Single Impurity in a Dipolar Bose-Einstein Condensate

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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.