Quantum dissipation in carbon-nanotube optomechanics

量子力学构成了我们目前对物理现实的理解的基本范式。虽然制定了近一个世纪前，并从那时起彻底测试到一个精致的精度，其基本原则的解释继续激发争议，直到今天。例如，它们的一个令人困惑的结果是，一个物体可以处于“量子叠加”状态，在这种状态下，它似乎同时处于两个不同的位置。虽然这些特征在微观领域已经得到了详细的证实，但它们肯定违背了我们对物理现实的日常概念，这就提出了一个问题：我们周围的经典“宏观”世界是如何从这种量子怪异中出现的。在过去的几十年里，人们发现宏观物体的这种量子到经典的转变，即所谓的“退相干”，是由它与周围环境不可避免的相互作用引起的。这最终导致了具有原子大小横截面的机械梁，由单片碳原子卷成无缝圆柱体，形成称为碳纳米管的狭窄管。它们的行为本质上就像敏感的弹簧，研究人员目前正在开发各种传感器，将它们的振荡运动转换为电信号或光信号。这样的传感器可以用来操纵这些“纳米弹簧”，考虑到它们的小尺寸和高质量，它们也是研究量子力学上述有趣方面的理想候选者。例如，研究人员正试图用它们实现宏观上可区分的振荡幅度的量子叠加。通常涉及换能器本身的类似叠加的配置。实现这一目标的一个重要先决条件是理解是什么决定了这些系统中的固有退相干，即，什么是它们的量子叠加的生存的基本限制。2为了达到这个效果，我们将预测在纳米管光学传感器中预期的本征退相干。后者是基于纳米管内被称为激子的带相反电荷的载流子的结合对。这些可以用光产生，并且它们的存在可以通过电力使管变形，使得在散射光和纳米管的振荡之间产生间接耦合。特别是，我们将阐明退相干，从事实上，把纳米管作为一个单一的理想弹簧是一个过于简单化。更准确的描述是把它想象成一个包含无数谐波的弹性弦，当我们试图用它把基本共振带入量子叠加时，这些谐波的振动都会影响和破坏“激子换能器”的工作。在量子环境中，振动能量被量子化，这些谐波的基本量子被称为声子，因此我们的重点将是声子诱导的纳米管激子的退相干。在第一阶段，我们将考虑这种现象的情况下，人们可以忽略的有限长度的纳米管，我们将研究声子如何影响散射光的属性。第二阶段将集中在纳米管悬挂在有限长度上并耦合到底层芯片的情况下，发挥作用。特别是，纳米管的振荡可以通过转换成弹性波来衰减，弹性波辐射到支持它的芯片中，这为激子换能器如何去聚的问题增加了进一步的转折。最后，除了进一步理解现实的基本结构，由此产生的见解将影响这些系统作为单粒子尺度传感器的使用，以及依赖于利用量子叠加的各种量子技术。

Quantum mechanics constitutes the basic paradigm that underlies our current understanding of physical reality. Though formulated almost a century ago and since then thoroughly tested to an exquisite precision, the interpretation of its basic principles continues to excite controversy to this very date. For example, a puzzling consequence of them is that an object can be in a "quantum superposition" state in which it seems to be simultaneously in two distinct locations. Though such features have been borne out in detail in the microscopic realm, they certainly defy our everyday notions of physical reality which raises the question of how the classical "macroscopic" world around us emerges from this quantum weirdness. Over the last decades it has transpired that this quantum-to-classical transition of a macroscopic object, known as its "decoherence", is effected by its unavoidable interaction with its surroundings.In turn, recent years have witnessed a miniaturisation of mechanical cantilevers used for sensing applications. This has culminated in mechanical beams having atomic-sized cross-sections, made from a single sheet of carbon atoms rolled into a seamless cylinder to form a narrow tube known as a carbon nanotube. These behave essentially like sensitive springs and researchers are currently developing a variety of transducers to convert their oscillatory motion into an electrical or optical signal. Such transducers can then be used to manipulate these "nanosprings", which given their small dimensions and high quality are also ideal candidates for studying the aforementioned intriguing aspects of quantum mechanics. For example researchers are trying to realise with them quantum superpositions of macroscopically distinguishable oscillation amplitudes. A configuration which often involves a similar superposition of the transducer itself. An important prerequisite for this goal is to understand what determines the intrinsic decoherence in these systems, i.e., what are the fundamental limits for the survival of their quantum superpositions.To this effect, we will predict the intrinsic decoherence expected in a nanotube optical transducer. The latter is based on bound pairs of oppositely charged carriers within the nanotube known as excitons. These can be generated with light and their presence can distort the tube via electrical forces so that an indirect coupling is generated between the scattered light and the oscillations of the nanotube. In particular, we will elucidate the decoherence that arises from the fact that regarding the nanotube as a single ideal spring is an oversimplification. A more accurate description is to think of it as an elastic string containing innumerable harmonics, whose vibrations will all influence and disrupt the workings of the "exciton transducer" while we try to use it to bring the fundamental resonance into a quantum superposition. Within a quantum setting, vibrational energy is quantised and the elemental quanta of these harmonics are known as phonons, so that our focus will be the phonon-induced decoherence of nanotube excitons. In a first phase, we will consider this phenomenon in situations where one can ignore the finite length of the nanotube, and we will study how the phonons influence the properties of the scattered light. A second phase will focus on situations where the fact that the nanotube is suspended over a finite length and coupled to an underlying chip, plays a role. In particular, the nanotube's oscillations can be damped by being converted into elastic waves that are radiated into the chip that supports it, and this adds a further twist to the problem of how the exciton transducer decoheres.Finally, beyond furthering our understanding of the basic fabric of reality, the resulting insights will impact on the usage of these systems as sensors at the single particle scale, and on a variety of quantum technologies that rely on exploiting quantum superpositions.