurements that could not have been done at all with parabolic traps.

Generally, we understand pretty well quantum many-body behaviour if the particles interact only weakly with each other and if the system is in equilibrium, or close to it. However, things become highly non-trivial if the interactions are very strong, leading to a highly correlated behaviour of the particles. Moreover, even weakly interacting systems can be hard to understand if they are far from equilibrium, for example because of constant changes in the environmental conditions, such as due to external forces that constantly drive the system.

We are focusing on such non-trivial behaviour, and in particular on some of its most extreme examples. One such example is the case of the unitary Bose gas, in which the interactions are as strong as possible, that is, as strong as allowed by the laws of quantum mechanics. Another example is turbulence, where the external forces drive the system in such a way that its behaviour becomes chaotic and extremely hard to characterise from first principles.

Beyond its fundamental importance, better understanding of many-body quantum physics is also relevant for practical purposes. For example, better understanding of the underlying principles of strongly-correlated behaviour in the practically relevant materials, such as the high-temperature superconductors, could allow for controllable design of superior materials with tailor-made properties. Moreover, there is now a number of emerging quantum technologies, where the quantum mechanical behaviour is fundamental for superior performance. Any quantum device that actually does something useful will during its operation inevitably be out of equilibrium, so understanding the general principles of out-of equilibrium quantum behaviour is of paramount importance for these emerging technologies.

In this project we have worked on a number of distinct, but often conceptually linked, research directions, and have had a number of breakthroughs published in the leading scientific journals (Nature, Science, Nature Physics, Physical Review Letters), presented in invited talks at the flagship international conferences, and also featured in more popular scientific journals, such as Physics World.

On one hand, we worked on "moderately" strongly interacting gases, which exhibit behaviour that is non-trivial, but is still tractable by existing theories. That is, it is believed to be tractable, but many such theories have not been experimentally tested. One highlight of our work (published in Physical Review Letters) was the first quantitative confirmation of the theory of so-called quantum depletion. This theory was developed by Bogoliubov already in 1947, and has been a cornerstone of our understanding of interacting fluids, but had until now been lacking explicit experimental verification.

On the other hand, working on the unitary Bose gases, with as-strong-as-possible interactions, we have pushed experiments beyond anything that could be calculated theoretically. These experiments are now posing new challenges for the theorists, but experimentally we have (in papers published in Nature, Science, and Physical Review Letters) firmly established some exciting properties of these exotic systems. For example, they are profoundly affected by simultaneous interactions of three (rather than just two) particles, due to the celebrated Efimov effect, first discussed in nuclear physics.

As another highlight, in our work on "shaken, not stirred" Bose gases (published in Nature and Science) we have established our box-trapped gases as a novel platform for studies of turbulence, which is a long-standing unsolved problem in both classical and quantum physics.

All of the above highlights of our work went significantly beyond the prior state of the art, either confirming for the first time decades-old textbook theories or opening completely new research directions and challenging theorists.

Looking forward, we expect even more exciting results on the newly established topics of the unitary Bose gases and turbulence in box-trapped gases. Our understanding of the unitary Bose gases is still in its infancy, and a lot of interplay between theory and experiments will be required to understand them better, but there are many exciting research avenues to pursue, including the possible superfluid properties of these systems. On the side of turbulence, one particularly exciting possibility is that in our system (unlike other systems where turbulence is studied) we can readily tune the dissipation, which in a classical fluid is set by the viscosity, which should allow a completely new class of experiments and better testing of the various theories that predict behaviour that crucially depends on the dissipation in the system.

Beyond understanding better these specific problems, there is now also an important newly emerged overarching grand challenge that our work significantly contributes to. This challenge is to develop a general understanding of the classification and universal features of out-of-equilibrium quantum many-body systems, in a way analogous to our understanding of the equilibrium states of matter and the universality classes that characterise the phase transitions between them. The basic idea is that the universality of some kind of behaviour implies that from experiments on one physical system, such as an atomic gas we have in the lab, we can learn something about behaviour in a system that is not experimentally accessible, such as the behaviour of the early universe. This idea is at the heart of "quantum simulation", but it was previously mainly discussed in the context of equilibrium states, and the new hope is that it can be extended to far-from-equilibrium behaviour. Our observation of the universal behaviour in the "prethermal" quasi-equilibrium unitary Bose gases, our work on the steady-state turbulence in a quantum gas, and our observation of universal dynamics in a weakly-interacting Bose gas, all significantly contribute to these hopes. A lot of the ideas are still speculative, and this remains a high-risk-high-gain research direction, but the scientific payoffs over the next few years could be formidable.

Periodic Reporting for period 4 - QBox (Quantum Gas in a Box)

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