Dilute Quantum Fluids Beyond the Mean-Field

If we peer deep inside nature to a microscopic level, we find a strange world governed by quantum mechanics where our intuition breaks down. In this fascinating regime, the position of a particle has inherent uncertainty and is perpetually fluctuating. Such quantum fluctuations lie at the heart of a number of physical phenomena, ranging from the van der Waals force to Hawking radiation in black holes, and may provide the ultimate limit to technologies based on quantum effects. However, quantum fluctuations are difficult to observe experimentally and to describe theoretically.

Since their realization in 1995, Bose-Einstein condensates (BECs) have provided a unique window through which to view the quantum world. A BEC is a gas of identical atoms cooled down to less than a millionth of a degree above absolute zero. At this point the uncertainty in an individual atom's position becomes greater than the separation between atoms and it is impossible to identify individual atoms. Instead, the gas behaves like a giant wave of matter dominated by quantum mechanics, and displays a range of striking quantum properties such as the ability to interfere with another BEC and the ability to flow without viscosity (superfluidity). In addition, BECs are amenable to a high degree of experimental control (for example, to manipulate and interrogate the system in time and space) and they can be imaged to high resolution.

The behaviour of BECs, including the properties above, are captured to a high degree of accuracy by considering just the average behaviour of all the atoms: the so-called "mean-field". Over the years since 1995, a synergy of experiments and theoretical works have established a deep understanding of the quantum mean-field and how it influences the system behaviour. However, in a BEC, quantum fluctuations are small compared to the mean-field, and as such, the merits offered by BECs have not extended to the realm of quantum fluctuations.

Enter the quantum liquid droplet. When two BECs co-exist, the mean-field quantum effects from each BEC can be made to cancel each other out, leaving behind the quantum fluctuations as the dominant effect within the system. This causes the system to change from a BEC gas to a liquid-like droplet. But this is far from your conventional liquid droplet: whereas, say, water is hard to compress because the electronic shells of neighbouring atoms refuse to overlap, in the quantum liquid it is because of quantum fluctuations. As such, the quantum droplet owes its existence to intrinsically quantum effects; this makes it a fascinating object to study. Moreover, it provides a platform to study quantum fluctuations, from their microscopic origins to their macroscopic manifestations.

We will engineer quantum droplets, for the first time in the UK, using a mixture of caesium and ytterbium BECs; this atomic combination will enable us to exert high levels of control over the liquid. Given that this state has only recently been discovered, there is much to study and learn. We will use our experimental capabilities to push the droplets to their limits. We will map out the regimes for which they are supported, as well as the details of how they form. We will experimentally interrogate them in a range of scenarios, effectively prodding and pushing them, to understand how they respond. We will pay particular attention to effectively 2D and 1D geometries where quantum fluctuations are predicted to be greatly enhanced. Alongside our experiments, we will develop and test theoretical models to describe our observations; this will allow us to address open questions regarding the underlying physics and quantify the precise role of the quantum fluctuations. The findings of our work will be of fundamental importance in deepening our understanding of quantum fluctuations and may motivate applications of quantum droplets such as in precision spectroscopy and deposition.

The research outlined in this proposal will have an impact across all four of the EPSRC-defined impact themes - knowledge, people, society and economy - in a number of ways:

1. Supply of highly trained personnel

Modern industry requires personnel with strong technical backgrounds and highly developed problem solving skills. Through this proposal, two PDRAs and three PhD students will acquire expertise in a range of state-of-the-art experimental and theoretical techniques in quantum science. Additionally, we have a strong track record of training undergraduates within the Joint Quantum Centre and we will involve students in the project wherever possible. All personnel connected with the project will gain professional and transferable skills highly sought after in the current job market (e.g. project and time management, supervision experience, communication and presentation skills). Such skills are widely applicable in, for example, the education, defence, R&D, technology and finance sectors.

2. Development of high-tech equipment

This proposal will drive the development of high-tech equipment with potential benefits to UK companies in areas such as photonics or lasers. The nature of our research often requires us to develop new techniques or devices that can lead to commercial exploitation. For example, in the past we have developed a simple resonant electro-optic modulator that is now commercially available from Photonic Technologies, a small UK based company. Similarly, working closely with manufacturers, our research can drive the improvement of existing products and the development of new product lines.

3. Public engagement

The general public will benefit from our efforts to communicate our research in simple terms, helping ensure that the public is fully engaged with science and recognises its enormous importance in the economy and society. In addition to public lectures, laboratory tours and outreach activities, we will add videos and non-technical synopses of our research publications to our web pages.

4. Knowledge generation

In the short term, our proposal will promote our fundamental understanding of quantum gases, beyond-mean-field physics and non-equilibrium dynamics. At the same time, we will develop advanced experimental and theoretical techniques for manipulating, controlling and modelling quantum systems. These will be of immediate benefit to the ultracold atom and quantum fluid communities. Our improved understanding of quantum fluctuations will contribute to a range of problems in condensed-matter physics (e.g. superfluid helium), cosmology (e.g. Hawking radiation in black holes) and quantum field theory (e.g. Casimir effect). Our research is relevant to two of the current Physics Grand Challenges: "Emergence and Physics Far From Equilibrium" and "Quantum Physics for New Quantum Technologies". The former through our investigation into the role of quantum fluctuations on non-equilibrium quantum dynamics, and the latter by improved understanding of quantum fluctuations (which are detrimental to quantum technologies that rely on quantum entanglement and coherence). We will promote this knowledge impact through publications, conference and seminar presentations, and the targeted workshop we will organise.

Overall, our research will contribute to the competitiveness of science research in the UK, which in turn will help attract highly skilled personnel, funding and even companies into the UK economy. It will probably take more than a decade for the knowledge generated in our research to spread beyond the academic community and on this time-scale it is hard to assess the full impact. Nevertheless, we can expect potential impacts in the areas of quantum metrology, precision measurement and quantum technology. Indeed, this research will contribute to maintaining the strong base of world-leading quantum science in the UK that is vital to the development of future quantum technologies.