Springer Theses Recognizing Outstanding Ph.D. Research Martin J.A. Schütz Quantum Dots for Quantum Information Processing: Controlling and Exploiting the Quantum Dot Environment Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent field of research. For greater accessibility to non-specialists, the published versions include an extended introduction, as well as a foreword by the student’s supervisor explaining the special relevance of the work for the field. As a whole, the series will provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. 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More information about this series at http://www.springer.com/series/8790 Martin J.A. Schütz Quantum Dots for Quantum Information Processing: Controlling and Exploiting the Quantum Dot Environment Doctoral Thesis accepted by Ludwig-Maximilian University, München, Germany 123 Author Supervisor Dr. Martin J.A. Schütz Prof. Ignacio Cirac Max-Planck-Institut für Quantenoptik Max-Planck-Institut für Quantenoptik Garching Garching Germany Germany ISSN 2190-5053 ISSN 2190-5061 (electronic) Springer Theses ISBN 978-3-319-48558-4 ISBN 978-3-319-48559-1 (eBook) DOI 10.1007/978-3-319-48559-1 Library of Congress Control Number: 2016956179 © Springer International Publishing AG 2017 This work is subject to copyright. 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Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer International Publishing AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Supervisor’s Foreword Since the discovery of quantum physics more than hundred years ago, our vision of nature has experienced a tremendous change. In the microscopic world, where most of its exotic phenomena manifest themselves, the properties of particles become fuzzy, and the observer is endorsed with an active role in how they change. Thanks to the technological progress experienced during the last 30 years, these phenom- ena, which used to attract more the attention of philosophers than physicists, may also lead to new paradigms in the way we process and transmit information. In fact, by now we know that if we are able to control and manipulate microscopic systems, so that we can exploit the laws of quantum physics, we will be able to build quantum computers and communication devices, leading to a revolution in our information society. Most of the pioneering experiments in this front of research were done in the realm of quantum optics. That is the theory that studies fundamental phenomena in the interaction of light with matter, typically atoms. In fact, atoms can reliably store qubits, the unit where (quantum) information is stored, whereas photons can be used to transmit those qubits from one place to another. During the last few years, a large amount of knowledge has been gained in how to manipulate atoms, transpass the information to photons, transmit them through optical fibers, and put it back into different atoms. The field of quantum optics is, in that sense, well established, and a great deal of control has been achieved during the last decade. In recent years, solid-state systems have appeared as alternative to build quan- tum computation and communication devices. In particular, electrons in quantum dots embedded in semiconducting materials can store qubits, which can be manipulated using external fields. Furthermore, quantum information can be stored in the nuclear spins of the atoms at the quantum dot. These situations are very reminiscent of what happens in quantum optical systems, if one identifies the electrons or nuclear spins with atoms. Thus, one may expect to observe similar v vi Supervisor’s Foreword phenomena with these solid-state systems as with atoms, or even use the tools and ideas developed in the field of quantum optics in order to learn how to manipulate or transport the information stored in quantum dots. This is precisely what has been achieved in this thesis. In the first part of Martin’s thesis, the interaction of the nuclear spins of a quantum dot with the electrons that go through it has been exploited in order to predict a very intriguing behavior. This system is very reminiscent of a set of atoms (whose role is played by the nuclear spins) interacting with propagating photons (here the electrons). In the quantum optics setup, one expects to observe phenomena associated with superradiance. Similarly, due to the collective nature of the cou- pling between the nuclear spins and the central electron spin in the quantum dot, the nuclear system may experience a strong correlation buildup, resulting in a sudden intensity burst in the electron current emitted from the quantum dot, giving rise to a new phenomenon in the solid state, electronic superradiance. In the second part of Martin’s thesis, the possibility of entangling two electrons in neighboring quantum dots has been analyzed. In particular, by sending electric current through both dots, one can obtain such a goal in the steady state. Once again, this situation is very similar to that of two atomic systems interchanging photons propagating from one to the other. In that context, it was predicted and experimentally observed that for some value of the external parameters, atomic entangled states could be produced. Based on that analogy, in this thesis an efficient way for achieving the same task with nuclear spins adjacent to quantum dots was put forward. In the third part, Martin continues to reveal and systematically explore previ- ously undiscovered connection points between the fields of quantum optics and modern solid-state semiconductor spin systems. In striking analogy to cavity QED, he proposes and analyzes phonon modes associated with surface acoustic waves (SAWs) in piezo-active materials as a universal mediator for long-range coupling between remote qubits. The proposal involves qubits interacting with a localized SAW phonon mode, defined by a high-quality resonator, which in turn can be coupled weakly to a SAW waveguide serving as a quantum bus (a concept well known from AMO physics with the role of photons replaced by SAW phonons). It is shown that the piezo-electric coupling between qubit and SAW phonon mode should enable a controlled mapping of the qubit state onto a coherent phonon superposition, which can then be converted to an itinerant SAW phonon in a waveguide, opening up the possibility to implement on-chip many quantum com- munication protocols well known in the context of optical quantum networks. The proposed combination of techniques and concepts known from quantum optics and quantum information, in conjunction with the technological expertise for SAW devices, should lead to further, rapid theoretical and experimental progress, opening up the avenue toward the widely anticipated field of quantum acoustics. Supervisor’s Foreword vii I do consider this thesis scientifically excellent and believe that the results motivate both (i) further research into what extent the proposed analogies between quantum optical and solid-state systems can be extended and generalized