This chapter presents the last interaction described by the Standard Model of particle physics, i.e. the weak interaction. A historical approach is followed, trying to explain the evolution of its theoretical description from the experimental discoveries: we start from Fermi theory before introducing the charged and neutral currents. The mixing matrices both in the quark sector and in the leptonic sector are described. The phenomenon of neutrino oscillation is also detailed. The chapter concludes with a detailed discussion of CP violation.

This chapter explains how we can reconcile massive particles within a gauge symmetry. The notion of spontaneous symmetry breaking is introduced, first in a simple model and then with the gauge group of the Standard Model. The Brout–Englert–Higgs mechanism is then presented in detail. The rest of the chapter is devoted to the experimental discovery of the Higgs boson and its properties with the most up-to-date experimental measurements.

Laser feedback interferometry, based on the self-mixing (SM) effect in quantum cascade lasers (QCLs), is one of the simplest coherent techniques, for which the emission source can also play the role of a highly-sensitive detector. The combination of QCLs and SM is particularly attractive for sensing applications, notably in the THz band where it provides a high-speed high-sensitivity detection mechanism which inherently suppresses unwanted background radiation. The SM phenomenon in QCLs has been exploited for a wide range of applications, including the measurement of internal laser characteristics, two- and three-dimensional imaging, materials analysis and near-field imaging. This chapter provides an overview of the SM effect in QCLs, and reviews the state of the art in sensing using this technique.

High-power terahertz quantum-cascade lasers (QCLs) are desired for a variety of applications in imaging and spectroscopy. The best performance at practical operating temperatures for single-mode terahertz QCLs is realized with metallic cavities due to a strong plasmonic mode confinement of the optical mode within the cavity. However, such plasmonic lasers suffer from poor beam shapes, low output power, and multi-mode spectral behavior. Development of distributed-feedback (DFB) techniques to improve spectral as well as modal properties becomes indispensable for terahertz QCLs to address targeted applications that typically require single-mode operation, frequency stability and specificity, and optimal far-field beam quality with single-lobed profile and low angular divergence. This chapter describes the theory, design methodologies, and key results from a sampling of a wide variety of DFB techniques that have been implemented in literature for monolithic terahertz QCLs with metallic cavities in both edge-emitting and surface-emitting configurations, either of which have their specific application areas and advantages much-like that for infrared diode lasers.

In this chapter we consider the problem of amplifying secrecy, or uncertainty. This is the problem of privacy amplification: given a partially secret string, how do we make it almost-perfectly secret? This task forms one of the key building blocks in the protocol for quantum key distribution that we develop in later chapters. It can be solved using a fundamental object from the theory of pseudorandomness called a randomness extractor. We introduce an extractor based on hashing and show that it can be used to perform privacy amplification.

Quantum information provides an advantage for cryptographic tasks in a wide variety of settings. In this chapter we focus on applications involving two parties and look beyond key distribution for other tasks where quantum information can play a role. This includes coin flipping, oblivious transfer, and other primitives in two-party cryptography.