This book covers the main ideas, methods, recent developments and applications of quantum-limit optical spectroscopy to quantum information, resolution spectroscopy, measurements beyond quantum limits, measurement of decoherence and entanglement. Quantum-limit spectroscopy lies at the frontier of current experimental and theoretical techniques, and is one of the areas of atomic spectroscopy where the quantization of the field is essential to predict and interpret the existing experimental results. Currently, there is an increasing interest in quantum and precision spectroscopy both...