Collective Quantum Thermodynamics

热机是我们工业化社会的发动机。通过将热能转化为机械功，它们启动汽车、飞机和轮船，并驱动发电机向我们的电脑和智能手机供电。如果没有200年前出现的一个基本理论，所有这些现代应用都不可能实现。从那时起，工程师们就可以开发出越来越强大和高效的机器：热力学。这个理论只配备了几个基本的概念和定律，制定了管理詹姆斯·瓦特18世纪蒸汽机和今天的汽车发动机性能的基本规则。在过去的20年里，一个新的时代已经开始，科学家们正在探索将小型化作为热力发动机的一种新的设计原则。在一系列具有里程碑意义的试验中，越来越小的发动机已经制造出来并成功运行。2016年，这一令人着迷的发展导致了只有一个原子的功能性热机的实现。如此微小的物体不再受我们经典世界的机械规则的约束；它们可以同时占据两个位置，穿过障碍，或者在没有直接互动的情况下远距离相互影响。这些违反直觉的现象是在原子尺度上统治世界的量子运动定律的表现。在这一领域工作的热机可以配备古典工程师无法想象的功能。描述这种新型机器并试图利用它们的技术潜力的科学学科仍处于初级阶段，被称为量子热力学。尽管量子发动机有可能克服经典的性能限制，但它离实际应用还很远，尤其是因为它们的能量输出很小；要移动一辆汽车，人们需要的单原子发动机的数量大致相当于一升水中的分子数量。这个数字大得离谱，主要是因为它比较了完全不同尺度的物体。尽管如此，很明显，即使要对自身规模的技术有用，量子引擎也需要发展。但是，当微小正是使它们成为量子的属性时，它们的大小如何才能增加呢？量子力学为这一困境提供了一个解决方案：集体行为。由于一种没有经典对应物的奇怪相互作用，像原子这样的物体可以像鸟群中的鸟一样以协调的方式行动。几十年来，这一非凡的现象一直让科学家着迷。在这里，我们建议将其用于下一代量子机。想象一下，一个引擎使用的是包含数百万个原子的集体量子气体，而不是只有一个。这样的设备可以受益于量子效应，同时仍能产生可观的功率输出。此外，这台发动机的活塞可以与它们周围移动的所有原子完美同步。因此，可以实现巨大的控制水平，这是普通气体不可能实现的，因为普通气体的原子遵循不可预测的轨迹。这些独特的特征使集体量子机器成为量子工程中一个引人入胜但尚未探索的课题。为设计和实施这种新型设备奠定概念基础是这个项目的主要目标。我们将在诺丁汉大学发展的理论将是集体量子现象世界中热力学的对应物：集体量子热力学。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。集体量子机器有潜力成为这一发展的蒸汽机。它们不会移动我们未来的汽车，但它们很可能为我们的量子计算机和加密设备提供电力。

Heat engines are the motors of our industrialised society. By converting thermal energy into mechanical work, they set cars, airplanes and ships in motion and drive the generators that deliver electricity to our computers and smartphones. None of these modern applications would be possible without one fundamental theory that emerged 200 years ago and has ever since enabled engineers to develop more and more powerful and efficient machines: thermodynamics. Equipped with only 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.During the past two decades, a new era has begun, in which scientists are exploring miniaturisation as a novel design principle for thermal engines. In a series of landmark experiments, smaller and smaller engines have been built and successfully operated. In 2016, this fascinating development led to the realisation of a functional heat engine with only one atom. Objects this tiny are no longer bound by the mechanical rules of our classical world; they can occupy two places at the same time, tunnel through barriers or influence each other at a distance without direct interaction. These counterintuitive phenomena are manifestations of the quantum laws of motion that govern the world at atomic scales. Heat engines operating in this realm can be equipped with features that no classical engineer could have imagined. The scientific discipline that describes this new type of machine and tries to harness their technological potential is still in its infancy and has been dubbed quantum thermodynamics. Although likely able to overcome classical performance limits, quantum engines are still far from practical applications, not least due to their minuscule energy output; to move a car, one would need roughly as many single-atom engines as there are molecules in one liter of water. This number is absurdly large, mainly because it compares objects at radically different scales. Still, it is clear that, even to be useful for technologies on their own scale, quantum engines need to grow. But how can their size be increased when smallness is precisely the property that makes them quantum? Quantum mechanics provides a solution to this dilemma: collective behaviour. Due to a strange interaction without a classical counterpart, objects like atoms can act in a coordinated way, like birds in a flock. This remarkable phenomenon has fascinated scientist for decades. Here, we propose to utilise it for the next generation of quantum machines. Imagine an engine working with a collective quantum gas containing millions of atoms instead of just one. Such a device could benefit from quantum effects while still producing significant power output. Moreover, the pistons of this engine could be perfectly synchronized with all the atoms they move around. Thus, an enormous level of control could be achieved, which would be impossible to realise with an ordinary gas, whose atoms follow unpredictable trajectories. Such unique features make collective quantum machines a fascinating yet unexplored subject of quantum engineering. Laying down the conceptual foundations for the design and implementation of this new type of device is the major goal of this project. The theory we will develop at the University of Nottingham will be the counterpart of thermodynamics in the world of collective quantum phenomena: collective quantum thermodynamics. Quantum technologies are widely expected to shape our century in a similar way as the industrial revolution changed 19th and 20th century. Collective quantum machines have the potential to become the steam engines of this development. They will not move our future cars, but they might well provide the power for our quantum computers and encryption devices.