Thermodynamics by Levitating Optomechanics

Optical control of nano- and micro-particles has recently found application in two relatively young fields of physics: Stochastic Thermodynamics and Quantum Optomechanics. In the former case, optical tweezers were used to control colloidal particles in liquid, to test new theoretical predictions concerning their far-from-equilibrium behaviour and to realize novel concepts, like a stochastic heat engine, that uses only single particles as a working medium. In Cavity-Optomechanics, light fields that are trapped between mirrors can control mechanical oscillators so delicately, that a control at the quantum level becomes possible. As optical levitation allows the realization of particularly high-quality mechanical oscillators, levitated cavity- Optomechanics has become a promising candidate for fundamental tests of quantum theory with massive particles. The idea behind this project is to enhance cross-fertilization between those flourishing and highly related fields by exploiting all-optical control of levitated nano-objects as a common experimental theme that allows access to the quantum regime. The central goal is to provide a testbed of unique flexibility for stochastic thermodynamics in the classical and in the quantum regime and to implement new concepts of quantum thermodynamics, to characterize and eventually optimize them, for example the idea of quantum heat engines. To achieve this, we will build on the technology existing for optical tweezers in liquid and even in cold atom experiments to implement complex optical potential landscapes in vacuum. This will enable a great level of control over the dynamics of a levitated nanoparticle. We further continue to fully develop levitating cavity optomechanics to additionally implement time-dependent anisotropic friction and/or temperature and to enable preparation of non-classical states and quantum state analysis. Combining these experiments in a single setup allows to implement thermodynamic processes with an extraordinary level of control and to implement completely new tests of thermodynamics with an unprecedented degree of generality. The major impact of such a new scientific tool surely is its value for understanding fundamental questions in thermodynamics, statistical physics and the foundations of quantum physics and as a model for new thermodynamic heat engines. In addition, however, optically levitating nanospheres in ultra-high vacuum have also been anticipated to serve as excellent sensors of force and mass and might therefore also find a direct way towards technological application.

When we talk about the efficiency of automotive engines or heat pumps, there is a powerful theory of physics at play: Thermodynamics provides fundamental laws to describe the efficiency of machines and many other phenomena . Thermodynamics also applies to the microscopic world, where random motions (thermal fluctuations) need to be considered in thermodynamic processes. The core of the project TheLO was the experimental development of methods that allow for the implementation of versatile, complex operations with microparticles while minimizing the influence of thermal fluctuations to the extent that quantum fluctuations become relevant. The tool used by TheLO for this purpose is laser light. It enables the levitation of small glass beads with a diameter only one thousandth that of a hair. Moreover, it allows for extremely precise measurement of particle movement. Within TheLO the researchers were able to generate a complex landscape of light with hills and valleys in a vacuum, changing in time. This allows for the implementation of much more general processes than previously possible. For example, using this technology, it was able to experimentally demonstrate conditions in which the generation of heat during a storage operation (such as erasing a bit) can be avoided. Once particle movement is measured and a force based on the measurement result is used for particle control (feedback control), the thermodynamic laws need to be adjusted. As part of the project, new fundamental limits imposed by time delays, which are not accounted for by the usual thermodynamic laws, were experimentally demonstrated. This insight has implications for understanding the performance and efficiency of feedback control, even in realistic scenarios. In collaboration with colleagues from Vienna, the method of feedback cooling has been refined to the point where particle movement could be reduced to the fundamental quantum physical limit. This milestone was achieved for the first time and even with two different approaches. It opens the door to quantum mechanical experiments with levitating nanoparticles, which are essential for our fundamental understanding of the quantum nature of massive objects. Based on the results and methods of TheLO research, a new approach has been proposed to control quantum mechanical behavior using temporally deformable "landscapes". This new approach brings the experimental verification of the wave nature of our glass beads within tangible reach and would increase the mass limit for objects that allow this by a factor of 10,000. In summary, TheLO has enabled a completely new level of control over the motion of mesoscopic particles, both in terms of spatial and temporal manipulation, as well as control in the quantum regime. It has expanded our understanding of the thermodynamics of feedback control and storage utilization, and paved the way for exploring complex quantum processes.