This special issue of physica status solidi (b) on single electron control contains a series of articles dealing with the manipulation and detection of single electrons. Based on the tremendous experimental and theoretical progress achieved within this field during the last years, the generation of single electrons on demand in coherent conductors and the full control over the quantum states of single electrons is now within reach. The knowledge of such techniques is an absolute prerequisite for the implementation of schemes for the controlled transfer of quantum information in electronic setups. The transfer of quantum information was up to now mostly applied in quantum-optical setups. Such quantum technologies are expected to open up a whole new field for basic science but also for future applications in quantum communication and quantum sensing. The idea to put in place this special issue arose during a focus session on “single-particle sources for electronic devices”, which we organized at the DPG spring meeting of the condensed matter section in Regensburg on March 7, 2016. During the focus session, it became evident that the advancement of this research field had been quite dramatic during the last years and that it would be helpful for the scientific community to document this progress in a special issue. The focus session in Regensburg included 6 invited and 8 contributed talks. Most of the research groups, which were active in that focus session at the DPG spring meeting, and several other groups have now contributed with manuscripts to this special issue. The leading role of European groups in the field of single-electron control is evident from the fact that most of the contributions are of European origin. Within Europe, in these days, a number of different activities are started to advance quantum technologies, to mention only as a prominent example, the announcement and planning of the 3rd EU Flagship Program on Quantum Technologies. The purpose of this issue is, among others, to bring the field of single-electron control to the attention of the scientific community interested in quantum technologies. This issue includes 10 feature articles, allowing the readers to get a broad overview, but also 11 original research papers, thereby presenting the most recent advancements of the field. The subjects that are covered range from the physics of single-electron wave packets, thermodynamics with single electrons, studies of entanglement and electron quantum optics, up to the realization of feedback control and investigations of single-electron charge transfer. Contributions from experimental groups and from theory groups are almost balanced, representing a complete picture of the field. We hope that all readers will find this issue interesting and stimulating. We dedicate this special issue to Prof. Dr. Tobias Brandes, who unexpectedly passed away on 2 February 2017 at the age of 50. With him we lost an excellent scientist with great international recognition who impressed by his extraordinary kindness and his modest, open and friendly way in meeting people. Throughout his whole scientific carreer, Tobias has worked to bridge the gap between coherent solid-state devices, as e.g. in the quantum Hall effect or in quantum dots, and quantum optics. A focus of his research were quantum mechanical aspects of dynamical effects in solid-state devices. After having studied physics at the University of GÖttingen, Tobias worked at a number of different places, in Germany, Japan and the UK, before he moved to the Technical University of Berlin as a full professor in 2016. Within this special issue one of his last manuscripts is published which deals with feedback control applied to single-electron devices. Feedback control, as he had in mind in some of his recent publications, is therefore also visualized in the picture of the title page of this issue. With the sudden death of Tobias, the whole scientific community lost a great scientist and we all lost a good friend. Janine Splettstoesser Rolf J. Haug Guest Editors
This special issue of physica status solidi (b) on single electron control contains a series of articles dealing with the manipulation and detection of single electrons. Based on the tremendous experimental and theoretical progress achieved within this field during the last years, the generation of single electrons on demand in coherent conductors and the full control over the quantum states of single electrons is now within reach. The knowledge of such techniques is an absolute prerequisite for the implementation of schemes for the controlled transfer of quantum information in electronic setups. The transfer of quantum information was up to now mostly applied in quantum-optical setups. Such quantum technologies are expected to open up a whole new field for basic science but also for future applications in quantum communication and quantum sensing. The idea to put in place this special issue arose during a focus session on “single-particle sources for electronic devices”, which we organized at the DPG spring meeting of the condensed matter section in Regensburg on March 7, 2016. During the focus session, it became evident that the advancement of this research field had been quite dramatic during the last years and that it would be helpful for the scientific community to document this progress in a special issue. The focus session in Regensburg included 6 invited and 8 contributed talks. Most of the research groups, which were active in that focus session at the DPG spring meeting, and several other groups have now contributed with manuscripts to this special issue. The leading role of European groups in the field of single-electron control is evident from the fact that most of the contributions are of European origin. Within Europe, in these days, a number of different activities are started to advance quantum technologies, to mention only as a prominent example, the announcement and planning of the 3rd EU Flagship Program on Quantum Technologies. The purpose of this issue is, among others, to bring the field of single-electron control to the attention of the scientific community interested in quantum technologies. This issue includes 10 feature articles, allowing the readers to get a broad overview, but also 11 original research papers, thereby presenting the most recent advancements of the field. The subjects that are covered range from the physics of single-electron wave packets, thermodynamics with single electrons, studies of entanglement and electron quantum optics, up to the realization of feedback control and investigations of single-electron charge transfer. Contributions from experimental groups and from theory groups are almost balanced, representing a complete picture of the field. We hope that all readers will find this issue interesting and stimulating. We dedicate this special issue to Prof. Dr. Tobias Brandes, who unexpectedly passed away on 2 February 2017 at the age of 50. With him we lost an excellent scientist with great international recognition who impressed by his extraordinary kindness and his modest, open and friendly way in meeting people. Throughout his whole scientific carreer, Tobias has worked to bridge the gap between coherent solid-state devices, as e.g. in the quantum Hall effect or in quantum dots, and quantum optics. A focus of his research were quantum mechanical aspects of dynamical effects in solid-state devices. After having studied physics at the University of GÖttingen, Tobias worked at a number of different places, in Germany, Japan and the UK, before he moved to the Technical University of Berlin as a full professor in 2016. Within this special issue one of his last manuscripts is published which deals with feedback control applied to single-electron devices. Feedback control, as he had in mind in some of his recent publications, is therefore also visualized in the picture of the title page of this issue. With the sudden death of Tobias, the whole scientific community lost a great scientist and we all lost a good friend. Janine Splettstoesser Rolf J. Haug Guest Editors
Precisely characterizing and controlling the dynamics of realistic open quantum systems has emerged in recent years as a key challenge across contemporary quantum sciences and technologies, with implications ranging from physics, chemistry and applied mathematics to quantum information processing (QIP) and quantum engineering. Quantum control theory aims to provide both a general dynamical-system framework and a constructive toolbox to meet this challenge. The purpose of this special issue of Journal of Physics B: Atomic, Molecular and Optical Physics is to present a state-of-the-art account of recent advances and current trends in the field, as reflected in two international meetings that were held on the subject over the last summer and which motivated in part the compilation of this volume—the Topical Group: Frontiers in Open Quantum Systems and Quantum Control Theory , held at the Institute for Theoretical Atomic, Molecular and Optical Physics (ITAMP) in Cambridge, Massachusetts (USA), from 1–14 August 2010, and the Safed Workshop on Quantum Decoherence and Thermodynamics Control , held in Safed (Israel), from 22–27 August 2010. Initial developments in quantum control theory date back to (at least) the early 1980s, and have been largely inspired by the well-established mathematical framework for classical dynamical systems. As the above-mentioned meetings made clear, and as the burgeoning body of literature on the subject testifies, quantum control has grown since then well beyond its original boundaries, and has by now evolved into a highly cross-disciplinary field which, while still fast-moving, is also entering a new phase of maturity, sophistication, and integration. Two trends deserve special attention: on the one hand, a growing emphasis on control tasks and methodologies that are specifically motivated by QIP, in addition and in parallel to applications in more traditional areas where quantum coherence is nevertheless vital (such as, for instance, quantum control of chemical reactions or high-resolution magnetic resonance spectroscopy); on the other hand, an unprecedented demand for close coupling between theory and experiment , with theoretical developments becoming more and more attuned to and driven by experimental advances as different quantum technologies continue to evolve at an impressive pace in the laboratory. Altogether, these two trends account for several of the recurrent themes in this volume, as well as in the current quantum control literature as a whole: namely, the quest for control strategies that can attain the highest degree of precision and robustness possible, while striving for efficiency and, ultimately, optimality in achieving the intended control task under realistic operational constraints. From a theory standpoint, this makes it imperative to take into account increasingly more realistic control settings; to assess the quantitative impact of limited control resources and/or system knowledge; and to provide a rigorous and general foundation for existing experimental approaches in order to further enhance applicability and performance. From an experimental standpoint, renewed emphasis is in turn placed on validating theoretical predictions and benchmarking performance, so that the limiting constraints can be singled out for additional theoretical analysis and guidance. This ongoing cross-talk is clearly reflected in this collection, which brings together theoreticians and experimentalists, with a significant fraction of the papers reporting on combined quantum control theory–experiment efforts. While a precise categorization would neither be possible nor desirable, contributions to this volume have been loosely grouped into five broad sections. This grouping has been made in the hope that connections between different problems and/or technical approaches will become more transparent, facilitating the transfer of concepts and methods. The special issue opens with a section devoted to open-loop control methods, with special emphasis on dynamical decoupling (DD), which is becoming an increasingly important tool for decoherence control at the physical 'quantum firmware' level. In addition to including original research results, the first two articles, by Brion et al and Biercuk et al , also serve to pedagogically review some background in their respective subjects. In particular, Brion et al revisit one of the conceptually simplest approaches to open-loop manipulation of both closed and open quantum systems, nonholonomic control , motivated by its broad applicability to QIP settings. A special instance of open-loop control based on sequences of (nearly) instantaneous `bang-bang' pulses is addressed by Biercuk et al , who reformulate the simplest DD scenario, suppression of phase decoherence in a single qubit, as a filter-design problem. Peng et al report on the implementation of 'concatenated' DD for arbitrary single-qubit decoherence in the context of nuclear magnetic resonance QIP. A dedicated analysis of the performance of different DD schemes in the presence of realistic pulse errors is given by Wang and Dobrovitski. DD is also one of the strategies used by Lucamarini et al to reduce polarization decoherence in a photon qubit. These authors additionally report on the use of active feedback to counter transmission noise, effectively setting the stage for the second section, which is centered on closed-loop control. Unlike in open-loop control, measurement is an essential ingredient in closed-loop schemes aimed at both reliably identifying features of the target quantum system and further modifying its dynamics. The importance of directly measuring the spectrum of the underlying system-environment coupling is stressed by Almog et al , who show how this knowledge is crucial, in particular, for predicting the performance of DD sequences in experiments and for optimizing performance. Riofrio et al address a weak-measurement protocol for implementing quantum state tomography , which is a necessary 'primitive' for inferring the target quantum state and thereby diagnosing the control performance. Next, the impact of realistic control and system imperfections in continuous-time Markovian feedback strategies for rapid state preparation is analyzed by Combes and Wiseman. A prominent role is played in the special issue by optimal control (OC) approaches, reflecting their central importance for quantum control and QIP. The OC contributions have been divided into two separate sections, depending on whether the target dynamics is modeled as Hamiltonian (section 3) or dissipative (section 4), respectively. The contribution by Beltrani et al deals with `control landscapes', which provide a foundation for analyzing the performance of numerical OC algorithms and their robustness against control errors. Specifically, this paper characterizes geometric properties of the control landscape, relevant to the optimal control of state-to-state transitions. Application of OC theory to the problem of population transfer and coherence enhancement in Λ-systems is studied by Kumar et al , whereas Goerz et al report on the OC-design of a high-fidelity controlled phase-gate in atomic qubits. The robustness of an OC solution is specifically addressed by Negretti et al , along with an approach for identifying easily implementable while still 'close-to-optimal' control pulses. Powerful relaxation-optimized OC schemes (based on so-called opengrape algorithms) for generating unitary target gates in the presence of known dissipation parameters are discussed by Schulte-Herbrüggen et al . Next, Lapert et al report on the problem of time-optimal control of spin-1/2 systems undergoing Bloch relaxation dynamics, highlighting the crucial role played by singular extremals in the control synthesis. Alternative approaches for optimized control of qubits exposed to various decoherence processes are developed by Esher et al and Xue et al , based on a perturbative 'bath-optimized' formalism and on numerical optimization via a genetic algorithm, respectively. Testifying to the richness of the field, the volume concludes with four contributions that address a diverse range of problems. The exploitation of properties of adiabatic quantum evolutions is common to the first two papers. In particular, Legthtas et al offer a rigorous explanation for the robustness of a control protocol, chirped pulsing , that is widely employed in 'adiabatic rapid passage' experiments, while Han et al present a theoretical framework for adiabatic Raman photo-association schemes relevant to ultracold atomic systems. In the context of cavity quantum electrodynamics, Montenegro and Orszag describe how to engineer a system of two atoms coupled to distant lossy cavities so that stable atomic entanglement is generated. Finally, still very little is known about the physical mechanisms that are responsible for and control the experimentally observed 'coherent' features of transport phenomena in biological systems. The last contribution by Alicki and Giraldi analyzes energy transport in dynamical systems that can be modeled as 'quantum networks', and points to this fascinating emerging frontier. It is our hope that the above papers may help readers to gain an overview of some of the main trends in current quantum control efforts, both theoretical and experimental. In closing, we take the opportunity to thank the organizations which sponsored the above-mentioned ITAMP Topical Group (the United States National Science Foundation and Harvard University) and the Safed Workshop (the Israeli Science Foundation, the Safed Scientific Workshop program, CECAM and ACAM). Last but not least our sincere gratitude goes to all of the contributors to the volume and the reviewers as well as the J. Phys. B staff, for their respective efforts in preparing the papers and ensuring the overall quality of this special issue.
In the last few years quantum information has played an increasingly central role in the research activities of many scientists within wide ranging areas of physics, mathematics and computer science as it known to be more efficient than its classical counterpart. The impact and advantages of quantum information protocols emerge in numerous situations. In the case of cryptography, quantum dynamics guarantees secure protocols, and in quantum computation, factorization of large numbers, intractable with classical algorithms, can be solved much faster with a quantum computer. It is now widely believed that quantum information will play a leading role in future technologies. Together with the ongoing development of more efficient schemes to solve both new and old tasks in information science, a great deal of interest has been devoted to selecting suitable physical systems where one could realize these ideas. In particular, the quest for large scale integrability has stimulated an increasing interest in the field of solid state physics. Nanoelectronics seems the natural arena to realize physical implementation of quantum hardware. Qubits made out of solid-state devices, such as spins/charges in quantum dots or superconducting nanocircuits, may offer a greater advantage in this respect because fabrication techniques allow for scalability to a large number of coupled qubits. Recent experimental breakthroughs in semiconducting and superconducting nanostructures constitute the first important steps towards the realization of a solid-state quantum computer. In addition the interest in solid state quantum computation has stimulated a large body of research aimed at understanding the properties of entanglement in solid state systems. This Focus issue of New Journal of Physics gathers together contributions from leading experts in the area of solid state quantum information with the aim of providing a panorama of the most exciting scientific questions currently being investigated in the field. Focus on Solid State Quantum Information Contents Quasiparticle entanglement: redfinition of the vacuum and reduced density matrix approach P Samuelsson, E Sukhorukov and M Buttiker Pseudospin quantum computation in semiconductor nanostructures V W Scarola, K Park and S Das Sarma Superconducting qubit network with controllable nearest-neighbour coupling M Wallquist, J Lantz, V S Shumeiko and G Wendin Weak coupling Josephson junction as a current probe: effect of dissipation on escape dynamics J M Kivioja, T E Nieminen, J Claudon, O Buisson, F Hekking and J P Pekola The single Cooper-pair box as a charge qubit K Bladh, T Duty, D Gunnarsson and P Delsing Quantum state transfer in arrays of flux qubits A O Lyakhov and C Bruder Spin filling of a quantum dot derived from excited-state spectroscopy L H Willems van Beveren, R Hanson, I T Vink, F H L Koppens, L P Kouwenhoven and L M K Vandersypen Clauser–Horne inequality for the full counting statistics F Taddei, R Fazio and E Prada Recent advances in exciton based quantum information processing in quantum dot nanostructures H J Krenner, S Stufler, M Sabathil, E C Clark, P Ester, M Bichler, G Abstreiter and J J Finley Spatially highly resolved study of AFM scanning tip-quantum dot local interaction S Kicin, A Pioda, T Ihn, M Sigrist, A Fuhrer, K Ensslin, M Reinwald and W Wegscheider Transfer of entanglement from electrons to photons by optical selection rules M Titov, B Trauzette, B Michealis and C W J Beenakker Non-Abelian Chern-Simons models with discrete gauge groups on a lattice B Doucot and L B Ioffe Landau–Zener transitions in qubits controlled by electromagnetic fields Martijn Wubs, Keiji Saito, Sigmund Kohler, Yosuke Kayanuma and Peter Hänggi Phase-slip flux qubits J E Mooij and C J P M Harmans Mediated tunable coupling of flux qubits Alec Maassen van den Brink, A J Berkley and M Yalowsky Divergent beams of nonlocally entangled electrons emitted from hybrid normal-superconducting structures Elsa Prada and Fernando Sols Decoherence from ensembles of two-level fluctuators Josef Schriefl, Yuriy Makhlin, Alexander Shnirman and Gerd Schön Semiconductor quantum dots for electron spin qubits W G van der Wiel, M Stopa, T Kodera, T Hatano and S Tarucha Rosario Fazio, NEST-INFM and Scuola Normale Superiore, Pisa, Italy
The ability to control the state of individual atoms is a new challenge for science and technology in the 21st century. Currently, experiments on individual quantum systems such as trapped ions, single molecules, quantum dots, superconducting qubits and photons are crucial for the rapidly growing field of quantum information processing and communication. In general, solid state systems are preferable for scaling and the choice of material plays a crucial role; for example, in classical electronic devices continual performance enhancement and miniaturization is strongly linked to the success of silicon-based technology. For quantum applications, diamond has the potential to become the material of choice, because its large bandgap enables the control of optically active impurities and higher operation temperature. This focus issue collates original research contributions from some of the leading groups in the field as a showcase for the very latest developments in diamond-based quantum technologies.
It is our great pleasure to have a Special Section entitled "Superconducting Electronics for Quantum Information Technologies" in IEICE Transactions on Electronics. The quantum information technology is strongly expected to develop in the fields of quantum computing, quantum communication, and quantum measurement. The superconducting electronics plays a number of important roles in these fields. In this Special Section, four invited papers, which describe superconducting quantum computers, digital electronics for controlling the quantum computers, and the quantum information processing by superconducting nanowire single-photon detectors and superconducting transition edge sensors, explained current states and future prospects in their fields. We hope the Special Section enhances the progress of related research fields and opens a new academic research area. Finally, I would like to appreciate this opportunity to the secretary and the guest associate editors for their dedicated activities to complete the Special Section.
The idea to organize a central-european workshop on quantum optics goes back to the activity of prof. V. Buzžek (Bratislava) and prof. J. Janszky (Budapest). The first meetings were held in Budapest and Bratislava and started a tradition bringing together people working on problems related to quantum optics. Later workshops have been held at Budmerice (Slovakia), Balaton (Hungary), Olomouc and Prague (Czech republic). The already eighth central-european workshop found his way back to Prague and was organized in cooperation with the Doppler Institute and the Department of Physics of the Faculty for Nuclear Science and Physical Engineering of the Czech Technical University in Prague. The contributions of the participants focus on problems in quantum communication, quantum information, quantum state reconstruction, atom optics and nonlinear quantum optics. The success of the workshop was enabled by the kind cooperation of all the participants during the conference. The present collection of contributions gives a good review of topics presented. We hope that the present issue will be able to transfer the constructive and stimulating atmosphere of the workshop to the readership. I would like to thank the editor prof. D. Lüst and co-editor prof. W. P. Schleich for opening up the possibility to publish the proceedings of the workshop in the Fortschritte der Physik. It surely helps to enlarge the readership of the proceedings and promotes the idea of the central-european workshop among colleagues around the world.
Though QD arrays have initially been mostly studied in an attempt to improve semiconductor lasers, much attention has been recently devoted to the revolutionary prospects opened by the unique properties of single QDs. These artificial atoms allow to mimic in solid-state systems some of the quantum optics experiments performed on atoms since the early 80’s. Recent demonstrations of coherent control experiments on QDs illustrate this fruitful analogy [10]. We will focus here our discussion on the impact of QDs on two other fields, solid-state cavity quantum electrodynamics (CQED) and the generation of quantum states of light. A single QD can be used to emit triggered single photon pulses and photon pairs, whereas its narrow emission spectrum permits to implement microcavities very efficiently to tailor its emission properties. By combining both kind of effects, it is possible to control simultaneously the number of emitted photons as well as their quantum state and to build a solid-state singlemode source of single photons (singlemode S4P) as proposed [11] and demonstrated recently by our group [12]. This novel device represents the ultimate step in a field which has been extremely active in the last 15 years, spontaneous emission (SE) control in optoelectronic devices using confined electronic and/or photonic systems [13]. Before reviewing the development of this single mode S4P, based on a single QD in a pillar microcavity, we will briefly discuss its potential applications in the field of quantum information processing and quantum communications (QIPC).