Superfluidity in a Two-Dimensional Bose Gas with Tuneable Interactions

当由相同原子组成的气体比空气薄100万倍时，冷却到极低的温度，大约比绝对零度高百万分之一度，量子力学效应就变得重要起来。在这样的温度下，气体可以表现出各种奇异的物质相，这可能在量子计算和精密传感器中有实际应用。此外，这些相在许多方面是普遍存在的——类似的现象在其他物理系统中也有发生，包括液氦、奇异的固体材料（如超导体），甚至中子星。原子气体通常比其他物理系统更容易在实验室中操作，使我们能够在高度控制的环境中研究基本的多体物理。这最终也可以设计出更好的真实材料用于实际应用。例如，在室温下具有超导性的材料（即在没有任何损耗的情况下传输电流）将大大节省能源。理解超冷气体物理学的核心是玻色-爱因斯坦凝聚和超流体的概念。前者指的是大量原子在单一量子力学状态下的积累，正如爱因斯坦在1925年预测的那样，最终于1995年在原子气体中直接观察到的那样。后者指的是气体无摩擦流动的迷人能力。自1937年发现液态氦的超流动性以来，这两个概念在概念上一直联系在一起，但它们显然是不同的，它们的确切定量联系往往是难以捉摸的。特别是，在许多最有趣的物理情况下，超流体和气体的冷凝部分可能是非常不同的。例如，这种情况包括粒子之间的相互作用非常强的系统，或者不在标准三维世界中运动的气体，而仅限于二维或一维。二维物理也被认为是仍未被理解的高温超导现象的核心。在这项工作中，我们将解决超冷原子气体领域的两个重要突出问题：首先，我们将开发一种二维（2D）原子气体，其中粒子之间的相互作用强度将通过施加外部磁场来调节。这将使我们能够对所谓的别列津斯基- kosterlitz - thouless （BKT）超流体跃迁进行第一次全面的研究。这种转变很吸引人，因为与二维超流动性通常的直觉相反，它可以在没有任何玻色-爱因斯坦凝聚的情况下发生。BKT跃迁的特征最初是在液氦薄膜中观察到的，在我们最近的工作中，它的微观起源在原子气体中得到了最直接的证实。然而，仍有许多问题有待解决，特别是因为这种转变预计从根本上取决于粒子之间相互作用的强度，而在以前的二维实验中，这种强度是无法控制的。第二，基于我们最近的理论建议，我们将开发直接测量原子气体超流体密度的实验方法。基础物理学往往是通用的，但不同子领域的实验方法却往往大不相同。所以在液氦中超流体密度是常规测量的，但是冷凝密度很难提取。另一方面，超冷原子气体因直接观察到玻色-爱因斯坦凝聚而闻名，但对它们的超流体密度的直接测量仍然难以捉摸。我们的工作将为在同一实验系统的各种物理情况下测量和直接比较这两个量提供可能性。

When a gas of identical atoms about million time thinner than air is cooled down to extremely low temperatures, about a millionth of a degree above absolute zero, quantum mechanical effects become important. At these temperatures the gas can exhibit various exotic phases of matter, which could have practical applications in quantum computation and precision sensors. Further, these phases are in many ways universal - analogous phenomena occur in a range of other physical systems, including liquid helium, exotic solid state materials such as superconductors, and even neutron stars. The atomic gases are often much easier to manipulate in the laboratory than those other physical systems, allowing us to study fundamental many-body physics in a highly controlled environment. This could eventually also allow the design of better real materials for practical applications. For example materials which would be superconducting (i.e. transmit current without any losses) at room temperature would allow dramatic energy savings.Central to the understanding of the physics of ultracold gases are the concepts of Bose-Einstein condensation and superfluidity. The former refers to the accumulation of a large fraction of atoms in a single quantum mechanical state, as predicted by Einstein in 1925 and finally directly observed in an atomic gas in 1995. The latter refers to the fascinating ability of the gas to flow without any friction. The two concepts have been conceptually linked ever since the discovery of superfluidity in liquid helium in 1937, but they are nevertheless clearly distinct and their exact quantitative connection is often elusive. In particular, in many of the most interesting physical situations the superfluid and the condensed fraction of the gas can be very different. Such situations for example include systems in which the interactions between the particles are very strong, or gases which do not move in the standard three-dimensional world, but are confined to only two or one dimension. Two-dimensional physics is also believed to be at the heart of the still not understood phenomenon of high-temperature superconductivity.In this work we will address two essential outstanding issues in the field of ultracold atomic gases:First, we will develop a two-dimensional (2D) atomic gas in which the strength of interactions between the particles will be tunable by applying an external magnetic field. This will allow us to perform the first comprehensive study of the so-called Berezinskii-Kosterlitz-Thouless (BKT) superfluid transition. This transition is fascinating because contrary to the usual intuition in 2D superfluidity can occur in the absence of any Bose-Einstein condensation. The signatures of the BKT transition were first observed in liquid helium films, and its microscopic origin was most directly confirmed in an atomic gas, in our recent work. However many issues remain open, in particular because this transition is expected to fundamentally depend on the strength of interactions among the particles, and in no previous 2D experiment could this strength be controllably tuned.Second, based on our recent theoretical proposal, we will develop experimental methods for a direct measurement of the superfluid density of an atomic gas. The fundamental physics is often universal, but the experimental methods of different sub-fields are often very different. So in liquid helium the superfluid density is routinely measured, but the condensed density is difficult to extract. On the other hand, ultracold atomic gases are celebrated for the direct observation of Bose-Einstein condensation, but a direct measurement of their superfluid density remains elusive. Our work will open the possibility for the two quantities to be measured and directly compared in a variety of physical situations in the same experimental system.