Phase Transitions and Non-equilibrium Dynamics in Homogeneous Quantum Gases

Lead Research Organisation: University of Cambridge
Department Name: Physics

Abstract

Atomic gases cooled to less than a millionth of a degree above absolute zero temperature offer flexible experimental systems for fundamental studies of quantum mechanical effects. In particular they offer excellent test-beds for theories of many-body phenomena, such as Bose-Einstein condensation and superfluidity, which arise from the collective behaviour of many interacting particles.

Over the past two decades, studies of many-body physics with ultracold atomic gases have been very successful. The resulting improved understanding of the fundamental principles of quantum mechanics is promising to also lead to many practical applications, including the development of novel materials, navigation instruments, and platforms for quantum information processing. A complete understanding of many-body physics, necessary for such developments, must include both equilibrium and non-equilibrium phenomena, and in the latter respect ultracold atomic gases offer some significant experimental advantages. Specifically they feature relatively long characteristic timescales (milliseconds to seconds), which allow for time-resolved studies of non-equilibrium processes.

Traditionally, an important difference between "conventional" many-body systems, such as liquid helium and solid-state materials, and ultracold gases has been that the former are usually spatially uniform, while the latter were produced in bowl-like harmonic traps. This difference can sometimes be addressed using the so-called local density approximation (LDA), which assumes that the gas density varies slowly in space. However, for studies of some very important problems this is a serious hindrance. In particular, LDA breaks down close to phase transitions between different states of matter, where the particles become correlated over very large distances, and where some of the most interesting non-equilibrium effects also emerge.

Recently, our group has made a major breakthrough in the field of ultracold atoms by creating first quantum gases in an essentially uniform trapping potential of an optical-box trap. This allows for closer connections with both other many-body systems and the theoretical calculations, and has opened up completely new research possibilities. It has already led to an important advance in the studies of non-equilibrium many-body physics, namely the most direct confirmation so far of the Kibble-Zurek theory of the dynamics of continuous phase transitions, which has implications as far reaching as understanding the formation of large structures in the early universe.

Under this Fellowship we will focus on fully capitalising on these exciting new developments. We will perform a comprehensive study of non-equilibrium phenomena in a homogeneous Bose gas with dynamically tuneable interactions, and will also collaborate with two other internationally leading groups, at College de France and MIT, to extend these studies to low-dimensional systems and Fermi gases. This will cement the UK's leadership in the emerging field of homogeneous quantum gases and more generally enhance the UK's position in the highly competitive field of ultracold atoms.

Planned Impact

Our research is primarily fundamental, but there are several ways in which it could in the long run have a large impact on practical applications that would be beneficial for the society at large. Specifically, our work will directly contribute to two already identified Grand Challenges of Physical Sciences - "Emergence and Physics Far from Equilibrium" and "Quantum Physics for New Quantum Technologies".

Research on ultracold atoms is already finding some applications in fields such as sensing and navigation (as pursued in one of the recently founded EPSRC Quantum Technology Hubs) and can in the future, on a timescale of a decade, find further practical applications in fields such as information technology and materials science. Due to this long timeframe, it is difficult to identify specific end users and beneficiaries at this stage. However, it is already possible to identify ways in which our fundamental work could contribute both to the next generation of the already-pursued quantum technologies and to completely new quantum-technology developments:

(1) We will very directly contribute to the fundamental understanding of non-equilibrium quantum phenomena, which will be essential for all future technologies that aim to harness quantum correlations for enhanced device performance. Specifically, the rate at which a quantum system can adjust to the changes in its parameters or its environment will determine the fundamental limits on the speed of operation of quantum devices. This issue will be central for future technology developments in a wide range of fields, including force sensing, navigation, and quantum information processing. It therefore falls within the interest areas of (future) hi-tech companies, as potential end users. The scientific outputs of our work will feed into R&D efforts of others, including industrial partners already interested in the participation in the UK Quantum Technologies Hubs and their satellites.

(2) Our work on the homogeneous Fermi gases (in collaboration with MIT) will significantly contribute to the on-going efforts to understand the origin of high-temperature superconductivity. Better understanding of this phenomenon should facilitate controlled design of novel materials with tailor-made properties for practical applications, and the ultimate goal is to design materials which are superconducting at room temperature. This would have an enormous economic impact in the energy sector, through large electrical-energy savings, and benefit the society as a whole.

A more immediate impact on the economy and society will be the training of highly skilled work force. We will train students and postdocs in a range of cutting-edge experimental techniques, and also provide them with opportunities and guidance to develop highly transferrable organisational and communication skills. The provision of such multi-skilled workforce will be absolutely essential both for keeping the quantum research in the UK at the international forefront, and for providing trained staff for industry at a crucial time when the Quantum Technologies are taking off.

Another immediate economic benefit will be bringing of foreign investments into the UK. Already at this stage the long-term potential of our research is attracting investments from the US and we expect such investments to grow in the future.

Publications

10 25 50
 
Description This grant is still ongoing, but we have already had several breakthroughs and have already published 14 high-profile papers, including 2 Nature papers, 2 Science papers, 1 Phys. Rev. X and 6 Phys. Rev. Letters.

To select just four highlights:

(1) We have demonstrated the possibility to use a homogeneous quantum gas as a new experimental platform for highly controllable and quantitative studies of turbulence, a topic that has been fascinating and challenging scientists for centuries. This work has led to two major publications, one in Nature and one in Science.

(2) We have made the first observation of the coherent effects of three-body interactions in a quantum gas, which promises to open a new era in the understanding of the interplay between few- and many-body physics. This work was published in Science.

(3) With our project partners at MIT we have now realised the first homogeneous Fermi gas, which was one of the proposed milestones for this grant. This work was published in Phys. Rev. Lett. as an Editors' Suggestion.

(4) We have observed universal non-equilibrium dynamics in a Bose gas quenched to the unitary, where the interactions between the a particles are as strong as theoretically possible. This is interesting both for the continuing progress in understanding the specific problem of the unitary Bose gas, which may be a truly novel state of matter, and for the general world-wide efforts to gain better understanding of quantum many-body systems far from equilibrium. This work was published in Nature and featured in Physics World as one of 'core physics' Breakthroughs of 2018.

Papers published under this grant have already been cited more than 300 times.
Exploitation Route Our work on non-linear and non-equilibrium phenomena (such as matter-wave collapse, turbulence and dynamics of quenched quantum systems) is of interest to many other fields, from mathematical physics and studies of non-linear differential equations, to industrially relevant studies of turbulent fluids. Our realisation of the first homogeneous Fermi gas significantly strengthens the links between atomic and solid-state physics communities.
Sectors Aerospace, Defence and Marine,Education,Electronics

URL http://www-amop.phy.cam.ac.uk/amop-zh/
 
Description Our work is blue sky, but beyond its immediate scientific value it has a great impact through training of extremely skilled people that are becoming new leaders in science, education and industry. In large part as a direct consequence of the work associated with this grant three of the PI's younger collaborators got very high profile jobs in academia and industry - an associate professorship at Oxford (Robert Smith), an assistant professorship at MIT (Richard Fletcher), a permanent research position with the French CNRS (Raphael Lopes) and a permanent PI position at Microsoft Research (Alexander Gaunt); as a result of Gaunt's appointment we have also established a collaboration with Microsoft Research.
First Year Of Impact 2017
Sector Digital/Communication/Information Technologies (including Software),Education
Impact Types Societal,Economic

 
Title Research data supporting "Observation of Weak Collapse in a Bose-Einstein Condensate" 
Description The repository contains figure data as well as additional raw data sets that were used to extract the figure data points. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Title Research data supporting "Observation of Weak Collapse in a Bose-Einstein Condensate" 
Description The repository contains figure data as well as additional raw data sets that were used to extract the figure data points. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Title Research data supporting "Universal Prethermal Dynamics of Bose Gases Quenched to Unitarity" 
Description The Figure folders contain the data points which reproduce each of the plots in the paper. The read-me files for each individual figure provide additional information. The Data folder contains the supporting data series (time- and momentum-resolved data used to extract the half-way times and quasi-steady-state (re)distributions). Additional information is available in the '?Overivew.txt' files. Contact details: ce330@cam.ac.uk / christoph.eigen@gmail.com zh10001@cam.ac.uk 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Title Research data supporting 'Quasiparticle energy in a strongly interacting homogeneous Bose-Einstein condensate' 
Description The supporting data contains both the data points of all paper figures and additionally the raw Bragg resonance spectra that were used in their production. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Research data supporting 'Two- and Three-body Contacts in the Unitary Bose Gas' 
Description The repository contains the raw Ramsey oscillation data that were used to extract the figure data points. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Supporting Data for 'Quantum Depletion of a Homogeneous Bose-Einstein Condensate' 
Description The supporting data contains both the data points of all paper figures and additionally the raw Bragg Rabi-oscillation data that were used in their production. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Supporting Data for 'Quantum Depletion of a Homogeneous Bose-Einstein Condensate' 
Description The supporting data contains both the data points of all paper figures and additionally the raw Bragg Rabi-oscillation data that were used in their production. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes