Entanglement is a quintessential quantum mechanical phenomenon with no classical equivalent. First discussed by Einstein, Podolsky, and Rosen and formally introduced by Schrödinger in 1935, entanglement has grown from a scientific debate to a radically new resource that sparks a technological revolution. This review focuses on fundamentals and recent advances in entanglement-based quantum information technology (QIT), specifically in photonic systems. Photons are unique quantum information carriers with several advantages, such as their ability to operate at room temperature, their compatibility with existing communication and sensing infrastructures, and the availability of readily accessible optical components. Photons also interface well with other solid-state quantum platforms. We first provide an overview on entanglement, starting with an introduction to its development from a historical perspective followed by the theory for entanglement generation and the associated representative experiments. We then dive into the applications of entanglement-based QIT for sensing, imaging, spectroscopy, data processing, and communication. Before closing, we present an outlook for the architecture of the next-generation entanglement-based QIT and its prospective applications.

Emerging applications in quantum information, microscopy, biosensing, depth sensing, and augmented reality demand miniaturized components in the visible (VIS) and near-infrared (NIR) spectrum with wavelengths between 380 and 1100 nm. Foundry silicon photonics, which has been optimized for telecommunication wavelengths, can be adapted to this wavelength range. In this article, we review recent developments in silicon photonics for VIS and NIR wavelengths, with a focus on platforms, devices, and photonic circuits fabricated in foundries. Foundries enable the creation of complex circuitry at a wafer scale. Platforms based on silicon nitride and aluminum oxide wave-guides compatible with complementary metal–oxide–semiconductor (CMOS) foundries are becoming available. As a result, highly functional photonic circuits are becoming possible. The key challenges are low-loss waveguides, efficient input/output coupling, sensitive detectors, and heterogeneous integration of lasers and modulators, particularly those using lithium niobate and other electro-optic materials. These elements, already developed for telecommunications, require further development forλ < 1100 nm. As short-wavelength silicon photonics technology advances, photonic integrated circuits can address a broader scope of applications beyond O- and C-band communication.

Since the invention of the silicon subwavelength grating waveguide in 2006, subwavelength metamaterial engineering has become an essential design tool in silicon photonics. Employing well-established nanometer-scale semiconductor manufacturing techniques to create metamaterials in optical waveguides has allowed unprecedented control of the flow of light in photonic chips. This is achieved through fine-tuning of fundamental optical properties such as modal confinement, effective index, dispersion, and anisotropy, directly by lithographic imprinting of a specific subwavelength grating structure onto a nanophotonic waveguide. In parallel, low-loss mode propagation is readily obtained over a broad spectral range since the subwavelength periodicity effectively avoids losses due to spurious resonances and bandgap effects. In this review we present recent advances achieved in the surging field of metamaterial integrated photonics. After briefly introducing the fundamental concepts governing the propagation of light in periodic waveguides via Floquet–Bloch modes, we review progress in the main application areas of subwavelength nanostructures in silicon photonics, presenting the most representative devices. We specifically focus on off-chip coupling interfaces, polarization management and anisotropy engineering, spectral filtering and wavelength multiplexing, evanescent field biochemical sensing, mid-infrared photonics, and nonlinear waveguide optics and optomechanics. We also introduce a nascent research area of resonant integrated photonics leveraging Mie resonances in dielectrics for on-chip guiding of optical waves, with the first Huygens’ metawaveguide recently demonstrated. Finally, we provide a brief overview of inverse design approaches and machine-learning algorithms for on-chip optical metamaterials. In our conclusions, we summarize the key developments while highlighting the challenges and future prospects.

Realization of single-mode, high-power and high-beam-quality (namely, high-brightness) semiconductor lasers, which can rival or even replace bulky lasers such as gas, solid, and fiber lasers, is one of the ultimate goals of laser physics and photonics. The demand for such ultimate single-mode high-brightness semiconductor lasers is increasing for a wide variety of emerging applications including next-generation remote sensing for smart mobility and high-precision laser processing for smart manufacturing. Photonic-crystal surface-emitting lasers (PCSELs) show promise to meet these demands, based on their broad-area coherent two-dimensional (2D) resonance at a singularity (Γ) point of their 2D photonic band structure. In this tutorial paper, the lasing principle, theoretical analysis, and experimental demonstration of PCSELs are described. Recent progress in PCSEL development, including the formulation of a design guideline for realizing 100-W-to-kW-class single-mode operation, the experimental demonstration of a brightness of 1 GW cm–2 sr–1, and an extension of the lasing wavelengths to telecommunication and mid-infrared wavelengths are also covered.

Non-Abelian optics has emerged as a promising research field with the potential to revolutionize our understanding of light–matter interactions and enable new applications in areas including topological photonic devices, quantum computing, optical sensing, and communications. This review provides an overall framework for the rapidly developing field of non-Abelian properties in optics, including the basic concepts of non-Abelian optics, the physical mechanism of non-Abelian statistics, the non-Abelian gauge field in optics, non-Abelian braiding in optics as a special phenomenon of the non-Abelian gauge field, and current challenges and opportunities. This review is intended to provide a new perspective on non-Abelian optics, summarize the current status and advanced progress in non-Abelian gauge fields and braiding in optics, and stimulate dialog about future perspectives.

Harnessing linear and angular momenta of light is one of the cornerstones in modern optics and has found tremendous applications in optical circuits, particle manipulation, metrology, quantum information processing, etc. Emerging theoretical protocols and experimental explorations have created a surge of interest in light lateral momenta and forces, which are perpendicular to the light wave propagation direction. However, there is yet a lack of a comprehensive and holistic overview of transverse momenta (both linear and angular) as well as of optical lateral forces (OLFs). In this article, we first review the most recent transverse momenta including the transverse spin angular momentum, optical skyrmions, as well as lateral momenta from directional side scattering, spin–orbit interaction, and surface plasmon polaritons. Since optical forces result from the momentum exchange between light and matter, the transverse momentum consequently gives rise to intriguing OLFs, which is the second topic of this article. Additional non-trivial lateral forces that combine optics with other effects from thermodynamics, electricity, and microfluidics, are also discussed. It should be emphasized that these momenta and forces ubiquitously exist in a broad range of optical phenomena and have often been neglected due to their unpredicted underlying physics and shortage of experimental means, especially prior to the last decade.

This tutorial–review on applications of artificial neural networks in photonics targets a broad audience, ranging from optical research and engineering communities to computer science and applied mathematics. We focus here on the research areas at the interface between these disciplines, attempting to find the right balance between technical details specific to each domain and overall clarity. First, we briefly recall key properties and peculiarities of some core neural network types, which we believe are the most relevant to photonics, also linking the layer’s theoretical design to some photonics hardware realizations. After that, we elucidate the question of how to fine-tune the selected model’s design to perform the required task with optimized accuracy. Then, in the review part, we discuss recent developments and progress for several selected applications of neural networks in photonics, including multiple aspects relevant to optical communications, imaging, sensing, and the design of new materials and lasers. In the following section, we put a special emphasis on how to accurately evaluate the complexity of neural networks in the context of the transition from algorithms to hardware implementation. The introduced complexity characteristics are used to analyze the applications of neural networks in optical communications, as a specific, albeit highly important example, comparing those with some benchmark signal-processing methods. We combine the description of the well-known model compression strategies used in machine learning, with some novel techniques introduced recently in optical applications of neural networks. It is important to stress that although our focus in this tutorial–review is on photonics, we believe that the methods and techniques presented here can be handy in a much wider range of scientific and engineering applications.

The generation, manipulation, storage, and detection of single photons play a central role in emerging photonic quantum information technology. Individual photons serve as flying qubits and transmit the relevant quantum information at high speed and with low losses, for example between individual nodes of quantum networks. Due to the laws of quantum mechanics, the associated quantum communication is fundamentally tap-proof, which explains the enormous interest in this modern information technology. On the other hand, stationary qubits or photonic states in quantum computers can potentially lead to enormous increases in performance through parallel data processing, to outperform classical computers in specific tasks when quantum advantage is achieved. In this review, we discuss in depth the great potential of semiconductor quantum dots in photonic quantum information technology. In this context, quantum dots form a key resource for the implementation of quantum communication networks and photonic quantum computers, because they can generate single photons on demand. Moreover, these solid-state quantum emitters are compatible with the mature semiconductor technology, so that they can be integrated comparatively easily into nanophotonic structures such as resonators and waveguide systems, which form the basis for quantum light sources and integrated photonic quantum circuits. After a thematic introduction, we present modern numerical methods and theoretical approaches to device design and the physical description of quantum dot devices. We then introduce modern methods and technical solutions for the epitaxial growth and for the deterministic nanoprocessing of quantum devices based on semiconductor quantum dots. Furthermore, we highlight the most promising device concepts for quantum light sources and photonic quantum circuits that include single quantum dots as active elements and discuss applications of these novel devices in photonic quantum information technology. We close with an overview of open issues and an outlook on future developments.

Non-Hermitian optics is a burgeoning field at the intersection of quantum physics, electrodynamics, and nanophotonics. It provides a new perspective of the role of gain and loss in optical systems. Leveraging the advanced designs inspired by non-Hermitian physics, classical optical platforms have been widely investigated to unveil novel physical concepts, such as parity-time symmetry and exceptional points, which have no counterparts in the conventional Hermitian settings. These investigations have yielded a plethora of new phenomena in optical wave scattering, optical sensing, and nonlinear optical processes. Non-Hermitian effects also have a profound impact on the lasing behaviors in the semiclassical framework of lasers, allowing for novel ways to engineer single-mode lasers, chiral laser emission, laser noise, linewidth, etc. Furthermore, over recent years, there has been increasing interest in the explorations of non-Hermitian physics in quantum optics, which addresses photon statistics, entanglement, decoherence, and quantum sensing in non-Hermitian systems. In this review, we review the most recent theoretical and experimental advances in non-Hermitian optics and photonics, covering the significant progress in both classical and quantum optics regimes.

Light transport in a highly multimode fiber exhibits complex behavior in space, time, frequency, and polarization, especially in the presence of mode coupling. The newly developed techniques of spatial wavefront shaping turn out to be highly suitable to harness such enormous complexity: a spatial light modulator enables precise characterization of field propagation through a multimode fiber, and by adjusting the incident wavefront it can accurately tailor the transmitted spatial pattern, temporal profile, and polarization state. This unprecedented control leads to multimode fiber applications in imaging, endoscopy, optical trapping, and microfabrication. Furthermore, the output speckle pattern from a multimode fiber encodes spatial, temporal, spectral, and polarization properties of the input light, allowing such information to be retrieved from spatial measurements only. This article provides an overview of recent advances and breakthroughs in controlling light propagation in multimode fibers, and discusses newly emerging applications.