Stochastic and Dissipative Dynamics of Ultracold Atoms

量子力学是一个令人印象深刻的成功理论，它告诉我们非常小的物体（原子和亚原子）是如何运动的。尽管到目前为止取得了成功，但量子力学仍有一些重要的方面违反直觉，需要更深入的理解。当前感兴趣的其中一个方面涉及对量子系统的测量。从根本上说，当测量量子系统的某些方面时，系统受到测量的干扰，使得系统的其他方面响应测量而改变。最著名的例子是“海森堡测不准原理”，例如，它指出，当一个人对原子的位置有更好的了解时，他对原子速度的了解必然会更差。更一般地说，这种扰动效应被称为“量子逆作用”。该项目的目标是利用超冷原子来观察和研究理论预测的量子逆作用的几种表现形式。从广义上讲，本研究解决了以下问题：在什么条件下，反向作用会显著影响系统的未来？精确设计的反作用力作为控制系统的工具有用吗？这些问题都很重要，因为逆作用通常指的是对量子系统的随机干扰。然而，在精心安排的条件下，反向作用的随机性可以以良好定义的方式控制量子系统。这项研究将促进对量子力学、测量和信息的基本理解，并将为未来的技术提供新的工具，如量子计算机和精确测量设备，如加速度和磁场，其性能最终将受到量子效应的限制。更具体地说，量子反作用对超冷原子动力学的影响将在三种不同的情况下进行研究。在第一种情况下，在激光场的驱动下，单个被捕获的原子在两个状态之间经历相干跃迁。自发发射不仅使原子在随机时间从激发态跃迁到基态，而且提供了有关原子当前状态的信息。通过只选择原子根本不自发发射的（随机）情况，通过原子处于每种状态的相对概率（即，测量结果不应该用不包括自发发射的理论来解释），该实验预计会显示自发发射仍然有影响。在第二种情况下，被限制在光学晶格中的原子进行自发发射，从而产生加热（动量扩散）。尽管动量扩散会增加原子在晶格中扩散的速率，但一个有趣的预测是，对于小的自发发射速率，原子的扩散应该由于量子隧道效应的抑制而受到抑制。对于更大的自发发射速率，原子的扩散应该会像预期的那样再次增加。在最后的场景中，预测的“喷灯”效应的实现将在光学晶格中的原子中实现：通过使原子的空间依赖“温度”（自发发射率），原子的泵送效应（作为原子的稳定电流实现）应该是可观察到的，证明了非平衡热力学的基本效应。

Quantum mechanics is an impressively successful theory that tells us how very small (atomic and subatomic) objects behave. Despite the success so far, quantum mechanics has important aspects that remain counterintuitive and require deeper understanding. One such aspect of current interest involves measurements on quantum systems. Fundamentally, when measuring some aspect of a quantum system, the system is disturbed by the measurement, such that other aspects of the system change in response to the measurement. The most famous example is "Heisenberg's uncertainty principle," which, for example, states that when one has better knowledge of the position of an atom, one must necessarily have worse information about the atom's velocity. More generally, this disturbance effect goes by the name of "quantum back-action." The goal of this project is use ultracold atoms to observe and study several manifestations of quantum back-action that have been predicted theoretically. Broadly speaking, this research addresses the following questions: Under what conditions does back-action significantly influence the future of the system? Can precisely engineered back-action be useful as a tool to control the system? These questions are nontrivial, as back-action generally refers to a random disturbance to a quantum system. However, under carefully arranged conditions, the randomness of the back-action can control quantum systems in well-defined ways. This research will advance fundamental understanding of quantum mechanics, measurement, and information, and it will provide new tools that may be useful in future technologies such as quantum computers and precision-measurement devices for such quantities as acceleration and magnetic field, whose performance will ultimately be limited by quantum effects.More specifically, the effects of quantum back-action on the dynamics of ultracold atoms will be studied in three separate scenarios. In the first scenario, a single trapped atom undergoes coherent transitions between two states when driven by a laser field. Spontaneous emission not only causes the atom to jump from the excited state to the ground state at random times, but also provides information about the present state of the atom. By selecting only the (random) cases where an atom does not spontaneously emit at all, the experiment is predicted to show that spontaneous emission nevertheless has an effect, via the relative probabilities for the atom to be in each state (i.e., the measurements should not be explainable by a theory that does not include spontaneous emission). In the second scenario, atoms confined to an optical lattice undergo spontaneous emission, which produces heating (momentum diffusion). Although momentum diffusion should increase the rate at which the atoms spread through the lattice, an interesting prediction is that for small spontaneous-emission rates, the atoms' spreading should be suppressed due to the inhibition of quantum tunneling. For larger spontaneous-emission rates, the spreading of the atoms should then again increase as expected. In the final scenario, a realization of the predicted "blowtorch" effect is to be realized with atoms in an optical lattice: by making a space-dependent "temperature" (spontaneous-emission rate) for the atoms, a pumping effect for the atoms (realized as a steady current of atoms) should be observable, demonstrating a fundamental effect in nonequilibrium thermodynamics.