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Scientists theoretically show infrared to X-ray sources that can be implemented on-chip by scattering high-energy electrons with graphene plasmons and predict that they are capable of producing tunable radiation.

Graphene plasmons have gained significant interest thanks to their high field confinement and low phase velocity. Here the authors show theoretically that charge carriers propagating in graphene can excite plasmons through a quantum Čerenkov emission process in two dimensions, in the form of plasmonic shock waves.

Self-accelerating beams are attractive for light-matter interaction applications but their propagation has been limited by absorption. Here, Schley et al.demonstrate self-healing in shape-maintaining, accelerating beams where the central peak intensity is preserved despite losses and apply these beams to particle manipulation.

Intense squeezed light with focusable intensities of 0.1 TW cm⁻² is created by propagating a classical, intense and noisy input beam through an optical fibre. The noise 4 dB below the shot-noise level is achieved by selecting a set of wavelengths whose intensity fluctuations are maximally anticorrelated.

Free-electron X-ray radiation is typically treated classically. Here, the authors identify unprecedented quantum features of X-ray emission from crystals, due to transversely recoiled electron-photon entanglement, enabling precisely tuned emission via quantum control of free electrons.

Free-electron Ramsey imaging enables space-, time- and phase-resolved electron imaging of weak optical near fields. Owing to its phase-resolving ability, this technique images chiral vortex–anti-vortex phase singularities of phonon-polariton modes in hexagonal boron nitride.

High-harmonic generation is a source of high-frequency radiation and is typically driven by strong, but classical, laser fields. A theoretical study now shows that using quantum light states as the driver extends the spectrum of outgoing radiation in a controllable manner.

The optical field inside a nanophotonic particle accelerator is revealed. To this end, the authors developed a field imaging technique for spatial and spectral resolution on the nanometer scale.

On-demand electron wavefront shaping is desirable for applications from nanolithography to imaging. Here, the authors present tunable photon-induced spatial modulation of electrons through their interaction with externally controlled surface plasmon polaritons.

Optical near-field microscopy has facilitated our understanding of nanophotonics. This Perspective explores the opportunities that near-field studies of terahertz fields provide for ultrafast phase transitions in condensed matter systems.

High-harmonic generation has so far been driven only by classical light. Now, its driving by a bright squeezed vacuum—a quantum state of light—has been observed and shown to be more efficient than using classical light.

Energy–momentum phase-matching enables strong interactions between free electrons and light waves. As a result, the wavefunction of the electron exhibits a comb structure, which was observed using photon-induced near-field electron microscopy.

Conventional models of high harmonic generation typically do not provide a full quantum description of all phenomena. Here, the authors develop a fully quantum theory for high harmonic generation and use it to study the emission from a quantum system in a strong field.

Cherenkov detectors are used to detect high energy particles and their performance capabilities depend heavily on the material used. Here, the authors propose use of a Brewster-optics-based angular filter for a detector with increased sensitivity and particle identification capability.

An approach called the Ramanujan Machine demonstrates the use of algorithms to find mathematical conjectures in the form of formulas of fundamental constants, some of which remain unproved.

Exceptional points are singularities in non-Hermitian systems that can produce unusual effects, and it is shown that a Dirac cone in a photonic crystal can generate a continuous ring of exceptional points through flattening the tip of the cone.

Here the authors show that radiation emitted by individual electrons can be controlled by shaping the electron wavepacket. They present feasible examples for applications including collimated and monochromatic X-ray emission from specially shaped electrons.

Tunable X-ray generation, from ultrathin van der Waals materials impacted by relativistic electrons, is demonstrated.

The strong interaction of coherent free electrons with a photonic-crystal cavity enables the measurement of the lifetimes of the cavity modes and provides a technique for multidimensional near-field imaging and spectroscopy.

When interfacing a graphene layer with a thin solid emitter, the quantum plasmonic vacuum allows each solid electron to access all unoccupied valence states through the nonlocality of their light-matter interaction, creating ultra-strong coupling alongside mass and bandgap renormalization.

The angle of Cherenkov radiation in one-dimensional photonic crystals can be controlled by making use of constructive interference. This feature allows new design of particle detectors with improved performance.

While modifications of emission and absorption rates are commonplace in photonics, similar manipulations of emitter transition frequencies are challenging. Here, 2D polaritons in graphene are predicted to enable non-vertical electronic transitions in a quantum well, controlling the transition frequencies by inducing an effective non-locality.

Vacuum fluctuations in the vicinity of nanophotonic structures can lead to the conversion of a free electron into a polariton and a high-energy photon, whose frequency can be controlled by the electromagnetic properties of the nanostructure.

Strongly driven light sources have become useful in many ways but are limited to classical emission. A quantum-optical theory now shows how non-classical states of light can be achieved from strongly-driven many-body systems, for example, non-coherent and correlated high-harmonic generation.

By engineering the electron wavefunction it is possible to create Aharonov–Bohm-like phases and relativistic effects such as length contraction and time dilation in a non-relativistic setting and in the absence of electromagnetic fields.

Photoemission experiments demonstrate that the photon number statistics of the exciting light can be imprinted on the emitted electrons, allowing the controlled generation of classical or non-classical electron number statistics of free electrons.

Wave-matter interaction suffers from fundamental limit to scattering cross section, referred to as the single-channel scattering limit. Here, the authors break this limit by exploiting gain metasurfaces and reveal the transient formation and relaxation of this phenomenon.

Free-electron quantum optics is an emerging field that requires a quantum-mechanical description of both the electronic and the optical contributions. This Perspective summarizes recent developments and discusses challenges and opportunities.

Strong-field approximation theory is extended to account for non-classical driving light. This extended theory predicts that ultrafast dynamics of strongly light-driven matter significantly depends on the quantum state of the driving light, particularly on its photon statistics.

A study demonstrates full energy–momentum matching, and enhanced interaction, between free electrons and photons through a continuum of flatband resonances, realized in a silicon-on-insulator photonic crystal slab.

Features much smaller than the wavelength are not expected to have a significant impact on the transport of a wave. Here, the authors show that Anderson localization can dominate light transport in a one-dimensional disordered system, even when the disordered features are a thousand times smaller than the wavelength.

While great progress has been made in object recognition, implementing them is typically based on conventional electronic hardware. Here the authors introduce a concept of neuro-metamaterials that enable a dynamic entirely-optical object recognition and mirage.

Synthetic biology and engineering approaches are harnessed to incorporate new capabilities in synthetic cells. Here, the authors designed bioluminescent signaling mechanisms for intracellular and intercellular synthetic-to-natural cell communication.

Extracting light from silicon is a longstanding challenge. Here, the authors report an experimental demonstration of free-electron-driven light emission from silicon nanogratings and investigates the feasibility of a compact, all-silicon tunable light source integrated with a silicon field emitter array.

The development of a quantitative and predictive theory of quantum light-matter interactions in ultrathin materials is both a conceptual and computational challenge. Here, the authors develop such a framework by combining density functional theory with macroscopic quantum electrodynamics, and use it to quantify the Purcell effect in van der Waals heterostructures.

Vortex electron beams are generated using single electrons but their low beam-density is a limitation in electron microscopy. Here the authors propose a scheme for the realization of non-diffracting electron beams by shaping wavepackets of multiple electrons and including electron–electron interactions.

The bidirectional interactions between metamaterials and artificial intelligence have recently attracted much attention. Here, the authors stand from a unified perspective to discuss intelligent metamaterials (AI for metamaterials) and metamaterials intelligence (metamaterials for AI).

Calculating the amount of radiation that can ultimately be extracted from free electrons near an arbitrary material structure is a challenge. Now, an upper limit to the spontaneous photon emission of electrons is demonstrated, regardless of geometry.

The authors theoretically investigate a novel form of a Doppler effect in homogeneous systems with positive refractive index that occurs under certain conditions. It is suggested that this Doppler effect can be experimentally separated from other Doppler effects by using polaritons such as those found in graphene.

We propose a multi-particle ‘which-path’ gedanken experiment with a quantum detector. Contrary to conventional ‘which-path’ experiments, the detector maintains its quantum state during interactions with the particles. We show how such interactions can create an interference pattern that vanishes on average, as in conventional ‘which-path’ schemes, but contains hidden many-body quantum correlations. Measuring the state of the quantum detector projects the joint-particle wavefunction into highly entangled states, such as GHZ’s. Conversely, measuring the particles projects the detector wavefunction into desired states, such as Schrodinger-cat or GKP states for a harmonic-oscillator detector, e.g., a photonic cavity. Our work thus opens a new path to the creation and exploration of many-body quantum correlations in systems not often associated with these phenomena, such as atoms in waveguide QED and free electrons in transmission electron microscopy.

The coupling of photons to material quasiparticles such as plasmons, phonons and excitons opens new possibilities in light–matter interactions. This Review presents a generalized view of such quasiparticles and the technique that describes their interactions with matter: macroscopic quantum electrodynamics.

X-ray self-focusing sources are demonstrated by free electrons coherently interacting with van der Waals heterostructures, bypassing the need for X-ray optics that are often inefficient and challenging to fabricate.