This book serves as an introduction to the intertwined subjects of quantum optics and quantum information. It explores how quantum mechanics allows for the encoding and manipulation of information in ways far more efficient than classical computers. Targeting students, lecturers in physics and informatics, experimental and theoretical physicists, as well as those in the industry, this book provides valuable insights into the foundations and applications of quantum information science.Quantum Information Science (QIS) is an exciting and rapidly growing field that holds tremendous potential for revolutionizing various scientific and engineering disciplines. It combines the principles of quantum mechanics with classical computing to explore new frontiers in computation, communication, precision measurement, and fundamental quantum science. The origins of QIS can be traced back to about twenty years ago when visionaries like Charles Bennett, Paul Benioff, and Richard Feynman began contemplating the implications of merging quantum mechanics with classical computing machines. Since then, the field has witnessed remarkable progress in terms of theoretical proposals for quantum computation, cryptography, and metrology, as well as experimental implementations of these concepts.