We introduce the task of key distribution, whose goal is to allow two mutually trusting users, Alice and Bob, to generate a random shared key that is unknown to any eavesdropper in the protocol. We start by precisely defining this task and our model for adversaries. We then show how to realize it in a simple toy scenario, which will help us demonstrate the key ideas. Finally we introduce information reconciliation, which is an important building block in the protocols that we will study in subsequent chapters.

A good knowledge of special relativity and quantum mechanics is essential for studying particle physics. Even if the reader is assumed to be already familiar with these two theories, a brief review of special relativity is given in this chapter with emphasis on the covariant and contravariant notations, which may be less well mastered but are very useful in particle physics. Important aspects of quantum mechanics for particle physics, such as the angular momentum, are also addressed.

The determination of particle properties relies mostly on experimental measurements based on their collisions and decays. This chapter introduces the concepts of reaction cross section and particle decay rate. For unstable particles, the origin of its lifetime as well as the notion of branching ratios is presented. Many formulas involving the reactions between two particles are derived in detail, and phase spaces involving three-body decays are presented with Dalitz diagrams. The notion of cross reactions is also presented.

In this chapter, we overview recent developments of a simulation framework capable of capturing the highly nonequilibrium physics of the strongly coupled electron and phonon systems in quantum cascade lasers (QCLs). In midinfrared (mid-IR) devices, both electronic and optical phonon systems are largely semiclassical and described by coupled Boltzmann transport equations, which we solve using an efficient stochastic technique known as ensemble Monte Carlo. The optical phonon system is strongly coupled to acoustic phonons, the dominant carriers of heat, whose dynamics and thermal transport throughout the whole device are described via a global heat-diffusion solver. We discuss the roles of nonequilibrium optical phonons in QCLs at the level of a single stage , anisotropic thermal transport of acoustic phonons in QCLs, outline the algorithm for multiscale electrothermal simulation, and present data for a mid-IR QCL based on this framework.

In this chapter we introduce a variant of the BB’84 quantum key distribution protocol, the E’91 protocol due to Ekert. We show that this protocol achieves a higher level of security called “device independent security.” What this means, informally, is that the new protocol’s security doesn’t rely on Alice and Bob performing trusted measurements on their qubit in each round. We sketch the proof of security of the E’91 protocol, which rests on the property of entanglement monogamy.

After more than 25 years of continuous and increasing investment in research and development, the quantum cascade laser (QCL) is now revolutionizing a multitude of applications which aim to address major global challenges, from climate change to the cost of health care, to the pollution of our atmosphere and oceans, to the protection of our soldiers and first-responders. This chapter provides an insider’s perspective and historical context to the present and rapid trajectory in widespread QCL deployment across multiple applications and markets which has been building over the past two decades and will likely continue to benefit our quality of life and standard of living over the next hundred years.

Quantum cascade lasers (QCL) can be powerful testing grounds of the fundamental physical parameters determined by their quantum nature. In this chapter we describe a set of experimental techniques to explore the linewidth, frequency and phase stability of far-infrared QCLs. By performing noise measurements with unprecedented sensitivity levels, we highlight the key role of gain medium engineering and demonstrate that properly designed semiconductor-heterostructure lasers can unveil the mechanisms underlying the laser-intrinsic phase noise, revealing the link between device properties and the quantum-limited linewidth. We discuss phase-locking of THz QCL to a free-space comb generated in a LiNbO3 waveguide, and present phase and frequency control of miniaturized QCL frequency combs. This work paves the way to novel metrological-grade THz applications, including high-resolution spectroscopy, manipulation of cold molecules, astronomy and quantum technologies. The physical processes and dynamics presented here open groundbreaking perspectives for the development of quantum sensors, quantum imaging devices and q-bits made by entangled teeth for photonic-based quantum computation.

Quantum cascade lasers provide a variety of challenges to theory, which are outlined in this review: (i) The choice of basis states is discussed, where energy eigenstates are badly defined if the full periodic structure is considered. (ii) The tunneling through barriers requires a treatment of quantum coherences, which sets particular demands on the formulation of the quantum kinetic equations. Here the advantages and disadvantages of different approaches such as rate equations, Monte-Carlo simulations, different density matrix approaches, and Green’s functions methods are addressed. (iii) The evaluation of gain is detailed, where broadening is of utmost importance. (iv) An overview regarding the electrical instabilities of the extended structure due to domain formation is given, which strongly affect the overall performance

In this chapter we present an alternative path to base security in challenging settings. We will discover that physical assumptions on the adversary, such that they have a bounded or a noisy quantum memory, can be leveraged to design secure protocols for tasks, such as 1-2 oblivious transfer, for which there cannot exist an unconditionally secure protocol. To prove security we make a fresh use of uncertainty relations introduced earlier in the context of quantum key distribution.