Quantum dynamics of opto- and nanomechanichal systems

In recent years the application of laser cooling techniques for micro- and nanomechanical systems has been successfully demonstrated and by now cooling of individual vibrational modes close to the quantum ground state has been achieved in many laboratories around the world. This is a first big step towards the study of quantum mechanics with macroscopic objects and might in the future enable the control of these mechanical degrees of freedom in analogy to what is currently achieved in cold atom or trapped ion experiments. In view of the rapid experimental progress in this field it is the aim of this project to develop a deeper theoretical understanding of opto- and nanomechanical systems in a regime where the quantum nature of the mechanical degree of freedom is significant. The general objectives of this project are on one hand to identify new and interesting physical effects which will be observable in these systems, but on the other hand also to describe specific applications for "quantum" mechanical systems which will provide a long-term perspective for this field. More specifically the project will be focused on three main topics. In the first part we will study the electron-phonon coupling between nitrogen-vacancy (NV) centers and confined phonon modes in diamond nano-structures. Here the goal is to use the NV centers to manipulate isolated phonon modes and to study potential application of phonon-mediated interactions between NV-centers for quantum information processing. In the second part we will analyze different variations of NV center based detectors for mechanical motion with the goal to investigate the role of quantum control techniques for nano-scale sensing applications. Finally, in the third part of this project we will study the dynamics of opto-mechanical systems in the single photon strong coupling regime. Here we will mainly focus on the fundamental properties of such systems and the non-trivial quantum effects that will arise under strong coupling conditions. In this context we will in particular consider various extensions to multi-mode opto-mechanical systems and opto-mechanical arrays and investigate the resulting few and many body effects in these settings.

In recent years the application of laser cooling techniques for micro- and nanomechanical systems has been successfully demonstrated and by now cooling of individual vibrational modes close to the quantum ground state has been achieved in many laboratories around the world. This is a first big step towards the study of quantum mechanics with macroscopic objects and might in the future enable the control of these mechanical degrees of freedom in analogy to what is currently achieved in cold atom or trapped ion experiments. Within this project, a series of theoretical studies have been performed, which contributed in various different ways to the development and the future of this field of quantum nano- and optomechanical systems. The project pursued, first of all, several analytic studies of cooling, quantum control and measurements schemes for mechanical systems, where in particular new approaches based on the coupling of nanomechanical resonators to individual defect centers in diamond have been investigated. As a potential future application of these systems, the more general concept of a mechanical quantum transducer was introduced and it was shown how quantum information stored in long-lived spin degrees of freedom can be mechanically converted into electrical or optical signals for long-distance quantum communication. We have further developed the basic theoretical models for describing quantum communication protocols in phononic quantum networks, where quantum information is transmitted solely via propagating mechanical vibrations. These ideas are particularly relevant for NV or SiV defect centers in diamond, which can be used as excellent quantum memories, but where today no efficient schemes for transmitting the quantum data are available. Apart from quantum technology related studies, the project has also revealed many basic physical effects, which can be experimentally probed in these systems for the first time. This includes, for example, a new type of optical nonlinearity, which arises from the tiny radiation pressure that even single photons exert on a vibrating mirror. Finally, we have shown that the application of optomechanical cooling and heating techniques in larger networks of coupled mechanical resonators can be used to investigate energy transport phenomena at the microscopic scale. Here we have discovered, for example, a new type of phase transition, where the energy current through a microscopic network suddenly switches between a noise-dominated and a coherent transport regime.