Abstracts and Keywords

Chapter 1: Introduction to Quantum Photonics

Richard J. Warburton*

*Corresponding author: richard.warburton@unibas.ch

A brief and non-technical introduction to contemporary quantum photonics is presented.

Keywords: photon; quantum optics; quantum technology.

Chapter 2: The Second Quantum Revolution: From Basic Concepts to Quantum Technologies

Alain Aspect*,┼

*Corresponding author: alain.aspect@institutoptique.fr

┼The 2022 Nobel Prize Laureate in Physics

The second quantum revolution was first conceptual, with the understanding and the experimental demonstration of the extraordinary character of entanglement, as evidenced by Bell inequalities violations. These experiments were permitted by the possibility to observe and manipulate individual quantum objects. The second quantum revolution is now at the stage of using these conceptual advances to develop quantum technologies that are able to go beyond those based on the first quantum revolution.

Keywords: second quantum revolution; entanglement; bell inequalities; quantum computing; quantum cryptography.

Chapter 3: Solid-State Quantum Emitters

A. Mark Fox*

*Corresponding author: mark.fox@sheffield.ac.uk

This chapter gives a tutorial overview of the key parameters that are used to assess solid-state quantum emitters for applications in quantum photonics. Specifically, it covers single-photon purity, coherence and photon indistinguishability, and the methods that are used to quantify them. The chapter concludes with a brief comparison of several of the more common material systems that are used in solid-state quantum optics.

Keywords: quantum emitter; single-photon source; photon statistics; photon anti-bunching; Purcell effect; Rabi rotations; Hanbury Brown-Twiss experiment; Hong-Ou-Mandel experiment; photon indistinguishability.

Chapter 4: Single-Photon Sources for Multi-photon Applications

Stefan Frick, Robert Keil, Vikas Remesh, and Gregor Weihs*

*Corresponding author: gregor.weihs@uibk.ac.at

Much of the recent development of single-photon sources is driven by the desire to apply them to protocols and technologies that use the interference of two or more photons, such as quantum repeaters or boson sampling. In all of these cases, the indistinguishability of the produced photons is a key requirement. For those applications that want to scale to larger photon numbers, source efficiency is equally important. In this chapter, we will discuss two competing solutions, sources based on nonlinear optics and quantum dots, as the most-used single-quantum emitters.

Keywords: photon-pair sources; quantum dots; multi-particle interference; indistinguishability; number purity; spectral purity; excitation schemes.

Chapter 5: Quantum Key Distribution Protocols

Álvaro Navarrete, Víctor Zapatero,* and Marcos Curty

*Corresponding author: vzapatero@com.uvigo.es

In contrast to public-key cryptography, whose security relies on computational assumptions, quantum key distribution (QKD) provides an information-theoretically secure solution for the task of distributing cryptographic keys between distant parties through an insecure channel. In this chapter, we provide a general introduction to QKD protocols and their security, starting from the theoretical foundations of QKD?-?based on fundamental properties of quantum mechanics, such as the no-cloning theorem or entanglement?-?and moving to state-of-the-art QKD solutions and the latest technological advances.

Keywords: quantum key distribution; quantum communication; quantum cryptography; quantum technologies.

Chapter 6: From Basic Science to Technological Development: The Case for Two Avenues

Matteo Carlesso and Mauro Paternostro*

*Corresponding author: m.paternostro@qub.ac.uk

We argue that, in the quest for the translation of fundamental research into actual quantum technologies, two avenues that have - so far - only partly explored should be pursued vigorously. On first entails that the study of energetics at the fundamental quantum level holds the promises for the design of a generation of more energy-efficient quantum devices. On second route to pursue implies a more structural hybridization of quantum dynamics with data science techniques and tools, for a more powerful framework for quantum information processing.

Keywords: quantum technologies; quantum thermodynamics; quantum machine learning.

Chapter 7: Quantum Networks in Space

Lisa Wörner*

*Corresponding author: lisa.woerner@dlr.de

The inherent properties of quantum mechanics render it an important subject for fundamental research and applications alike. Entanglement stands out among those, allowing for novel secure communication and more rapid computation. To enable global secure communication, space based networks are required. A space network with trusted nodes for every user is an unviable feat, and could lead, given the amount of potential civil, governmental, and military users, to a cluttering of space. Additionally, global networks of trusted nodes exclude smaller to medium players from profiting from the quantum revolution and the benefits of secure communication. To remain inclusive to small businesses and enable broad usage of the technology, the operation of untrusted nodes in space is inevitable. Consequently, untrusted nodes need to be capable of receiving and re-transmitting the information with the possibility of storage of the information in-between. Hence, space-based quantum repeater, consisting of a receiver, a transmitter, and a coherent memory, could operate as untrusted nodes.

In this chapter, the challenges, necessities, and opportunities for global quantum networks will be discussed. This includes novel techniques for quantum repeater and their current technology readiness level.

Keywords: quantum network; untrusted node; quantum repeater.

Chapter 8: Fluorescence Spectroscopy in Planar Dielectric and Metallic Systems

Alexey I. Chizhik, Daja Ruhlandt, and Jörg Enderlein*

*Corresponding author: jenderl@gwdg.de

In this chapter, we present an overview of our recent theoretical and experimental studies of the fluorescence modulation of quantum emitters by planar nanostructures. We will mainly focus on two methods that are based on this phenomenon: (i) axial localization of a fluorophore using energy transfer from the fluorophore to a planar metal film or graphene and (ii) absolute quantum yield measurements using a plasmonic nanocavity. Both methods have been used in a large number of recent studies. We present the full theoretical modeling of these experiments and give an overview of recent experimental results.

Keywords: single-molecule fluorescence; nanocavity electrodynamics; fluorescence lifetime spectroscopy; defocused single-molecule imaging; metal- and graphene-induced energy transfer.

Chapter 9: Single Trapped Neutral Atoms in Optical Lattices

Andrea Alberti and Dieter Meschede*

*Corresponding author: meschede@uni-bonn.de

Controlling neutral atoms one by one has opened the path to create new experimental platforms for investigating their individual as well as their collective dynamics in a bottom-up approach. We discuss the tools for precisely detecting and preparing all degrees of freedom of trapped atoms at the ultimate quantum level, including their internal spin state, their motional state, and also their position, which has relevant quantum character. We summarize applications including single-atom interferometry and quantum walks.

Keywords: cold atoms; trapped neutral atoms; quantum information; quantum walks; single atoms; atom interferometry.

Chapter 10: Long-Distance Entanglement of Atomic Qubits

Kai Redeker, Wenjamin Rosenfeld, and Harald Weinfurter*

*Corresponding author: h.w@lmu.de

Quantum networks connecting future quantum computers and enabling efficient long-distance quantum communication require as most important resource the distribution of entanglement. This chapter details how this can be achieved for the particular example of atomic quantum memories implemented with single, trapped, neutral rubidium atoms.

Keywords: quantum network; quantum communication quantum memory; entanglement.

Chapter 11: Collective Light Emission of Ion Crystals in Correlated Dicke States

Ferdinand Schmidt-Kaler* and Joachim von Zanthier

*Corresponding author: fsk@uni-mainz.de

We give an overview of collective light scattering of trapped ion crystals serving as arrays of correlated emitters of single, indistinguishable photons into free space. The collectively emitted light is recorded in the far field by detectors, which feature high spatial and high temporal resolution. We observe interference in the first- and second-order photon correlation functions as well as, bunched and antibunched photon statistics. We analyze theoretically how the observed features of collective light scattering off arrays of correlated...