Collective Quantum Thermodynamics: Quantum vs Classical

像汽车发动机、飞机涡轮机和家用冰箱这样的热机长期以来一直是我们现代社会必不可少的。通过将热量转化为机械功或反之亦然，它们驱动汽车和飞机运动，驱动发电机向我们的电脑供电，并为我们的食物、生活空间和数据中心降温。如果没有一个200年前出现的基本理论，这些现代应用都不可能实现。从那以后，热力学使工程师们能够开发出越来越先进的机器。在一些基本概念和定律的帮助下，这一理论奠定了18世纪詹姆斯·瓦特的蒸汽机和今天的汽车发动机的基本规则。随着纳米和量子世界的下一次技术革命正在进行，现在越来越需要开发新一代的热机，这些热机可以在极小的长度尺度上运行，以推动纳米机器人或冷却需要超低工作温度的量子计算机的构建块。在过去的十年里，我们看到了一系列具有里程碑意义的实验，在这些实验中，越来越小的热机被实现到了单原子的水平。这些微小的物体不再受古典世界规则的约束；它们可以同时占据两个位置，或者在没有直接互动的情况下在一定距离内相互影响。这些现象是在原子尺度上支配世界的量子运动定律的表现。旨在描述在这个世界上运行的热机并寻求利用其技术潜力的学科被称为量子热力学，这是我的主要研究领域。量子热机的技术应用仍然面临着重大的概念和实践挑战。其中一个挑战是它们有限的能量周转率，它太小，无法满足目前大多数设想应用的几个数量级的需求。支持我的奖学金的关键思想是通过利用物质集体状态的特性来解决这个问题，当大量量子物体开始以一种协调的方式行动时，就会出现这种状态，有点类似于一群鸟。为实现利用这些现象来提高其性能的新型量子热机奠定理论基础是我研究计划的中心目标。在我们目前的成果基础上，我的团队、我在理论和实验方面的合作伙伴和我正在研究三个主要课题，它们的主题是寻求量子物理学和经典物理学之间的协同作用。首先，为了开发描述承载集体效应的量子系统所需的方法，我们研究了这些系统的经典类似物，这些系统可以用经典计算机有效地模拟；这个想法类似于使用经典水波作为量子粒子波动特性的模型。其次，为了将集体量子热机与其输出的经典消费者整合在一起，我们研究了热力学量（如热机产生的功）如何从量子世界传递到经典世界。第三，为了找到由集体量子效应产生的热力学优势的定量测量方法，我们探讨了这些现象如何使克服限制经典小型热机（如分子发动机）的功率、效率和精度的一般权衡关系成为可能。人们普遍预计，量子技术将像工业革命改变19世纪和20世纪一样，塑造我们的世纪。我们正在为集体量子热机的发展奠定概念基础，它有可能成为这一发展的蒸汽机。它们不会驱动我们未来的汽车，但它们很可能有助于运行我们的量子计算机和加密设备。

Thermal machines like car engines, airplane turbines and household refrigerators have long been essential to our modern society. By converting heat into mechanical work or vice versa, they set cars and airplanes in motion, drive the generators that deliver electricity to our computers and cool our food, living spaces and data centers. None of these modern applications would be possible without one fundamental theory that emerged 200 years ago and has since then enabled engineers to develop more and more advanced machines: thermodynamics. Equipped with a few elementary concepts and laws, this theory lays down the basic rules that govern the performance of James Watt's 18th century steam engine and today's car engines alike.With the next technological revolution underway in the nano and quantum world, there is now an increasing need to develop a new generation of thermal machines that operate on extremely small length-scales to propel nano-robots or cool the building blocks of quantum computers that require ultra-low working temperatures. The last decade has seen a series of landmark experiments, in which ever smaller thermal machines were realized down to the level of single atoms. Such tiny objects are no longer bound by the rules of the classical world; they can occupy two places at the same time or influence each other at a distance without direct interaction. These phenomena are manifestations of the quantum laws of motion that govern the world at atomic scales. The discipline that aims to describe thermal machines operating in this world and seeks to harness their technological potential has been called quantum thermodynamics and forms my main area of research. Technological applications of quantum thermal machines are still facing major conceptual and practical challenges. One of these challenges is their limited energy turnover, which is too small to match the needs of most currently envisaged applications by several orders of magnitude. The key idea underpinning my fellowship is to address this problem by harnessing the properties of collective states of matter, which emerge when large numbers of quantum objects begin to behave in a coordinated way, somewhat similar to a flock of birds. Laying the theoretical groundwork to realize new types of quantum thermal machines that exploit these phenomena to enhance their performance is the central aim of my research program. Building on our results so far, my team, my partners in theory and experiment and I are working on three major topics, which are connected by the theme of seeking synergies between quantum and classical physics. First, to develop the methods required to describe quantum systems hosting collective effects, we investigate classical analogues of these systems, which can be efficiently simulated with classical computers; this idea is similar to using classical water waves as models for the wave character of quantum particles. Second, with the aim of integrating collective quantum thermal machines with classical consumers of their output, we investigate how thermodynamic quantities, like the work produced by a heat engine, can be transmitted from the quantum world into the classical one. Third, to find quantitative measures for the thermodynamic advantage generated by collective quantum effects, we explore how these phenomena make it possible to overcome general trade-off relations that constrain the power, efficiency and precision of classical small-scale thermal machines such as molecular motors. Quantum technologies are widely expected to shape our century in a similar way as the industrial revolution changed 19th and 20th century. Collective quantum thermal machines, for the development of which we are helping to lay the conceptual foundations, have the potential of becoming the steam engines of this development. They will not move our future cars, but they might well help to run our quantum computers and encryption devices.