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.

Physicists know how to produce gases at a few billionths of a degree above absolute zero. The cooling methods apply not only to atoms but also to ions and molecules. This field of research has three times been awarded the Nobel Prize. The field experienced remarkable growth when experimentalists learned how to vary at will the interactions between particles, trapping them with optical tweezers or in optical gratings with adjustable geometry. Artificial crystals made of atoms or molecules can be built to simulate the structure of matter and elucidate some of its magnetic properties, hopefully contributing to the understanding of high-temperature superconductivity. The phenomenon of quantum entanglement is the basis for new devices for the storage and transmission of quantum information. Spectacular progress is constantly being made in metrology. For example, ultra-cold atom or ion clocks measure time to better than one second over the lifetime of the Universe. New types of industrial gravimeters and gyroscopes are improving the sensitivity of seismology and navigation in space. In addition, the extreme precision of the measurements allows tests of the fundamental laws of physics, such as quantum electrodynamics, Lorentz invariance or possible variations of the fundamental constants. The field of ultra-cold particles has now reached the stage where it provides insights in the fields of condensed matter, chemistry and even cosmology.

This book illustrates the history of Atomic Physics and shows how its most recent advances allow the possibility of performing precise measurements and achieving an accurate control on the atomic state. Written in an introductory style, this book is addressed to advanced undergraduate and graduate students, as well as to more experienced researchers who need to remain up-to-date with the most recent advances. The book focuses on experimental investigations, illustrating milestone experiments and key experimental techniques, and discusses the results and the challenges of contemporary research. Emphasis is put on the investigations of precision physics: from the determination of fundamental constants of Nature to tests of General Relativity and Quantum Electrodynamics; from the realization of ultra-stable atomic clocks to the precise simulation of condensed matter theories with ultracold gases. The book discusses these topics while tracing the evolution of experimental Atomic Physics from traditional laser spectroscopy to the revolution introduced by laser cooling, which allows the manipulation of atoms at a billionth of a degree above absolute zero and reveals new frontiers of precision in atomic spectroscopy.

Space-based laboratory research in fundamental physics is an emerging research discipline that offers great discovery potential and at the same time could drive the development of technological advances which are likely to be important to scientists and technologists in many other different research fields. The articles in this review volume have been contributed by participants of the international workshop ?From Quantum to Cosmos: Fundamental Physics Research in Space? held at the Airlie Center in Warrenton, Virginia, USA, on May 21?24, 2006. This unique volume discusses the advances in our understanding of fundamental physics that are anticipated in the near future, and evaluates the discovery potential of a number of recently proposed space-based gravitational experiments. Specific research areas covered include various tests of general relativity and alternative theories, search of physics beyond the Standard Model, investigations of possible violations of the equivalence principle, search for new hypothetical long- and short-range forces, variations of fundamental constants, tests of Lorentz invariance and attempts at unification of the fundamental interactions. The book also encompasses experiments aimed at the discovery of novel phenomena, including dark matter candidates, and studies of dark energy.

This book presents a comprehensive overview of the spectacular advances seen in atomic physics during the last 50 years. The authors explain how such progress was possible by highlighting connections between developments that occurred at different times. They discuss the new perspectives and the new research fields that look promising. The emphasis is placed, not on detailed calculations, but rather on physical ideas. Combining both theoretical and experimental considerations, the book will be of interest to a wide range of students, teachers and researchers in quantum and atomic physics.

The Les Houches Summer School in August 2015 covered the emerging fields of cavity optomechanics and quantum nanomechanics. Optomechanics is flourishing and its concepts and techniques are now applied to a wide range of topics. Modern quantum optomechanics was born in the late 1970s in the framework of gravitational wave interferometry, with an initial focus on the quantum limits of displacement measurements. Carlton Caves, Vladimir Braginsky, and others realized that the sensitivity of the anticipated large-scale gravitational-wave interferometers (GWI) was fundamentally limited by the quantum fluctuations of the measurement laser beam. After tremendous experimental progress, the sensitivity of the upcoming next generation of GWI will effectively be limited by quantum noise. In this way, quantum-optomechanical effects will directly affect the operation of what is arguably the world's most impressive precision experiment. However, optomechanics has also gained a life of its own with a focus on the quantum aspects of moving mirrors. Laser light can be used to cool mechanical resonators well below the temperature of its environment. After proof-of-principle demonstrations of this cooling in 2006, a number of systems were used as the field gradually merged with its condensed matter cousin (nanomechanical systems) to try to reach the mechanical quantum ground state, eventually demonstrated in 2010 by pure cryogenic techniques and just one year later by a combination of cryogenic and radiation-pressure cooling. The book covers all aspects -- historical, theoretical, experimental -- of the field, with its applications to quantum measurement, foundations of quantum mechanics and quantum information. It is an essential read for any new researcher in the field.

Fundamental Tests of Physics with Optically Trapped Microspheres details experiments on studying the Brownian motion of an optically trapped microsphere with ultrahigh resolution and the cooling of its motion towards the quantum ground state. Glass microspheres were trapped in water, air, and vacuum with optical tweezers; and a detection system that can monitor the position of a trapped microsphere with Angstrom spatial resolution and microsecond temporal resolution was developed to study the Brownian motion of a trapped microsphere in air over a wide range of pressures. The instantaneous velocity of a Brownian particle, in particular, was studied for the very first time, and the results provide direct verification of the Maxwell-Boltzmann velocity distribution and the energy equipartition theorem for a Brownian particle. For short time scales, the ballistic regime of Brownian motion is observed, in contrast to the usual diffusive regime. In vacuum, active feedback is used to cool the center-of-mass motion of an optically trapped microsphere from room temperature to a minimum temperature of about 1.5 mK. This is an important step toward studying the quantum behaviors of a macroscopic particle trapped in vacuum.