Theory of Ultracold Matter

广义地说，我的研究涉及的是被限制在太空中的稀薄原子气体，并冷却到极低的温度。这些条件的结合产生了引人注目的物理特性，吸引了世界各地物理学家的注意。这方面的一个例子是玻色-爱因斯坦凝聚(BEC)现象，在这种现象中，玻色原子凝聚成最低可用量子态--凝聚态。在这种新的物质状态下，原子相互作用，在接近宏观尺度的尺度上表现出量子行为。BEC更具戏剧性的后果之一是流动性过剩：有可能在没有阻力或耗散的情况下流动。 这项提议中所描述的许多研究都是流动性过剩的基础。也许最简单的例子是凝聚体在零温度下的无阻尼振荡运动。然而，在高温下，由于冷凝物与热激发的相互作用，振荡被抑制，即所谓的热云。我们发展的理论解释了这种行为。当考虑涡旋的动力学时，也观察到类似的耗散效应。涡旋是原子围绕中心核的量子化循环。如果在被困的玻色气体中产生这样的漩涡，由于它与热云的相互作用，它将向气体的边缘螺旋上升，最终将消失。我们开发的理论也描述了这一点。 冷原子令人着迷的一个方面是，它们可以通过各种方式进行操纵。例如，如果施加驻留光波，原子实际上发现自己处于周期势中，我们称之为光学晶格。在这方面，也存在过剩流动性。然而，如果光学晶格加深，原子就会转变为绝缘态，即完全不同的量子相。我的研究的一个方向是关于当玻色原子的混合物驻留在晶格中时发生的量子相变。所有这些工作的最终目标是提高我们进一步控制和操纵冷囚禁原子的能力。

Broadly speaking, my research deals with dilute atomic gases confined in space and cooled to extremely low temperatures. This combination of conditions results in remarkable physical properties that have captured the attention of physicists around the world. One example of this is the phenomenon of Bose-Einstein condensation (BEC) in which bosonic atoms condense into the lowest available quantum state, the condensate. In this novel state of matter, the atoms act coherently, and exhibit quantum behaviour on scales approaching macroscopic dimensions. One of the more dramatic consequences of BEC is superfluidity: the possibility of flow without resistance or dissipation. Superfluidity underlies much of the research described in this proposal. Perhaps the simplest example is the undamped oscillatory motion of a condensate at zero temperature. However, at elevated temperatures, the oscillations are damped as a result of the interaction of the condensate with the thermal excitations, the so-called thermal cloud. The theory we have developed accounts for this behaviour. Similar dissipative effects are also observed when considering the dynamics of a vortex - the quantized circulation of atoms around a central core. If such a vortex were to be created in a trapped Bose gas, it would spiral out towards the edge of the gas as a result of its interaction with the thermal cloud, and would eventually disappear. This too is described by the theory we have developed. One of the fascinating aspects of cold atoms is that they can be manipulated in various ways. For example, if a standing light wave is imposed, the atoms effectively find themselves in a periodic potential which we call an optical lattice. Here, too, superfluidity exists. However, if the optical lattice is deepened, the atoms make a transition to an insulating state, a completely different quantum phase. One direction of my research concerns the quantum phase transitions that occur when a mixture of bosonic atoms resides in the lattice. The ultimate objective of all this work is to enhance our ability to further control and manipulate cold trapped atoms.