Single Impurity in a Dipolar Bose-Einstein Condensate

My research involves using cold atomic gases to study the strange world of quantum mechanics where particles behave as waves. In the 20th century, it was the understanding of the quantum nature of how electrons behave that enabled the development of computers, iPhones and most of modern technology. However, there are many quantum phenomena that are not fully understood. This is due to the difficulty of understanding a system with so many particles all interacting with each other. For example, there are more electrons in a cubic centimetre of metal than stars in the observable universe, while even super-computers struggle to simulate more than a few hundred particles. What makes ultracold atomic gases so useful in this quest to understand quantum many-body systems is that they can be manipulated using tools such as lasers and magnetic fields in a highly controlled environment.

The particular system that I wish to study is that of a single impurity atom embedded in the 'quantum bath' of an ultracold atomic gas. Anybody who has tried to drive or walk through a crowd of people will know that the interactions between the individual and the crowd leads to significant changes of behaviour (most obviously it slows the person or vehicle down!). The same is true of an impurity particle interacting with a bath of particles, so much so that the embedded particle can be thought of as a 'new' particle - a quasiparticle which has new properties (for example a modified mass). The nature of the new particle depends on both the properties of the bath it which it sits and also on how it interacts with that bath. This general problem is a rich many-body paradigm that is relevant across a wide sweep of fields from condensed matter physics to quantum information theory to particle physics.

This grant will allow me to add an impurity atomic species (potassium) to my existing erbium cold-atom machine (which was funded from an EPSRC programme grant). Erbium atoms have the special feature of long-range dipole-dipole interactions and the addition of a potassium impurity species will result in a unique experimental system with distinct advantages for the study of quantum impurity problems that will enhance our understanding of both materials and quantum information technology, making us better placed to develop the new technologies of the 21st century. Below I outline two specific experiments that I plan to carry out.

First, I will investigate how the coupling of a potassium impurity atom to the dipolar bath changes both its energy and its mass. This is closely related to the problem of an electron in a metal or semiconductor and so could provide insights that help us to explain and control phenomena such as colossal magnetoresistance (used for data storage) and superconductivity.

Second, setting up the impurity as a qubit (quantum bit) I plan to investigate the physics of de-coherence in quantum systems coupled to a reservoir and explore something known as non-Markovian dynamics whereby information can be recovered from the reservoir (rather than there just being a one-way information flow). This both addresses interesting fundamental questions in quantum mechanics as well as potentially having an impact on quantum information processing.

The initial focus of this project is to enhance our fundamental understanding of quantum impurity systems and quantum information flow. This will directly contribute to the identified Grand Challenges of Physical Sciences - "Emergence and Physics Far from Equilibrium" and "Quantum Physics for New Quantum Technologies". While initially, our impact in this area will mainly be in the academic community - strengthening the UK's competitiveness and adding to and stimulating new research in these areas - in the longer term our 'fundamental' work should also contribute to new technologies in identifiable ways as outlined below.

Polarons are thought to play a key role in understanding a range of important materials including high-Tc superconductors, transparent conducting oxides and manganites displaying colossal magnetoresistance. These phenomena have a multitude of current and potential applications including in touchscreens, photovoltaics, liquid crystal displays, energy transportation and storage and computer memory. Our work on polaron physics will enable a greater understanding of polarons and polaron interactions which could facilitate further developments in these highly functional materials.

A key ingredient in successfully implementing a large-scale quantum computer will be an understanding of how quantum systems interact with their environments. Our work on non-Markovian dynamics of qubits coupled to their environment, decoherence in quantum systems more generally, and the optimal methods to control decoherence will add to this understanding and thus further the quest for a useable quantum computer.

A further important impact of this research will be in training highly skilled researchers (i.e. the postdoctoral research associate on the grant and the two PhD students who will be aligned with it) in a range of interdisciplinary techniques. As well as specific skills in quantum physics, these will also include experimental skills in optics, electronics and ultra-high vacuum as well as proficiency in data analysis and written and oral presentation. Such as skill set is highly valuable for a range of high-tech sectors and in particular for the emerging industries related to quantum technologies (for example, Oxford Quantum Circuits and Oxford Ionics have recently spun out of Oxford University and are founded and staffed by such early career scientists).

The wider public will also benefit from the engagement events that are planned in association with the grant. These include participation in quantum themed open days as well as publicising our results to a more general audience using platforms such as Oxford Sparks (https://www.oxfordsparks.ox.ac.uk/).