The realization of scalable, real world devices in quantum technologies requires myriad new approaches and hardware components. Integrated photonics play a crucial role in many applications in quantum information and quantum sensing, enabling the realization of key components such as new light sources, waveguides, resonators, and detectors. The field of integrated quantum photonics is booming, and an increasing number of research groups are contributing to the accelerating efforts to study fundamental and technological challenges at the intersection of quantum optics, nanophotonics, and hybrid integrated devices. This Special Topic Collection is dedicated to highlighting important progress in the field of integrated quantum photonics and presenting new cutting-edge results, both experimental and theoretical, toward the common goal of exploring quantum photonic phenomena.
Integrated sources of photons come in many different types but can generally be split into two types: sources of photon pairs based on nonlinear optical processes and on-demand quantum light sources based on defects in solids. This collection contains articles tackling both. On the nonlinear source side, McCutcheon presents a theoretical treatment of coupled cavities in integrated nanophotonics to generate correlated pairs of photons.1 Here, they consider micro-ring resonators in a complementary metal–oxide–semiconductor (CMOS), which can enhance photon pair generation. They tackle the particular issue of backscattering and roughness in these structures and how this can adversely affect the efficiency of photon generation and the spectral purity of the photons themselves. The paper by Thomas et al. further analyzes how imperfections in nanophotonic devices with a large number of optical elements affect the operation of the device as a whole.2 They do so by employing a multimode Gaussian quantum optics theoretical framework that includes linear and nonlinear optical elements, filtering, and detection.
Equally important are on-demand sources that offer remarkable opportunities for integrated quantum photonics. There are many different quantum emitters spanning single atoms3 and molecules4 to defects in diamond and two-dimensional materials, as well as semiconductor quantum dots.5,6 The perspective article by Rodt and Reitzenstein in this Special Topic Collection takes an in depth look at the requirements to build fully functional quantum circuits that contain quantum emitters, with special attention being paid to the use of integrated quantum dots.7 While huge leaps in functionality have been recently achieved, the authors also stress that there are many challenges to be overcome to create a photonic quantum computer using quantum dots. Other articles in this collection investigate the spatial, temporal, and spectral engineering of quantum dot emission, which is crucial if photons from separate quantum emitters are to be simultaneously employed in quantum photonic technology. Kim et al. investigate the use of multi-exciton cascades in quantum dots placed in optical cavities,8 which allows for adjustments of the temporal and spatial mode of the emission, while Schnauber et al. explore the use of electric fields to Stark tune the emission of pre-selected quantum dots to show that they can be made spectrally compatible with one another,9 a prerequisite for scaling technology up to many quantum dots in a reliable manner. Fuchs et al. investigate enhancing the emission from single tin vacancies in diamond.10 They model a structure based on a Fabry–Pérot cavity around a thin membrane of diamond, find that this can enhance the collection of emission, and show preliminary enhancements from a coated diamond membrane in experiments.
Finally, there has been a lot of recent interest in the use of defects in silicon as quantum emitters, mainly because these can leverage the huge amount of investment seen in silicon electronics and photonics. In this collection, Yan et al. present a perspective on how one would use such emitters to build a full quantum information processing platform.11 To do so, they look at building spin–photon interfaces, quantum gates, photon sources, and cluster state creation systems all on integrated photonic structures. They assess the capabilities of various quantum emitters and set out a roadmap to overcome the challenges facing this exciting new technology. Keeping with the theme of large-scale integrated waveguide devices, the contribution Bell and Walmsley looks at novel photonic waveguide and beam splitter layouts, which can create more compact photonic networks to perform unitary operations.12 These beam splitter networks can be used for boson sampling, quantum walks, quantum simulations, and quantum computing.
Clearly huge progress is ongoing in university laboratories around the world in integrated quantum photonics, and we are also seeing photonic quantum technology leaving laboratories and becoming more widespread in industry. Now is the time for a quantum revolution in which integrated quantum photonics will no doubt play a major front-line role.
