More than 60 people from all over the world, including students, researchers and lecturers, gathered in Varenna for the 204 Course of the International School of Physics “E. Fermi” dedicated to Nanoscale Quantum Optics. The course was organized in collaboration with the COST Action MP1403 “Nanoscale Quantum Optics”, a network that involved 28 European countries and more than 500 researchers.

Recent developments aiming at the realization of new technologies based on quantum physics have been recognized by the European Commission as priorities, with the launch of the Quantum Technology Flagship Programme. These are, for example, new cryptographic techniques for security in telecommunications, new computing hardware that can solve problems so far inaccessible even to the latest generation of supercomputers, and new precision standards and sensors capable of measuring for instance extremely weak magnetic fields, with applications ranging from materials science to medical diagnostics. Nanoscale quantum optics combines these themes with nanophotonics, which addresses the control of light and its coupling with matter on a nanometer scale, a miniaturization comparable to the transition from valve-based electronics to integrated circuits. Structured materials provide confinement much beyond the wavelength scale, the interaction of light with nanoscale object offers novel means for interfacing light with different degrees of freedom, and quantum optics experiments are being upgraded in miniaturized nanophotonic platforms.

Based on such advances, the Course was therefore an opportunity to train new gener- ations of scientists, who will have the privilege of doing research on topics that promise great innovations in science and technology.

This proceedings book contains the following lecture and seminars held during the school:

– “Basic concepts for quantum optics and quantum technologies”, by I. D’Amico, introduces the background concepts.

– “Materials for quantum nanophotonics”, by N. P. de Leon, outlines the requirements for a range of quantum nanophotonics applications, and describes the key material characteristics that affect physical properties related to these requirements.

– “Quantum optics and nonclassical light generation”, by J. Vuckovic et al., discusses the theoretical underpinnings of the seminal experiments in solid-state quantum optics.

– “Creating quantum correlations between quantum-dot spins”, by M. Atatüre, describes how to generate nonlocal quantum correlations between electron spins in semiconductor quantum dots.

– “Platforms for telecom entangled photon sources”, by F. Sciarrino et al., reviews different platforms used to generate telecom-entangled photon pairs, focusing in particular on an integrated source realized by the femtosecond laser writing technique.

– “Quantum optics with single spins”, by L. C. Bassett, describes the quantum-mechanical coupling between atom-like spin states and light, using the diamond nitrogen-vacancy (NV) center as a paradigm. Moreover, it explains various methods that serve as the basis for advanced protocols at the heart of many emerging quantum technologies.

– “Nanoscale sensing and quantum coherence”, by F. Reinhard, summarizes concepts and techniques about single-qubit sensors, including an outlook to the major trends of the field.

– “ Many-body physics and quantum simulations with strongly interacting photons”, by D. G. Angelakis and Jirawat Tangpanitanon, focuses on interacting photons in superconducting circuits for quantum simulation of both in and out-of-equilibrium quantum many-body systems.

– “Nano-optomechanics”, by E. Verhagen, introduces the basic physical description of optomechanical interactions at a tutorial level, and highlight several directions of current research.

In addition, among all poster contributions the following were selected for this book:

– “Photostable molecules on chip: a scalable approach to photonic quantum technologies”, by M. Colaiutti et al., presents the design and characterization of the evanescent coupling between dibenzoterrylene molecules and a ridge waveguide made of silicon nitride.

– “Environment spectroscopy with an NV center in diamond”, by S. Hernández-Gómez et al., describes in detail a method to spectroscopically characterize the spin bath around an NV center and identify the coherent coupling with the nearest nuclear spins.

– “Ultrafast photonic quantum correlations mediated by individual phonons”, by S. Tarragó Vélez and C. Galland, outlines a new technique to prepare and measure the lifetime of the first phonon Fock state in diamond using single photon time-correlated Raman spectroscopy.

In this collection of chapters we hope the readers will find a valuable overview of the state-of-the-art and current trends in nanoscale quantum optics.

Mario Agio, Irene D’Amico, Costanza Toninelli and Rashid Zia