Probing Non-Equilibrium Quantum Many-Body Dynamics with Bright Matter-Wave Solitons

Lead Research Organisation: Durham University
Department Name: Physics


Dilute gases of alkali atoms are now routinely cooled to within a millionth of a degree of absolute zero using laser light, permitting them to be confined in traps formed due to the interaction of the atom with either an applied magnetic field or a far-detuned off-resonant laser beam. Further cooling by evaporation in such traps leads, in the case of bosonic atoms, to the creation of a new state of matter, known as a Bose-Einstein condensate, in which the quantum mechanical nature of the particles dominates over their classical behaviour. Such condensates are often viewed as the atomic or matter-wave equivalent of coherent laser light.

Since their first observation in 1995, Bose-Einstein condensates have been used with great success to investigate a vast range of physical phenomena from fundamental studies of superfluidity to strongly correlated many-body states in optical lattices, providing insight into more complicated condensed matter systems. This success stems from two important features of ultracold quantum gases. Firstly, from an experimental stand-point, ultracold atomic gases are readily manipulated and controlled with external electromagnetic fields (dc, radio-frequency, microwave and optical) permitting a very high degree of real-time flexibility in the experimental configuration and highly sensitive detection. Secondly, Bose-Einstein condensates have proved theoretically tractable, due largely to their dilute, weakly interacting nature, leading to a deeper understanding of experimental observations. This makes ultracold quantum gases an ideal testing ground for the cutting-edge developments in our theoretical understanding of the behaviour of many-body quantum systems.

Here, we propose a program of fundamental research intended to yield a better general understanding of the dynamics of non-equilibrium interacting quantum many-body systems, using atomic Bose-Einstein condensates of 85Rb. Specifically, we will exploit a collision resonance (known as a Feshbach resonance) between two 85Rb atoms to tune the atomic interactions in the condensate to be attractive, thereby generating bright matter-wave solitons; robust, non-dispersive atomic wave-packets confined to propagate in one dimension, in which the attractive atomic interactions exactly compensate the usual dispersion. Solitons arise as solutions to nonlinear partial differential equations describing a diverse range of physical systems. First observed in the shallow water of the Union Canal in Scotland in 1834, solitons have since been studied in many other contexts, including nonlinear optics, biophysics, astrophysics and particle physics. In the atomic context, the underlying quantum nature of the system provokes sophisticated many-body quantum treatments to accurately capture the essential physics. This proposal describes a systematic, closely interlinked experimental-theoretical study of such "quantum" bright matter-wave solitons with a view to exposing the coherence and entanglement properties of bright solitons, whilst developing new advanced theoretical treatments applicable to other quantum many-body systems. Working together with the leading international experts in the field, we aim ultimately to assess the feasibility of using quantum bright solitons to generate Schrödinger cat states for quantum-enhanced interferometry. The proposed research falls within the remit of two of the identified current Grand Challenges in Physics, "Emergence and Physics Far From Equilibrium" and "Quantum Physics for New Quantum Technologies", and thereby contributes to UK science in areas where there is recognised potential for significant societal and economic impact.

Planned Impact

The research outlined in this proposal will have an impact on society and the economy in a number of ways, both in the short and long term:

1. Supply of highly trained personnel.
Modern high-tech industry requires personnel with strong technical backgrounds and highly developed problem solving skills. Through this proposal we will train at least two PDRAs and two PGs in state-of-the-art experimental and theoretical techniques in atomic and molecular physics. Moreover, they will gain professional and transferable skills highly sought after in the current job market (e.g. project and time management, communication and presentation skills). The skills they acquire could be applied in the education, defence, R&D, technology and finance sectors, for example.

2. The 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 which can lead to commercial exploitation. For example, in the past we have developed a simple resonant electro-optic modulator which 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.

3. Presentation of our research to the wider community.
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. Long term impact of knowledge generation.
Our proposal will yield a better fundamental understanding of non-equilibrium interacting quantum many-body systems, relevant to two of the current Physics Grand Challenges. Our efforts 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 identify several areas of potential impact:
(i) Quantum metrology and precision measurement.
Our studies of Schrödinger-Cat states are important to applications in quantum-enhanced interferometry and metrology; areas of research which ultimately may lead to improved clocks (relevant to GPS, for example) and improved precision measurement experiments which affect our fundamental understanding of the Universe.
(ii) Quantum technology.
Controlling practical devices at the quantum level is undoubtedly going to be important in future technologies. The fundamental insights we gain into simple quantum systems and the role of decoherence in them, will form the basis for a deeper understanding of more complex systems of commercial relevance.
(iii) Quantum information.
Quantum information based protocols are already used for secure communication, for example, in the financial sector. Again, our research into the fundamental aspects of quantum many-body systems will help underpin the development of more sophisticated devices and protocols in this emerging area.
Description We have for the first time observed quantum reflection and splitting of bright matter wave solitons from an attractive potential (where there is no classical turning point in the motion of the particles. This is a fascinating demonstration of the wave nature of matter at a macroscopic level.

We have also used repulsive Gaussian potentials to split a single soliton into two daughter solitons and have then studied the recombination of the daughter solitons when they collide again on the barrier following an oscillation in the trap. We find regimes in which either velocity selection or interference mediated recombination dominates. Interference mediated recombination can be used for matter-wave interferometry. We have established that the recombination is very sensitive to the position of the repulsive Gaussian barrier and have put limits on the stability of the barrier needed to successfully implement soliton interferometry.

In separate experiments we have studied the formation and dynamics of vortex lattices created in a Bose-Einstein condensate by rotating the confining potential.

On the theory side, we have explored several novel aspects of quantum solitons and investigated new schemes for building rotational sensors using solitons in a ring trap. This opens up new avenues for future investigation.
Exploitation Route Our work will be of most interest to the academic community interested in nonlinear matter-waves and soliton dynamics. Our proposals for new interferometry schemes may be taken up by other research groups with a particular technological interest in inertial sensing, but probably need to be researched further scientifically first.
Sectors Aerospace, Defence and Marine,Education

Description Soliton Conference - Durham 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact We organised an international conference in Durham connected to the research grant on "non-equilibrium quantum dynamics in low dimensions". The conference was attended by approximately 100 participants from all over the world and significantly raised the profile of our research activities.
Year(s) Of Engagement Activity 2015