INTRODUCTION Physical systems that exhibit topological invariants are naturally endowed with robustness against perturbations, as was recently demonstrated in many settings in condensed matter, photonics, cold atoms, acoustics, and more. The most prominent manifestations of topological systems are topological insulators, which exhibit scatter-free edge-state transport, immune to perturbations and disorder. Recent years have witnessed intense efforts toward exploiting these physical phenomena in the optical domain, with new ideas ranging from topology-driven unidirectional devices to topological protection of path entanglement. But perhaps more technologically relevant than all topological photonic settings studied thus far is, as proposed by the accompanying theoretical paper by Harari et al ., an all-dielectric magnet-free topological insulator laser, with desirable properties stemming from the topological transport of light in the laser cavity. RATIONALE We demonstrate nonmagnetic topological insulator lasers. The topological properties of the laser system give rise to single-mode lasing, robustness against fabrication defects, and notably higher slope efficiencies compared to those of the topologically trivial counterparts. We further exploit the properties of the active topological platform by assembling topological insulator lasers from S -chiral microresonators that enforce predetermined unidirectional lasing even in the absence of magnetic fields. RESULTS Our topological insulator laser system is an aperiodic array of 10 unit cell–by–10 unit cell coupled ring resonators on an InGaAsP quantum wells platform. The active lattice uses the topological architecture suggested in the accompanying theoretical paper. This two-dimensional setting is composed of a square lattice of ring resonators coupled to each other by means of link rings. The intermediary links are judiciously spatially shifted to introduce a set of hopping phases, establishing a synthetic magnetic field and two topological band gaps. The gain in this laser system is provided by optical pumping. To promote lasing of the topologically protected edge modes, we pump the outer perimeter of the array while leaving the interior lossy. We find that this topological insulator laser operates in single mode even considerably above threshold, whereas the corresponding topologically trivial realizations lase in multiple modes. Moreover, the topological laser displays a slope efficiency that is considerably higher than that in the corresponding trivial realizations. We further demonstrate the topological features of this laser by observing that in the topological array, all sites emit coherently at the same wavelength, whereas in the trivial array, lasing occurs in localized regions, each at a different frequency. Also, by pumping only part of the topological array, we demonstrate that the topological edge mode always travels along the perimeter and emits light through the output coupler. By contrast, when we pump the trivial array far from the output coupler, no light is extracted from the coupler because the lasing occurs at stationary modes. We also observe that, even in the presence of defects, the topological protection always leads to more efficient lasing compared to that of the trivial counterpart. Finally, to show the potential of this active system, we assemble a topological system based on S -chiral resonators, which can provide new avenues to control the topological features. CONCLUSION We have experimentally demonstrated an all-dielectric topological insulator laser and found that the topological features enhance the lasing performance of a two-dimensional array of microresonators, making them lase in unison in an extended topologically protected scatter-free edge mode. The observed single longitudinal-mode operation leads to a considerably higher slope efficiency as compared to that of a corresponding topologically trivial system. Our results pave the way toward a new class of active topological photonic devices, such as laser arrays, that can operate in a coherent fashion with high efficiencies.

Topological insulator laser: Experiments

Silicon photonics research can be dated back to the 1980s. However, the previous decade has witnessed an explosive growth in the field. Silicon photonics is a disruptive technology that is poised to revolutionize a number of application areas, for example, data centers, high-performance computing and sensing. The key driving force behind silicon photonics is the ability to use CMOS-like fabrication resulting in high-volume production at low cost. This is a key enabling factor for bringing photonics to a range of technology areas where the costs of implementation using traditional photonic elements such as those used for the telecommunications industry would be prohibitive. Silicon does however have a number of shortcomings as a photonic material. In its basic form it is not an ideal material in which to produce light sources, optical modulators or photodetectors for example. A wealth of research effort from both academia and industry in recent years has fueled the demonstration of multiple solutions to these and other problems, and as time progresses new approaches are increasingly being conceived. It is clear that silicon photonics has a bright future. However, with a growing number of approaches available, what will the silicon photonic integrated circuit of the future look like? This roadmap on silicon photonics delves into the different technology and application areas of the field giving an insight into the state-of-the-art as well as current and future challenges faced by researchers worldwide. Contributions authored by experts from both industry and academia provide an overview and outlook for the silicon waveguide platform, optical sources, optical modulators, photodetectors, integration approaches, packaging, applications of silicon photonics and approaches required to satisfy applications at mid-infrared wavelengths. Advances in science and technology required to meet challenges faced by the field in each of these areas are also addressed together with predictions of where the field is destined to reach.

https://iopscience.iop.org/article/10.1088/2040-8978/18/7/073003/pdf

Roadmap on silicon photonics

Quantum cryptography is arguably the fastest growing area in quantum
information science. Novel theoretical protocols are designed on a regular
basis, security proofs are constantly improving, and experiments are
gradually moving from proof-of-principle lab demonstrations to in-field
implementations and technological prototypes. In this paper, we provide
both a general introduction and a state-of-the-art description of the
recent advances in the field, both theoretical and experimental. We start
by reviewing protocols of quantum key distribution based on discrete
variable systems. Next we consider aspects of device independence,
satellite challenges, and protocols based on continuous-variable systems.
We will then discuss the ultimate limits of point-to-point private
communications and how quantum repeaters and networks may overcome these
restrictions. Finally, we will discuss some aspects of quantum
cryptography beyond standard quantum key distribution, including quantum
random number generators and quantum digital signatures.

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Advances in quantum cryptography

Some years ago quantum hacking became popular: devices implementing the unbreakable quantum cryptography were shown to have imperfections which could be exploited by attackers. Security has been thoroughly enhanced, as a consequence of both theoretical and experimental advances. This review gives both sides of the story, with the current best theory of quantum security, and an extensive survey of what makes quantum cryptosystem safe in practice.

Secure quantum key distribution with realistic devices

Quantum technologies comprise an emerging class of devices capable of controlling superposition and entanglement of quantum states of light or matter, to realize fundamental performance advantages over ordinary classical machines. The technology of integrated quantum photonics has enabled the generation, processing and detection of quantum states of light at a steadily increasing scale and level of complexity, progressing from few-component circuitry occupying centimetre-scale footprints and operating on two photons, to programmable devices approaching 1,000 components occupying millimetre-scale footprints with integrated generation of multiphoton states. This Review summarizes the advances in integrated photonic quantum technologies and its demonstrated applications, including quantum communications, simulations of quantum chemical and physical systems, sampling algorithms, and linear-optic quantum information processing. This Review covers recent progress in integrated quantum photonics (IQP) technologies and their applications. The challenges and opportunities of realizing large-scale, monolithic IQP circuits for future quantum applications are discussed.

Integrated photonic quantum technologies

We demonstrate various silicon-on-insulator polarization management structures based on a polarization rotator-splitter that uses a bi-level taper TM0-TE1 mode converter. The designs are fully compatible with standard active silicon photonics platforms with no new levels required and were implemented in the IME baseline and IME-OpSIS silicon photonics processes. We demonstrate a polarization rotator-splitter with polarization crosstalk < -13 dB over a bandwidth of 50 nm. Then, we improve the crosstalk to < -22 dB over a bandwidth of 80 nm by integrating the polarization rotator-splitter with directional coupler polarization filters. Finally, we demonstrate a polarization controller by integrating the polarization rotator-splitters with directional couplers, thermal tuners, and PIN diode phase shifters.

Polarization rotator-splitters in standard active silicon photonics platforms.

We review and present additional results from our work on multilayer silicon nitride (SiN) on silicon-on-insulator (SOI) integrated photonic platforms over the telecommunication wavelength bands near 1550 and 1310 nm. SiN-on-SOI platforms open the possibility for passive optical functionalities implemented in the SiN layer to be combined with active functionalities in the SOI. SiN layers can be integrated onto SOI using a front-end or back-end of line integration process flow. These photonic platforms support low-loss SiN waveguides, low-loss and low-crosstalk waveguide crossings, and low-loss interlayer transitions using adiabatic tapers. Novel ultra-broadband and efficient grating couplers as well as polarization management devices are enabled by the close coupling between the silicon and SiN layers.

Multilayer Silicon Nitride-on-Silicon Integrated Photonic Platforms and Devices

A dynamic model for the transmission of a microring modulator based on changes in the refractive index, loss, or waveguide-ring coupling strength is derived to investigate the limitations to the intensity modulation bandwidth. Modulation bandwidths approaching the free spectral range frequency are possible if the waveguide-ring coupling strength is varied, rather than the refractive index or loss of the ring. The results illustrate that via controlled coupling, resonant modulators with high quality factors can be designed to operate at frequencies much larger than the resonator linewidth.

Dynamics of microring resonator modulators.

Silicon (Si) photonics is forming a fabless ecosystem, which is enabling low-cost and densely integrated components for optical communications and quantum information We present a Si optical transmitter for polarization-encoded quantum key distribution (QKD) The chip was fabricated in a standard Si photonic foundry process and integrated together a pulse generator, intensity modulator, variable optical attenuator, and polarization modulator in a 13  mm×3  mm13  mm×3  mm die area The devices in the photonic circuit meet the requirements for QKD The transmitter was used in a proof-of-concept demonstration of the BB84 QKD protocol over a 5 km long fiber link This work shows the potential of using foundry Si photonics for low-cost, wafer-scale fabricated components for quantum information

Silicon photonic transmitter for polarization-encoded quantum key distribution

We propose and experimentally demonstrate fiber-to-chip grating couplers with aligned silicon nitride (Si(3)N(4)) and silicon (Si) grating teeth for wide bandwidths and high coupling efficiencies without the use of bottom reflectors. The measured 1-dB bandwidth is a record 80 nm, and the measured peak coupling efficiency is -1.3 dB, which is competitive with the best Si-only grating couplers. The grating couplers are integrated in a Si(3)N(4) on silicon-on-insulator (SOI) integrated optics platform with aligned waveguides in both the Si(3)N(4) and Si, and we demonstrate a 1 × 4 tunable multiplexer/demultiplexer using the Si(3)N(4)-on-SOI dual-level grating couplers and thermally-tuned Si microring resonators.

Wesley D. Sacher

Papers

Polarization rotator-splitters in standard active silicon photonics platforms.

Multilayer Silicon Nitride-on-Silicon Integrated Photonic Platforms and Devices

Dynamics of microring resonator modulators.

Silicon photonic transmitter for polarization-encoded quantum key distribution

Wide bandwidth and high coupling efficiency Si3N4-on-SOI dual-level grating coupler.