COMPRESSIVE QUANTIZATION FOR SCALABLE CLOUD RADIO ACCESS NETWORKS

With the proliferation of new mobile devices and applications, the demand for ubiquitous wireless services has increased dramatically in recent years. The explosive growth in the wireless traffic requires the wireless networks to be scalable so that they can be efficiently extended to meet the wireless communication demands. In a wireless network, the interference power typically grows with the number of devices without necessary coordination among them. On the other hand, large scale coordination is always difficult due to the low-bandwidth and high-latency interfaces between access points (APs) in traditional wireless networks. To address this challenge, cloud radio access network (C-RAN) has been proposed, where a pool of base band units (BBUs) are connected to the distributed remote radio heads (RRHs) via high bandwidth and low latency links (i.e., the front-haul) and are responsible for all the baseband processing. But the insufficient front-haul link capacity may limit the scale of C-RAN and prevent it from fully utilizing the benefits made possible by the centralized baseband processing. As a result, the front-haul link capacity becomes a bottleneck in the scalability of C-RAN. In this dissertation, we explore the scalable C-RAN in the effort of tackling this challenge. In the first aspect of this dissertation, we investigate the scalability issues in the existing wireless networks and propose a novel time-reversal (TR) based scalable wireless network in which the interference power is naturally mitigated by the focusing effects of TR communications without coordination among APs or terminal devices (TDs). Due to this nice feature, it is shown that the system can be easily extended to serve more TDs. Motivated by the nice properties of TR communications in providing scalable wireless networking solutions, in the second aspect of this dissertation, we apply the TR based communications to the C-RAN and discover the TR tunneling effects which alleviate the traffic load in the front-haul links caused by the increment of TDs. We further design waveforming schemes to optimize the downlink and uplink transmissions in the TR based C-RAN, which are shown to improve the downlink and uplink transmission accuracies. Consequently, the traffic load in the front-haul links is further alleviated by the reducing re-transmissions caused by transmission errors. Moreover, inspired by the TR-based C-RAN, we propose the compressive quantization scheme which applies to the uplink of multi-antenna C-RAN so that more antennas can be utilized with the limited front-haul capacity, which provide rich spatial diversity such that the massive TDs can be served more efficiently

Topological Photonics

Topological photonics is a rapidly emerging field of research in which geometrical and topological ideas are exploited to design and control the behavior of light. Drawing inspiration from the discovery of the quantum Hall effects and topological insulators in condensed matter, recent advances have shown how to engineer analogous effects also for photons, leading to remarkable phenomena such as the robust unidirectional propagation of light, which hold great promise for applications. Thanks to the flexibility and diversity of photonics systems, this field is also opening up new opportunities to realize exotic topological models and to probe and exploit topological effects in new ways. This article reviews experimental and theoretical developments in topological photonics across a wide range of experimental platforms, including photonic crystals, waveguides, metamaterials, cavities, optomechanics, silicon photonics, and circuit QED. A discussion of how changing the dimensionality and symmetries of photonics systems has allowed for the realization of different topological phases is offered, and progress in understanding the interplay of topology with non-Hermitian effects, such as dissipation, is reviewed. As an exciting perspective, topological photonics can be combined with optical nonlinearities, leading toward new collective phenomena and novel strongly correlated states of light, such as an analog of the fractional quantum Hall effect.Comment: 87 pages, 30 figures, published versio

Publications of the Jet Propulsion Laboratory 1989

This bibliography describes and indexes by primary author the externally distributed technical reporting, released during 1989, that resulted from scientific and engineering work performed, or managed, by JPL. Three classes of publications are included: JPL publications in which the information is complete for a specific accomplishment; articles from the quarterly Telecommunications and Data Acquisition (TDA) Progress Report; and articles published in the open literature

Colloquium on Solid State Devices

[no abstract

High-fidelity, compact readout of spins in silicon quantum dots

Silicon has become one of the leading platforms for quantum computation, having demonstrated qubits with long coherence times and high fidelity operations. Moreover, the similarities between silicon quantum dots and transistors give hope for mass production of qubits easily integrable with control electronics. However, to fully leverage their scalability potential, the footprint of the additional circuits for control and readout needs to be minimised. Here, we introduce a compact spin-readout method based on spin-dependent tunnelling combined with a dispersive charge sensor: the radio-frequency single-electron box (SEB). Opposite to traditional charge sensors, the SEB only requires a single lead, reducing its footprint. Using this sensing technique, we demonstrate spin readout of a single electron spin in a CMOS device manufactured at the 300mm wafer-scale using industrial processes, in which we measure long single spin relaxation times (up to 9 s). Next, we focus on achieving a high readout fidelity, since it is essential to perform error correction and ultimately sets the fidelity of qubit operations. The readout fidelity is partly set by the ability of the sensor to detect rapid events with high accuracy. We demonstrate that a low-loss high-impedance resonator highly coupled to the SEB, together with a Josephson Parameter Amplifier, are central for optimal performance. With these modifications, we obtain an integration time τm = 100 ns for a signal to noise ratio equal to 1, which facilitates single-shot spin readout, reaching a measurement fidelity FM = 99.54%, above the fault-tolerant threshold, in a readout time Δt = 250 μs. We identify that the readout time is limited by the choice of the spin-to-charge conversion mechanism. In the last part of the thesis, we work towards performing Pauli spin blockade spin readout, which does not have such time limitation

Artificial gauge fields with ultracold atoms in optical lattices

Diese Doktorarbeit befasst sich mit der Erzeugung von künstlichen Magnetfeldern für ultrakalte Atome in optischen Gittern mithilfe von Laser-induziertem Tunneln sowie mit der ersten experimentellen Bestimmung der Chernzahl in einem nicht-elektronischen System. Kalte Atome in optischen Gittern lassen sich experimentell sehr gut kontrollieren, was sie zu guten Modellsystemen für die Simulation von Festkörpern macht, wobei die Atome die Rolle der Elektronen übernehmen. Allerdings können Magnetfeldeffekte in diesen Systemen nicht direkt im Experiment simuliert werden, da die Atome elektrisch neutral sind, weshalb auf sie keine Lorentzkraft wirkt. Im Rahmen dieser Doktorarbeit wird eine neue Methode vorgestellt künstliche Magnetfelder basierend auf Laser-induziertem Tunneln zu erzeugen um somit die Physik geladener Teilchen in realen Magnetfeldern nachzuahmen. Dabei verursachen Laserstrahlen eine periodische Modulation der einzelnen Gitterplätze, deren Phase von der Gitterposition abhängt und dadurch zu komplexen Tunnelkopplungen führt. Ein Atom, welches sich entlang einer geschlossenen Bahn in diesem System bewegt, erfährt eine Phase, die als Aharonov-Bohm-Phase eines geladenen Teilchens in einem Magnetfeld interpretiert werden kann. Das modulierte Gitter wird durch einen zeitabhängigen Hamilton-Operator beschrieben, der typischerweise durch einen effektiven zeitunabhängigen Floquet Hamilton-Operator genähert wird. Im Rahmen dieser Arbeit wird darüber hinaus die vollständige Zeitabhängigkeit innerhalb einer Modulationsperiode beschrieben und mit den experimentellen Daten verglichen. Mithilfe des Laser-induzierten Tunnelns wurden alternierende sowie gleichgerichtete Magnetfelder im Experiment erzeugt, wobei letztere eine Realisierung des Harper-Hofstadter-Modells für einen Fluss Phi=pi/2 pro Gittereinheitszelle darstellen. Durch die Verwendung eines zusätzlichen Pseudospin-Freiheitsgrades konnte zudem der Spin-Hall-Effekt in einem optischen Gitter beobachtet werden. Unter Benutzung der einzigartigen Detektions- und Manipulationstechniken eines zweidimensionalen Übergitters konnte die Stärke und Verteilung des künstlichen Magnetfeldes auf lokaler Ebene durch die Beobachtung von Zyklotronorbits experimentell bestimmt werden. Die Bandstruktur in einem periodischen Potential mit externem Magnetfeld weist interessante topologische Eigenschafen auf, die durch Chernzahlen beschrieben werden, welche beispielsweise dem Quanten-Hall-Effekt zugrunde liegen. Um topologische Bandeigenschaften mit kalten Atomen beobachten zu können, wurden die genannten experimentellen Techniken weiterentwickelt. Mit einem neuen Aufbau, der nur auf optischen Potentialen beruht, konnte erstmals die Chernzahl in einem nicht-elektronischen System bestimmt werden. Die vorgestellten experimentellen Methoden eröffnen einzigartige Möglichkeiten die Eigenschaften von topologischen Materialien mit kalten Atomen in optischen Gittern zu untersuchen. Die Techniken wurden mit bosonischen Atomen implementiert, sie lassen sich allerdings ohne weiteres auch auf fermionische Systeme anwenden.This thesis reports on the generation of artificial magnetic fields with ultracold atoms in optical lattice potentials using laser-assisted tunneling, as well as on the first Chern-number measurement in a non-electronic system. The high experimental controllability of cold atoms in optical lattices makes them suitable candidates to study condensed matter Hamiltonians, where the atoms play the role of the electrons. However, the observation of magnetic field effects in these systems is challenging because the atoms are charge neutral and do not experience a Lorentz force. In the context of this thesis a new experimental technique for the generation of effective magnetic fields with laser-assisted tunneling was demonstrated, which mimics the physics of charged particles in real magnetic fields. The applied laser beams create a periodic on-site modulation whose phase depends on the position in the lattice and leads to complex tunnel couplings. An atom that hops around a closed loop in this system picks up a non-zero phase, which is reminiscent of the Aharonov-Bohm phase acquired by a charged particle in a magnetic field. The corresponding time-dependent Hamiltonian is typically described in terms of an effective time-independent Floquet Hamiltonian. In this work a theoretical description of the underlying full-time dynamics that occurs within one driving period and goes beyond the simple time-independent picture is presented. In the experiment the laser-assisted-tunneling method was implemented for staggered as well as uniform flux distributions, where the latter is a realization of the Harper-Hofstadter model for a flux Phi=pi/2 per lattice unit cell. By exploiting an additional pseudo-spin degree of freedom the same experimental setup led to the observation of the spin Hall effect in an optical lattice. Using the unique experimental detection and manipulation techniques offered by a two-dimensional bichromatic superlattice potential the strength of the artificial magnetic field and its spatial distribution could be determined through the observation of quantum cyclotron orbits on the level of isolated four-site square plaquettes. The band structure in the presence of a uniform magnetic field is topologically non-trivial and is characterized by the Chern number, a 2D topological invariant, which is at the origin of the quantized Hall conductance observed in electronic systems. In order to probe the topology of the bands the techniques mentioned above were refined by developing a new all-optical laser-assisted tunneling setup, which enabled the first experimental determination of the Chern number in a non-electronic system. The presented measurements and techniques offer a unique setting to study the properties of topological systems with ultracold atoms. All experimental techniques that were developed in the context of this thesis with bosonic atoms can be directly applied to fermionic systems