Phonon Quantum Networks in Diamond

The development of practical quantum technologies, such as quantum computers or secure quantum communication, is one of the big scientific challenges of today. Apart from identifying suitable systems for storing and manipulating quantum information, this goal will also require the realization of efficient ways for communicating between such quantum bits. The overall goal of this project is to investigate new ways for transmitting quantum information by using the quantized lattice vibrations in solids, so-called phonons, as information carriers. In particular, we will analyze the transfer of quantum states between separated electronic defect centers in nanostructured diamond. Defect centers in diamond are already used today for encoding and storing quantum superposition states in well- isolated spin states of the localized electrons and by doing so, to improve the sensitivity of nanoscale magnetic field sensors. However, so far there exists no efficient scheme to interconnect two or more defect centers, which is a crucial requirement for further progress in diamond-based quantum technologies. Phonons, which are guided along predefined diamond beams, offer a promising and natural way to implement such quantum communication abilities. As a first step in this direction, we will develop in this project a detailed theoretical model for phonon networks in diamond and identify optimized quantum state transfer protocols for this system. Beyond potential technological applications, this work will also provide the basis for new experimental techniques that will enable the detection and manipulation of sound and heat on the level of single vibrational quanta.

The development of practical quantum technologies, such as quantum computers or secure quantum communication, is one of the big scientific challenges of today. Apart from identifying suitable systems for storing and manipulating quantum information, this goal will also require the realization of efficient ways for communicating between such quantum bits. The overall goal of this project was to investigate new ways for transmitting quantum information by using the quantized lattice vibrations in solids, so-called phonons, as information carriers. Specifically, we analyzed the transfer of quantum states between separated electronic defect centers in nanostructured diamond and showed how optimized control pulses can be applied to enhance the transfer rate while simultaneously suppressing detrimental noise from the environment. Our analytical and numerical simulations showed that by using this trick, quantum gate operations with a residual error of less then 0.0001 can be implemented. This value is in general considered as the threshold, below which large-scale quantum computations become possible. In addition, we have investigated energy transport phenomena in phononic and more general types of quantum networks. Similar to regular computers, future quantum computers will be limited by heating processes. However, this problem is much more sever for quantum devices, since in such highly isolated systems, dissipating heat is very difficult. Our theoretical calculations have shown that the problem of heat flow in quantum networks is very peculiar and can even lead to phase transitions. A deeper understanding of such phenomena will be very important for the development and operation of large-scale quantum computing architectures.