This chapter reviews typical waveguide and active region designs for quantum cascade lasing in the terahertz (THz) frequency range. Operating principles are analyzed in details with special attention paid to the most recent developments with the state-of-the-art device performance. The maximum operation temperature of THz QCL is still the main obstacle for its wide employment in applications, although it has been lifted to 250 K, allowing cryogenic-free THz coherent radiation for potentially portable applications. Optimization of various limiting factors in the most advanced resonant-phonon designs or the combined designs with scattering-assisted injection scheme could be promising for further breakthroughs in achieving higher temperature operations. The discussions in this chapter mainly focus on the matured GaAs/AlGaAs material system, but the design strategies can be applied to THz QCLs utilizing other material systems, which may overcome the main challenges of the GaAs/AlGaAs material system and achieve better performance in the future.

Optical gas sensing is a promising alternative to analytical, electrochemical and semiconductor sensors that can offer fast responses times, minimal drift, high gas specificity, with zero cross-response to other gases. Quantum cascade lasers represent the optimal choice as mid-IR sources due to their high output power, compactness, narrow spectral linewidth and broad wavelength tunability. Among optical techniques, Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) has been demonstrated to be a leading-edge technology for real-world gas detection applications, thanks to its modularity, ruggedness, portability and real-time operation capability. QEPAS sensors typically achieve gas detection limits of few parts-per billion level. The basic principles of PAS are provided with a discussion on optoacoustic waves generation and detection. Quartz tuning forks physics is presented in detail, covering aspects like flexural modes resonance, including overtone, quality factor and microresonator tubes configuration. Finally, an overview of QCL-based QEPAS gas sensors for real-world applications, like environmental monitoring, breath sensing, leak detection and multi-gas detection is provided.

The advent of optical frequency combs revolutionized many research fields from metrology to high precision spectroscopy. It was recently demonstrated that broadband quantum cascade lasers can operate as frequency combs. As such, they operate under direct electrical pumping at both mid-infrared and terahertz frequencies, reaching powers in the watt range with multi-terahertz bandwidths. As their key application field, they unlock the advantages in speed and accuracy of the dual-comb spectroscopy technique in a frequency range where molecules have their fundamental vibrational and rotational bands. In this Chapter we review the design and basic functioning principles of these devices, the characterization of their coherence properties as well as few example applications.

This chapter shows how gauge theories underlie all elementary interactions described in the Standard Model. Surprisingly, this necessitates encompassing electromagnetism and the weak interaction into a unified theory called the electroweak interaction theory. A modern description of the weak neutral current is then formulated with the introduction of the Weinberg angle. The various Feynman rules are derived step by step in detail.

This chapter introduces how we can use the quantum fields introduced in the previous chapter to access amplitudes and, thus, measurable quantities, such as the cross sections and the particle lifetime. More specifically, an educational tour of quantum electrodynamics (QED), which describes the interaction of electrons (or any charged particles) with photons, is proposed. Although this chapter uses concepts from quantum field theory, it is not a course on that topic. Rather, the aim here is to expose the concepts and prepare the reader to be able to do simple calculations of processes at the lowest order. The notions of gauge invariance and the S-matrix are, however, explained. Many examples of Feynman diagrams and the calculation of the corresponding amplitudes are detailed. Summation and spin averaging techniques are also presented. Finally, the delicate concept of renormalisation is explained, leading to the notion of the running coupling constant.

“We revisit the quantum one-time pad and investigate the possibility of shortening the key used for quantum encryption. We first provide an impossibility result, and then show how it can be circumvented in two different ways: using approximate encryption, and by opening the door to the fascinating world of computational security. We also discuss a new possibility for quantum encryption, which is known as certified deletion: this is the possibility for the encrypter of a secret to request that the ciphertext is provably and irrevocably erased.”

Quantum cascade lasers are based on Intersubband transitions between quantum confined states in semiconductor heterostructures. The origin of these states is briefly described in this chapter starting with linear combination of atomic orbitals and then proceeding to the k.P theory. The relations between the interband and Intersubband transitions including their oscillator strength and selection rules are established. It is shown that “giant” Intersubband dipole owes its existence to the confinement induced band mixing. Aside from the radiative Intersubband transitions investigated in this chapter, nonradiative transitions also play important roles in QCL operation, hence most relevant of these processes: electron phonon, electron-electron, interface roughness and alloy disorder are also described in detail.

Entanglement is one of the most fundamental, and intriguing, properties of quantum mechanics. It is also at the heart of quantum cryptography! In this chapter we start by giving a clear mathematical definition of entanglement. We give two classic applications, to superdense coding and to secret sharing. We then investigate two complementary properties of entanglement that we will use deeply in cryptographic applications. The first is nonlocality, which we investigate through the famous CHSH game. The second is the monogamy of entanglement, which we demonstrate using a three-player version of the CHSH game.

A quick introduction to the standard model of particle physics is given. The general concepts of elementary particles, interactions and fields are outlined. The experimental side of particle physics is also briefly discussed: how elementary particles are produced with accelerators or from cosmic rays and how to observe them with detectors via the interactions of particles with matter. The various detector technologies leading to particle identification are briefly presented. The way in which the data collected by the sensors is analysed is also presented: the most frequent probability density functions encountered in particle physics are outlined. How measurements can be used to estimate a quantity from some data and the question of the best estimate of that quantity and its uncertainty are explained. As measurements can also be used to test a hypothesis based on a particular model, the hypothesis testing procedure is explained.