Quantum information processing (QIP) has seen tremendous progress in the XXI century, primarily with the development of commercial quantum key distribution systems and prototypical quantum processors based on trapped ions and superconducting qubits [1]. Still, the quantum information community is facing the need of developing a wide range of quantum circuitries with simultaneous applications such as information processing, storage, and long-range communications [2]. Achieving multi-quantum functionalities, however, may require physically disparate platforms that are difficult to interface and scale. These challenges may be addressed by integrating QIP infrastructure through on-chip quantum photonics [3,4]. Using photons as messenger quantum bits and leveraging on the variety of existing photonic and optoelectronic devices is one of the most promising paradigms for a future QIP system. Further material integration with quantum emitters, such as quantum dots, color centers, cold atoms, and even mechanical modes provide promising routes toward quantum nonlinear light-matter interfaces. Developing these hybrid material platforms is of fundamental importance for the practical applications of quantum nanophotonics, and calls for new processes in material fabrication and integration.

In solid-state material platforms, there has been much recent work on novel hybrid designs that integrate low loss dielectrics, as host photonic environment for quantum emitters, with ultra-compact optical cavities to realize efficient light-matter interfaces. Plasmonic cavities or optical metamaterials, in particular, promise deep sub-wavelength light confinement and broadband operations that may find important applications in future room temperature solid-state quantum technologies. In two Optical Materials Express Spotlight articles, Kelaita et al. [5] from Stanford University and the University of Washington and Harats et al [6] from the Free University of Berlin and the Hebrew University of Jerusalem, leverage on such a hybrid approach and bring innovative concepts to fruition. The US team presents detailed studies on a metal-dielectric nano-antenna structure, featuring large Purcell enhancement and an efficient surface-emitting channel, while the German and Israeli team investigates the robust design and high-yield fabrication of nano-antennas with a single nanocrystal quantum dot. As another promising hybrid platform, in an Editors’ Pick article [7], Andersen and colleagues at the University of Southern Denmark studied enhanced photon emission of a nanodiamond nitrogen-vacancy center via coupling to a plasmonic silver nanocube.

Integrating photonic platforms with plasmonics or metamaterials can potentially offer more unprecedented properties for manipulating light-matter interactions than do conventional materials. In particular, in the so-called epsilon-and-mu-near-zero (EMNZ) materials, light propagates with nearly zero phase advancement within the material. As such, radiative interaction between two distant quantum emitters can be treated as if they are located in close proximity, thus breaking geometric limitations of the underlying photonic platform. This ability may greatly facilitate QIP in a complex quantum network. Mahmoud and colleagues [8] at the University of Pennsylvania and the American University in Cairo discuss the possibility of mediating dipole-dipole supercoupling between quantum emitters within EMNZ materials, in both a proof-of-concept microwave experiment and an all-dielectric platform operating in the optical domain.

In dielectric materials, coupling quantum emitters to ultrahigh-Q cavities has remained one of the most robust approaches in solid-state QIP. Finding novel materials with large refractive index and implementing them in new nanocavity designs can lead to breakthroughs in material platform research by improving optical confinements and quality factors. In this feature issue, Chung et al [9], an Australian-US team, make use of titanium dioxide with a high refractive index (n~2) to form a compact, high-Q nanobeam cavity structure that can operate at the frequency of nitrogen-vacancy centers in diamond. This new hybrid material promises exciting new possibilities for room temperature, dielectric quantum devices.

Interfacing cold atoms with nanophotonics has emerged in recent years as a promising hybrid platform, providing on-chip integration for cold-atom quantum technologies. Stable atom trapping, excellent light confinement, and flexible modal engineering with nanophotonics are among the key features that lead to recent successes in achieving strong quantum non-linearity with atoms and guided photons. Along these lines, Optical Materials Express highlighted the article by Bappi et al. at the University of Waterloo [10]. The authors discuss exciting prospects of using integrated hollow-core anti-resonant reflection optical waveguides (ARROWs), loaded with atomic ensembles, as an on-chip quantum platform to achieve nonlinear quantum operations controlled by single photons. In an Editors’ Pick article [11], Stievater and colleagues from U.S. Naval and Army Research Laboratories demonstrated a very promising scheme of achieving stable atom trapping in the evanescent field of a nanophotonic rib waveguide that has important applications in creating efficient atom-light interfaces.

One surprising feature of light propagation in nanophotonic structures is the onset of chirality. Unlike in freespace, tightly confined light necessarily develops an out-of-phase axial component that makes the rotation of polarization, and thus the chirality of light, dependent on the direction of photon propagation. By coupling to polarization-sensitive quantum emitters and thus breaking the so-called Lorentz reciprocity, a novel chiral photonic platform may lead to tremendous applications such as quantum photonic isolators or circulators. This issue features a Spotlight article [12] by Moahmoodian and colleagues from the University of Copenhagen, who are also the co-founders of a successful quantum photonic start-up Sparrow Quantum. The team investigated a slow-light mode in a novel photonic crystal design that can induce near-unity directional coupling with a quantum emitter. This chiral crystal may find important applications in future studies of chiral quantum nonlinear optics.

On-chip quantum applications and QIP depend critically on robust single-photon sensing. In this issue, Vorobyov et al [13], a Russian-US team, experimentally demonstrate a new superconducting single-photon detector that is optimized for a wide wavelength range overlapping with solid-state and atom-like quantum emitters such as nitrogen- and silicon-vacancy centers in diamond. The new detector thus provides important technical advancement with possible chip-scale integration with a number of quantum photonic systems.

Last but not least, an Editors¹ Pick article [14] by Bogdanov et al at the Purdue University provides an overview on several material platforms that can host future quantum photonic computer and network nodes, and offers comprehensive discussions on the degree of integration in individual and composite platform approaches.

We hope that this feature issue can provide a taste of diverse optical material research in the emergent field of quantum nanophotonics and can lead to new ideas towards material integration for future quantum applications. We are thankful to all of the authors and reviewers for their contributions. We also thank the OSA staff for their outstanding work throughout the review and production processes.