Collective Quantum Thermodynamics: Quantum vs Classical

长期以来，汽车发动机、飞机涡轮机和家用冰箱等热力机械一直是我们现代社会必不可少的设备。通过将热量转化为机械功或反之亦然，它们可以启动汽车和飞机，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200年前出现的一个基本理论，这些现代应用都不可能实现，从那时起，工程师们就能够开发出越来越先进的机器：热力学。配备了一些基本的概念和定律，这一理论制定了支配詹姆斯·瓦特18世纪蒸汽机和当今类似汽车发动机的性能的基本规则。随着纳米和量子世界正在进行的下一次技术革命，现在越来越有必要开发新一代热机，这些机器在极小的长度范围内运行，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积木。在过去的十年里，见证了一系列里程碑式的实验，在这些实验中，越来越小的热机被实现到单个原子的水平。这样的微小物体不再受经典世界规则的约束；它们可以同时占据两个位置，也可以在没有直接互动的情况下远距离相互影响。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。旨在描述热机在这个世界上运行并寻求利用其技术潜力的学科被称为量子热力学，构成了我的主要研究领域。量子热机的技术应用仍然面临着重大的概念和实践挑战。这些挑战之一是它们有限的能量周转量，太小了，无法满足目前最设想的应用的几个数量级的需求。支持我成为研究员的关键思想是通过利用物质集体状态的属性来解决这个问题，集体状态是当大量量子物体开始以一种协调的方式行为时出现的，有点类似于一群鸟。为实现利用这些现象来提高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。在我们迄今成果的基础上，我的团队、我的理论和实验合作伙伴和我正在研究三个主要主题，这三个主题与寻求量子和经典物理之间的协同效应的主题有关。首先，为了发展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模拟，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相结合，我们研究了热力学参数如何从量子世界传输到经典世界，就像热机产生的功一样。第三，为了找到集体量子效应产生的热力学优势的定量衡量标准，我们探索了这些现象如何使克服制约经典小型热机(如分子马达)功率、效率和精度的一般权衡关系成为可能。人们普遍预计，量子技术将以类似于工业革命改变19世纪和20世纪的方式塑造我们的世纪。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。它们不会移动我们未来的汽车，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.