Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - QBox (Quantum Gas in a Box)
Teaser
\"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...
Summary
\"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) as 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 ultracold 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 different 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 (almost) inevitably be out of equilibrium, so understanding the general principles of out-of equilibrium quantum behaviour is of paramount importance for these emerging technologies.
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Work performed
\"Since the beginning of this project we have worked on a number of distinct, but often conceptually linked, research directions, and have had a number of breakthroughs. These results have been published in the leading scientific journals (Nature, Science, Physical Review Letters) and presented in invited talks at the flagship international conferences (such as \"\"Frontiers in Quantum Gases\"\" and the \"\"International Conference on Atomic Physics\"\"). They have also received press coverage 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 (meaning that the correlations between the behaviour of the different particles are significant), but is still tractable by existing theories. That is, it is believed to be tractable, but many such already developed theories have not been experimentally tested. One of the highlights of our work (published in Physical Review Letters) is the first quantitative confirmation of the theory of so-called quantum depletion. This theory was developed by Bogoliubov already in 1947, and has conceptually been a cornerstone of our understanding of interacting Bose gases, but has 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 so far been calculated theoretically. These experiments are now posing new challenges for the theorists, but purely experimentally we have (in a series of papers published in Nature, Science, and Physical Review Letters) already 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. Moreover, while a Bose gas in which the interactions are suddenly tuned to the unitary regime is fundamentally a non-equilibrium system, we have shown that it attains an exciting (quasi-)equilibrium state that is both very strongly correlated and has a non-zero fraction of atoms that are in a Bose-Einstein condensate. All this suggests a very exciting possibility that these systems could be a very exotic new kind of superfluids.
As a final highlight, in our work on \"\"shaken, not stirred\"\" Bose gases (published in Nature) we have established our box-trapped gases as a novel platform for studies of turbulence. Turbulence is a long-standing unsolved problem in both classical and quantum physics, and our system may now offer a new perspective on it. There have been previous qualitative studies of turbulence in (parabolically trapped) atomic gases, but we can now for the first time observe and quantitatively study in a quantum gas the so-called turbulent cascades, which are the hallmark of turbulence.\"
Final results
\"All of the above highlights of our work so far 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, by the end of the project we expect many further exciting results. Even with \"\"moderately strongly interacting gases\"\" there are still things to explore and understand, such as the effect of the interaction strength on the \"\"critical behaviour\"\" near the phase transition between a normal thermal gas and a Bose-Einstein condensed gas, but we now expect even more exciting results on the more 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 coherence between atoms and Efimov trimers and 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 we hope to significantly contribute 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. Both our observation of the universal behaviour in the \"\"prethermal\"\" quasi-equilibrium unitary Bose gases and our work on the steady-state turbulence in a quantum gas already significantly contribute to these hopes. Several internationally leading groups are now focusing on these important questions, and the recent boost in progress has been particularly highlighted by the three papers published back-to-back in Nature, in November 2018, by our group, the Oberthaler group from Heidelberg, and the Schmiedmayer group from Vienna. 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.\"