SINGLE IMPURITY IN A DIPOLAR BOSE-EINSTEIN CONDENSATE

单一杂质与量子浴相互作用是一个简单的（状态）但丰富的多体范式，涉及从凝聚态物理到量子信息理论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种物理。这些研究的平台将是一个现有的铒冷原子机。在垂直方向上使用强谐波约束，在水平面上使用箱形阱，将超冷铒原子限制在均匀的准二维几何结构中。铒原子的特殊之处在于它们具有较大的磁偶极矩，除了在冷原子系统中常见的短程接触相互作用外，还能产生远距离和各向异性的偶极-偶极相互作用。在这个项目中，我们将在这个实验中加入第二种杂质原子。由此产生的系统将比目前最先进的系统有几个优势，并将为探索开辟许多途径，但对于这个项目，我们有两个主要主题/目标，这将是我们的重点。偶极态BEC中极化子的形成。极化子，正如最初设想的那样，是电子在穿过晶格时被声子包裹而形成的准粒子。浸没在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。虽然在过去十年中，BEC中的杂质一直是重要实验研究的主题，但直到最近才清楚地观察到玻色极化子（这样命名与费米极化子相反，费米极化子发生在费米气体中）。至少对于弱杂质-槽相互作用，玻色极化子有望被一个考虑杂质与声子相互作用的Frohlich模型很好地描述（在BEC的情况下，声子是长波长的波哥留波夫激发）。杂质与偶极BEC相互作用的情况进一步丰富，有几个原因：(i)偶极模式变得各向异性，（ii）我们有第二种低能量激发（即旋转），它也应该包裹杂质，（iii）旋转模式的软化也导致量子涨落的作用增强，甚至在弱杂质浴相互作用中也有可能进入强耦合状态。我建议通过测量这个奇异极化子的静态和输运性质来探索这个问题。虽然理论上研究了很多非马尔可夫行为，但直到最近才在实验中进行探索——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层的一种可能方法是使用偶极原子的准二维BEC；实际上，正是激发谱中的类似旋转的特征导致了这种特性。我建议实现这种情况，并测试信息流的性质（使用杂质原子作为量子比特），因为BEC激发谱的形式发生了变化。此外，研究通过提高温度来破坏非马尔可夫行为有多容易，这对理解量子信息的存储也很有趣，也很重要。将非马尔可夫行为的研究扩展到原子系统将是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，例如增强自旋压缩和可控耗散。更普遍的是，一个与工程储存库具有可控耦合的量子系统可能能够解决有关叠加和纠缠以及量子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.