Progress in Optics, Volume 68 highlights new advances in the field of optics, with this updated volume presenting interesting chapters on a variety of timely topics in the field. Chapters in this release include Nonlinear Optical Polarimetry with application in biomicroscopy, Single-photon Sources, Introduction to Tensor Networks and Matrix Product States with Applications in Cavity and Waveguide Quantum Electrodynamics, Rotated frames, Phase retrieval, and more. Each chapter is written by an international board of authors who review the latest developments in optics.