The Physical Review Journals Celebrate the International Year of Light2015 has been designated the International Year of Light and Light-based Technologies by UNESCO. This global initiative aims to highlight the importance of light in our everyday lives and how it has contributed to the development of society. To celebrate this yearlong event, the editors of the Physical Review journals have organized a collection of papers that represent important breakthroughs in the field of optics, from fundamental insights into how light behaves, to findings that were critical in the development of everyday technologies. These papers will be free to read throughout 2015.For a shorter list aimed at a broader public audience, visit the Physics International Year of Light page.Superradiant Atom EmissionThe probability that an excited two-level system (e.g., an excited atom) will emit a photon decreases exponentially with time. However, this emission rate is much higher if a second atom is placed nearbyeven if the second atom is in its ground state. Robert Dicke made this surprising theoretical discovery, which occurs because the quantum states of nearby atoms are correlated, in 1954. Dicke also showed that emission events in a large group of particles are not independent, leading to a significant increase in the radiative power of the system, a behavior he named superradiance. Since Dicke's exploratory study, the phenomenon of superradiance has been observed in many physical systems such as optically pumped hydrogen fluoride gas, quantum dots, and superconducting qubits. The effect has recently been used to make a superradiant laser, where the correlated atomic emission boosted photon emission by a factor of 10,000.Coherence in Spontaneous Radiation ProcessesR. H. DickePhys. Rev. 93, 99 (1954)Defense Advanced Research Projects Agency (via Wikimedia)Photonic Crystals Make Their DebutLosses can occur in devices, such as semiconductor lasers, when light of unwanted frequencies is spontaneously emitted. In the late 1980s, Eli Yablonovitch and Sajeev John were separately working on ways to avoid these losses. Yablonovitch observed that losses didnt occur in mediums that prevented the unwanted frequencies from propagating. He proposed that such a medium could be made by carving voids into an optically transparent material, where the material and voids had different refractive indices. Later termed photonic crystals by Yablonovitch and John, these structures reduced losses because the periodic voids caused waves with certain frequencies to destructively interfere. Photonic crystals are now widely used in optical fibers, light-emitting diodes, high-energy particle accelerators, and medical devices.Strong localization of photons in certain disordered dielectric superlatticesSajeev JohnPhys. Rev. Lett. 58, 2486 (1987)Inhibited Spontaneous Emission in Solid-State Physics and ElectronicsEli YablonovitchPhys. Rev. Lett. 58, 2059 (1987)See Physics article: Focus: Landmarks The Birth of Photonic CrystalsThe Quantum Side of LightSince the 19th century, light was known to be a wave of electric and magnetic fields. In the early 20th century, the physics revolution known as quantum mechanics included the alternative description of light as a collection of particles called photons. But Roy Glauber of Harvard University realized that with the invention of the laser and the detectors capable of sensing a single photon, a more complete quantum theory of light was needed. His theory, published in 1963, explained that photons are not entirely independent objects and that detecting a photon from a light beam affects the probability of detecting another photon. His theory marked the birth of the field of quantum optics, which led to a wide range of advances, such as a type of optics-based cryptography that has already been used for some bank transactions. Glauber shared the 2005 Nobel Prize in physics for his work. [Reprinted from Physics].