SINGLE IMPURITY IN A DIPOLAR BOSE-EINSTEIN CONDENSATE

单个杂质与量子浴相互作用是一种简单但丰富的多体范例，涉及从凝聚态物理到量子信息论再到粒子物理的广泛领域。这个项目的目的是创造一个高度可控的环境来研究这个物理学。这些研究的平台将是一台现有的铒冷原子机。超冷的Er原子将被限制在均匀的准2D几何结构中，该几何结构是由垂直方向的强谐限制和水平面上的盒陷阱产生的。铒原子的特殊特征是它们的大磁偶极矩，这导致了长程和各向异性的偶极-偶极相互作用，以及冷原子系统中常见的短程接触相互作用。在这个项目中，我们将在这个实验中添加第二个杂质原子物种。由此产生的系统将比目前最先进的系统有几个优势，并将开辟许多探索的途径，但对于这个项目，我们有两个主要主题/目标将是我们的重点。极化子在偶极BEC中的形成。正如最初设想的那样，极化子是电子在穿过晶格时被声子覆盖而形成的准粒子。处于简并玻色气体中的杂质原子是研究极化子物理的可调环境。虽然BEC中的杂质在过去十年中一直是重要的实验研究的主题，但直到最近才清楚地观察到玻色极化子(与费米极化子相反，当浴液是费米气体时出现的费米极化子)。至少对于弱的杂质-浴相互作用，玻色-极化子有望用Frohlich模型很好地描述，该模型考虑了杂质与声子的相互作用(在BEC的情况下，声子是长波长的Bogoliubov激发)。由于几个原因，杂质与偶极BEC相互作用的情况进一步丰富：(I)偶极模变得各向异性，(Ii)我们有第二种低能激发(即roton)，它也应该修饰杂质，以及(Iii)roton模的软化也导致量子涨落的作用增强，即使对于弱的杂质-槽相互作用，也有可能进入强耦合区域。我建议通过测量这种奇异极化子的静态和输运性质来探索这个问题。虽然在理论上进行了很多研究，但非马尔科夫行为直到最近才被实验探索-到目前为止，主要是在光子系统中。最近的一项提议表明，实现这种非马尔可夫储存库的一种可能方法是利用偶极原子的准2DBEC；实际上，正是激发光谱中的类似旋子的特征导致了这种特征。我建议认识到这种情况，并测试信息流的性质(使用杂质原子作为量子比特)，因为BEC激发光谱的形式发生了变化。此外，对于理解量子信息的存储，研究通过提高浴的温度来破坏非马尔可夫行为也是很有趣的，也是很重要的。将非马尔科夫行为的研究扩展到原子系统将代表着开放量子系统研究的关键一步，并可能对量子信息技术产生重要影响，例如在增强自旋压缩和可控耗散方面。更广泛地说，一个与工程水库可控耦合的量子系统可能能够解决关于叠加和纠缠以及量子与经典交叉的基本问题。

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.