SINGLE IMPURITY IN A DIPOLAR BOSE-EINSTEIN CONDENSATE

A single impurity interacting with a quantum bath is a simple (to state) yet rich many-body paradigm that is relevant across a wide sweep of fields from condensed matter physics to quantum information theory to particle physics. The aim of this project is to create a highly controllable setting in which to study this physics. The platform for these studies will be ann existing erbium cold-atom machine. The ultracold erbium atoms will be confined in a homogeneous quasi-2D geometry generated using strong harmonic confinement along the vertical direction and box trap in the horizontal plane. The special feature of erbium atoms is their large magnetic dipole moments which result in long-range and anisotropic dipole-dipole interactions in addition to the short-range contact interactions more normally seen in cold atom systems. In this project we will add a second, impurity, atomic species to this experiment. The resulting system will have several advantages over the current state-of-the-artand will open up many avenues for exploration, but for this project we have two main themes/objectives that will be our focus. Polaron' formation in a dipolar BEC. A polaron, as originally conceived, is the quasiparticle formed when an electron is dressed by phonons as it moves through a crystal lattice. An impurity atom immersed in a degenerate Bose gas is a tuneable setting in which to study polaron physics. While impurities in a BEC have been the topic of significant experimental investigation in the past decade it is only very recently that the Bose polaron (so named in contrast to Fermi polarons which occur when the bath is a Fermi gas) have been clearly observed. At least for weak impurity-bath interactions the Bose-polaron is expected to be well described by a Frohlich model which considers an impurity interacting with phonons (which in the case of a BEC are long-wavelength Bogoliubov excitations). The case of an impurity interacting with a dipolar BEC is enriched further for several reasons: (i) the dipole modes become anisotropic, (ii) we have a second type of low-energy excitation (namely the roton) which should also dress the impurity, and (iii) the softening of the roton mode also leads to an enhanced role for quantum fluctuations and the possibility to enter the strong-coupling regime even for weak impurity - bath interactions. I propose to explore this problem by measuring both the static and transport properties of this exotic polaron.While much studied theoretically non-Markovian behaviour is only recently being explored experimentally - so far mainly in photonic systems. A recent proposal has suggested that one possible way to realise such a non-Markovian reservoir would be with a quasi- 2D BEC of dipolar atoms; indeed it is the roton-like feature in the excitation spectrum that results in such characteristics. I propose to realise such a situation and test the nature of information flow (using an impurity atom as the qubit) as the form of the BEC excitation spectrum is changed. Also it would be interesting, and important for understanding the storage of quantum information, to investigate how easily the non-Markovian behaviour is destroyed by increasing the temperature of the bath. Extending the studies of non-Markovian behaviour to atomic systems would represent a key step in the study of open quantum systems and could have important consequences for quantum information technology for example in enhanced spin squeezing and controllable dissipation. Even more generally, a quantum system with a controllable coupling to an engineered reservoir may be able to address fundamental questions about superposition and entanglement and the quantum-classical crossover.