On-chip Quantum Photonics Research Articles

Deterministic Fabrication of a Coupled Cavity-Emitter System in Hexagonal Boron Nitride.

Light-matter interactions in optical cavities underpin many applications of integrated quantum photonics. Among various solid-state platforms, hexagonal boron nitride (hBN) is gaining considerable interest as a compelling van der Waals host of quantum emitters. However, progress to date has been limited by an inability to engineer simultaneously an hBN emitter and a narrow-band photonic resonator at a predetermined wavelength. Here, we overcome this problem and demonstrate deterministic fabrication of hBN nanobeam photonic crystal cavities with high quality factors over a broad spectral range (∼400 to 850 nm). We then fabricate a monolithic, coupled cavity-emitter system designed for a blue quantum emitter that has an emission wavelength of 436 nm and is induced deterministically by electron beam irradiation of the cavity hotspot. Our work constitutes a promising path to scalable on-chip quantum photonics and paves the way to quantum networks based on van der Waals materials.

Silicon/graphene–ITO multiple heterojunctions and 1D PhC waveguide-based photodetection for mid-NIR IPE with high responsivity

A nanophotonic photodetector is proposed with a multi-heterojunction structure based on silicon–ITO (Indium Tin Oxide) slot waveguide covered with 2D (two-dimensional) graphene sheets. A heavily n-doped ITO layer covers the whole device and works as an electrode to reduce the light loss at the metal–graphene interfaces. Different waveguides are arranged to design a one-dimensional photonic crystal (1D PhC) and the slot is defined by creating a defect in it. The PhC provides strong confinement of light, increasing the light–matter interaction, and thus generating more electron–hole (e–h) pairs or charge carriers under light illumination. Moreover, the multiple heterojunctions enhance the e–h generation and help enhancing the transportation of charge carriers. As a result, high photocurrent, fast response across a large bandwidth, strong electron-photon interaction, and also enhancement of photocarrier multiplications can be achieved. The photodetector realizes a photoresponsivity beyond 0.6 A/W at around 1550 nm wavelength. From 0.5 V–2 V of the bias voltage, the difference between the photocurrent and the dark current for the device with the PhC is more than 5 times that for the device without the PhC. Moreover, a 25 Gbit/s non-return to zero optical transmission is measured at 10 km transmission. The proposed device has diverse applications such as telecommunications, optical computing, interconnects, laser radar, lab-on-chip for sensing, and on-chip quantum photonics.

Quantum emitters in 2D materials: Emitter engineering, photophysics, and integration in photonic nanostructures

Quantum emitters have become a vital tool for both fundamental science and emerging technologies. In recent years, the focus in the field has shifted to exploration and identification of new quantum systems enabled by the emerging library of atomically thin, two dimensional materials. In this review, we highlight the current state of the art in engineering of quantum emitters in 2D systems, with an emphasis on transition metal di-chalcogenides (TMDCs) and hexagonal boron nitride. We start by reviewing progress in TMDCs, with focus on emitter engineering, ability to tune their spectral properties, and observation of interlayer excitons. We then discuss emitters in hBN and focus on emitters' origin, engineering, and emerging phenomena—spanning super-resolution imaging and optical spin readout. We summarize by discussing practical advances of integration of emitters in 2D hosts with plasmonic and dielectric photonic cavities, underpinned by quantum light–matter interactions. We conclude by outlining pathways for practical on-chip quantum photonics applications and highlight challenges and opportunities within this field of research.

Experimental Optical Properties of Single-Photon Emitters in Aluminum Nitride Films

Single-photon emitters (SPEs) are one of the building blocks in quantum information processing. Here, we report detailed experimental optical properties of the SPEs in aluminum nitride (AlN) films at 10 K. The high-quality AlN films are grown by metal–organic chemical vapor deposition on graphene/sapphire substrates, which can conquer the large lattice and thermal mismatches between the sapphire and AlN. We report the defects in AlN with a relatively high Debye–Waller factor up to ∼29% and near-perfect linear polarization SPEs with a saturation count rate of 1.43 × 10⁶ counts/s. The power-dependent second-order autocorrelation measurements are used to study the transition kinetics, which can be described using a three-level model. The polarization measurements of absorption and emission reveal the optical cycle mechanism, where a particular zero-phonon line may be excited via multiple mechanisms. This work provides some insight into the nature of the optical properties and energy-level structures of AlN defects, which pave the way to integrated on-chip quantum photonics.

Purcell-enhanced single-photon emission from a strain-tunable quantum dot in a cavity-waveguide device

On-chip quantum photonics is a promising route toward the implementation of complex photonic architectures on a small footprint. Therefore, different photonic components demonstrated for off-chip operation must be realized in an integrated manner. An essential building block for the realization of this goal is the integration of efficient on-demand single-photon sources within waveguide circuits. Here, we address this challenge by demonstrating the Purcell-enhanced single-photon emission from an In(Ga)As quantum dot coupled to a high-Q cavity-waveguide device. The combination with a piezoelectric actuator further enables the strain-induced emission energy tuning of the quantum dot as well as the cavity mode. We observe wavelength shifts up to 0.85 nm for the quantum dot, with a differential tuning factor of four between emitter and cavity. This allows for the full compensation of the spectral mismatch between a selected quantum dot and the cavity resonance. A nearly twofold enhancement of the spontaneous emission rate is observed at resonance with the on-demand generation of single photons. This demonstration of a strain-tunable emitter in a waveguide-coupled cavity device represents an essential building block for large scale quantum photonic circuits, especially if combined in the future with miniaturization approaches based on recently developed micromachined piezoelectric actuators.

Photorefractive effect in LiNbO3-based integrated-optical circuits for continuous variable experiments.

We investigate the impact of the photorefractive effect on lithium niobate integrated quantum photonic circuits dedicated to continuous variable on-chip experiments. The circuit main building blocks, i.e. cavities, directional couplers, and periodically poled nonlinear waveguides, are studied. This work demonstrates that photorefractivity, even when its effect is weaker than spatial mode hopping, might compromise the success of on-chip quantum photonics experiments. We describe in detail the characterization methods leading to the identification of this possible issue. We also study to which extent device heating represents a viable solution to counter this effect. We focus on photorefractive effect induced by light at 775 nm, in the context of the generation of non-classical light at 1550 nm telecom wavelength.

Hybrid integration methods for on-chip quantum photonics

The goal of integrated quantum photonics is to combine components for the generation, manipulation, and detection of nonclassical light in a phase-stable and efficient platform. Solid-state quantum emitters have recently reached outstanding performance as single-photon sources. In parallel, photonic integrated circuits have been advanced to the point that thousands of components can be controlled on a chip with high efficiency and phase stability. Consequently, researchers are now beginning to combine these leading quantum emitters and photonic integrated circuit platforms to realize the best properties of each technology. In this paper, we review recent advances in integrated quantum photonics based on such hybrid systems. Although hybrid integration solves many limitations of individual platforms, it also introduces new challenges that arise from interfacing different materials. We review various issues in solid-state quantum emitters and photonic integrated circuits, the hybrid integration techniques that bridge these two systems, and methods for chip-based manipulation of photons and emitters. Finally, we discuss the remaining challenges and future prospects of on-chip quantum photonics with integrated quantum emitters.

4H-silicon-carbide-on-insulator for integrated quantum and nonlinear photonics

Optical quantum information processing will require highly efficient photonic circuits to connect quantum nodes on-chip and across long distances. This entails the efficient integration of optically addressable qubits into photonic circuits, as well as quantum frequency conversion to the telecommunications band. 4H-silicon carbide (4H-SiC) offers unique potential for on-chip quantum photonics, as it hosts a variety of promising colour centres and has a strong second-order optical nonlinearity. Here, we demonstrate within a single, monolithic platform the strong enhancement of emission from a colour centre and efficient optical frequency conversion. We develop a fabrication process for thin films of 4H-SiC, which are compatible with industry-standard, CMOS nanofabrication. This work provides a viable route towards industry-compatible, scalable colour-centre-based quantum technologies, including the monolithic generation and frequency conversion of quantum light on-chip. Monolithic photonics devices based on SiC are fabricated by a wafer bonding and thinning technique. The strong enhancement of single-photon emission from a colour centre and optical frequency conversion with an efficiency of 360% W−1 are demonstrated.

Directional couplers with integrated carbon nanotube incandescent light emitters.

We combine on-chip single-walled carbon nanotubes (SWNTs) emitters with directional coupling devices as fundamental building blocks for carbon photonic systems. These devices are essential for studying the emission properties of SWNTs in the few photon regime for future applications in on-chip quantum photonics. The combination of SWNTs with on-chip beam splitters herein provides the basis for correlation measurements as necessary for nanoscale source characterization. The employed fabrication methods are fully scalable and thus allow for implementing a multitude of functional and active circuits in a single fabrication run. Our metallic SWNT emitters are broadband and cover both visible and near-infrared wavelengths, thus holding promise for emerging hybrid optoelectronic devices with fast reconfiguration times.