Interferometry, the most precise measurement technique known today, exploits the wave-like nature of the atoms or photons in the interferometer. As expected from the laws of quantum mechanics, the granular, particle-like features of the individually independent atoms or photons are responsible for the precision limit, the shot noise limit. However this “classical” bound is not fundamental and it is the aim of quantum metrology to overcome it by employing entanglement among the particles. This work reports on the realization of spin-squeezed states suitable for atom interferometry. Spin squeezing was generated on the basis of motional and spin degrees of freedom, whereby the latter allowed the implementation of a full interferometer with quantum-enhanced precision.

Cover -- Contents -- CHAPTER 1 Weak Superconductivity8212; Phenomenological Aspects -- 146;1 Macroscopic Quantum System -- 146;2 Coupled Superconductors -- 146;3 Single Electron Tunneling -- 146;4 Josephson Equations -- 146;5 Magnetic Field Effects -- 146;6 Barrier Free Energy -- 146;7 Electrodynamics of the Josephson Junction -- 146;8 Other Josephson Structures -- CHAPTER 2 Microscopic Theory -- 1 Tunneling Hamiltonian Formalism -- 2 General Expression for the Total Current -- 3 Tunneling Current for Constant Voltage -- 4 Expressions of Iqpi44; Iqp44; IJ144; IJ2 -- 5 Tunneling Current in the B46;C46;S46; Approximation -- 6 The 34;cos w34; Problem -- CHAPTER 3 Magnitude and Temperature Dependence of the Critical Current -- 346;1 Josephson Current for V61;0 -- 346;2 B46;C46;S46; Approximation -- 346;3 Strong Coupling Effects -- 346;4 Effects of Paramagnetic Impurities -- 346;5 Measurement Techniques -- CHAPTER 4 34;Small34; Junctions in a Magnetic Field -- 446;1 Josephson Penetration Depth -- 446;2 Small Junctions -- 446;3 Uniform Tunneling Current Distribution -- 446;4 Nonuniform Tunneling Current Density -- CHAPTER 5 Large Junctions8212;Static Self45;Field Effects -- 546;1 Approximate Analysis -- 546;2 Analysis of Owen and Scalapino -- 546;3 Effects of the Junction Geometrical Configuration -- CHAPTER 6 Voltage Current Characteristics -- 646;1 V45;I Curves of Various Weak Links -- 646;2 Resistively Shunted Junction Model58; Autonomous Case -- 646;3 Current Biased Tunneling Junction -- 646;4 Effects of Thermal Fluctuations -- CHAPTER 7 Other Superconducting Weak Link Structures -- 746;1 Metal Barrier Junctions -- 746;2 Semiconducting Barrier Junctions -- 746;3 Bridge45;Type Junctions -- 746;4 Point Contact Weak Links -- CHAPTER 8 Device Fabrication Technology -- 846;1 Josephson Tunneling Junctions -- 846;2 Junction Electrodes -- 846;3 Oxide Barriers -- 846;4 Junction Patterning -- 846;5 Simple Procedures for Preparing Oxide Barrier Junctions -- 846;6 Semiconductor Barriers -- 846;7 Bridge45;Type Weak Links -- 846;8 Point Contact Structures -- CHAPTER 9 Resonant Modes In Tunneling Structures -- 946;1 Josephson Junction as a Transmission Line -- 946;2 Resonant Modes for Low Q Junctions -- 946;3 Junction of Infinite Length -- 946;4 Nonuniform Current Density Distribution -- CHAPTER 10 Fluxon Dynamics -- 1046;1 The Sine Gordon Equation -- 1046;2 Nonlinear Standing Waves on a Rectangular Junction -- 1046;3 Effects of Losses and Bias -- 1046;4 Zero Field Steps -- 1046;5 Perturbative Analysis of Fluxon Dynamics -- 1046;6 Effects of Flux Flow on D46;C46; Voltage45;Current Characteristics -- 1046;7 Two Dimensional Junctions -- CHAPTER 11 High Frequency Properties and Applications of the Josephson Effect -- 1146;1 Simple Voltage Source Model -- 1146;2 Tunneling Junctions in External Microwave Radiation -- 1146;3 Current Source Model -- 1146;4 Emission of Radiation -- 1146;5 Detection of Radiation -- 1146;6 Parametric Amplification -- 1146;7 The Determina.

This thesis presents a theoretical investigation into the creation and exploitation of quantum correlations and entanglement among ultracold atoms. Specifically, it focuses on these non-classical effects in two contexts: (i) tests of local realism with massive particles, e.g., violations of a Bell inequality and the EPR paradox, and (ii) realization of quantum technology by exploitation of entanglement, for example quantum-enhanced metrology. In particular, the work presented in this thesis emphasizes the possibility of demonstrating and characterizing entanglement in realistic experiments, beyond the simple “toy-models” often discussed in the literature. The importance and relevance of this thesis are reflected in a spate of recent publications regarding experimental demonstrations of the atomic Hong-Ou-Mandel effect, observation of EPR entanglement with massive particles and a demonstration of an atomic SU(1,1) interferometer. With a separate chapter on each of these systems, this thesis is at the forefront of current research in ultracold atomic physics.

In a new branch of physics and technology, called spin-electronics or spintronics, the flow of electrical charge (usual current) as well as the flow of electron spin, the so-called "spin current", are manipulated and controlled together. This book is intended to provide an introduction and guide to the new physics and applications of spin current.

Quantum mechanics entails effects like superpositions and entanglement, which have no classical counterparts. From a technological standpoint these counterintuitive quantum aspects can be viewed as an unexploited resource that can be harnessed to support various tasks, e.g. in the domains of computation, communication, and metrology. In many applications, however, the potential of nonclassical states cannot practically be exploited due to detection inefficiencies. The authors address this limitation by experimentally realizing a novel detection scheme in which entangling interactions are time reversed. In this way, nonclassical many-particle states are disentangled, allowing them to be detected in a robust and technically feasible manner. In the context of quantum metrology, these nonlinear readout techniques extend the class of entangled probe states that can be leveraged for sensing applications without being limited by finite detector resolution. The authors present an active atom interferometer, where both the entangled state preparation and disentangling readout involve parametric amplification. This “SU(1,1)” interferometer is implemented with the help of spinor Bose–Einstein condensates, where amplification is implemented by atomic collisions leading to spin exchange.