Optical Properties of Stressed Silicon Nitride Films and Their Impact on High Contrast Grating Performance

In cavity-based optomechanical platforms, the coupling between the optical modes of a cavity and the vibrational modes of a mechanical resonator is mediated by radiation pressure. High contrast gratings (HCGs) have attracted a lot of interest for such platforms because they offer a way to make high reflectivity (> 99.5%), low mass mirrors. In its simplest form, a high contrast grating is a high index dielectric slab that has been patterned with sub-wavelength scale features to create a periodic modulation of the refractive index in one or two dimensions. Optomechanical platforms also need these low mass reflectors to act as mechanical resonators with a high mechanical quality factor Q. Stressed silicon nitride on silicon has emerged as a leading candidate for such devices because these films possess mechanical quality factors in excess of 10^5. Due to the stress that these silicon nitride films are under, 2D HCGs end up being more tolerant of the microfabrication process than their 1D counterparts. Additionally, those based on a symmetric 2D photonic crystal lattice are expected to be insensitive to the polarization of light at normal incidence. This thesis looks at the performance of 2D silicon nitride HCGs, as well as examines some of the properties of stressed silicon nitride films that limit their performance. We present a new method to separate transmission losses from dissipative losses in an HCG and find that dissipative losses are not the dominant factor in limiting the reflectivity of 2D HCGs. Our results also show that a slight anisotropy in the refractive index of stressed silicon nitride films can make these HCGs birefringent, thereby breaking the degeneracy between polarization eigenmodes when the HCGs are used as the end mirror in an optical cavity

Silicon Photonic Microresonator-Based High-Resolution Line-by-Line Pulse Shaping

Optical pulse shaping stands as a formidable technique in ultrafast optics, radio-frequency photonics, and quantum communications. While existing systems rely on bulk optics or integrated platforms with planar waveguide sections for spatial dispersion, they face limitations in achieving finer (few- or sub-GHz) spectrum control. These methods either demand considerable space or suffer from pronounced phase errors and optical losses when assembled to achieve fine resolution. Addressing these challenges, we present a foundry-fabricated six-channel silicon photonic shaper using microresonator filter banks with inline phase control and high spectral resolution. Leveraging existing comb-based spectroscopic techniques, we devise a novel system to mitigate thermal crosstalk and enable the versatile use of our on-chip shaper. Our results demonstrate the shaper's ability to phase-compensate six comb lines at tunable channel spacings of 3, 4, and 5 GHz. Specifically, at a 3 GHz channel spacing, we showcase the generation of arbitrary waveforms in the time domain. This scalable design and control scheme holds promise in meeting future demands for high-precision spectral shaping capabilities.Comment: 21 pages, 12 figure

Fine-Resolution Silicon Photonic Wavelength-Selective Switch Using Hybrid Multimode Racetrack Resonators

In this work, we describe a procedure for synthesizing racetrack resonators with large quality factors and apply it to realize a multi-channel wavelength-selective switch (WSS) on a silicon photonic chip. We first determine the contribution of each component primitive to propagation loss in a racetrack resonator and use this data to develop a model for the frequency response of arbitrary order, coupled-racetrack channel dropping filters. We design second-order racetrack filters based on this model and cascade multiple such filters to form a 1x7 WSS. We find good agreement between our model and device performance with second-order racetrack that have ~1 dB of drop-port loss, ~2 GHz FWHM linewidth, and low optical crosstalk due to the quick filter roll-off of ~ 5.3 dB/GHz. Using a control algorithm, we show three-channel operation of our WSS with a channel spacing of only 10 GHz. Owing to the high quality factor and quick roll-off of our filter design, adjacent channel crosstalk is measured to be <-25 dB for channels spaced on a 10 GHz grid. As a further demonstration, we use five of seven WSS channels to perform a demultiplexing operation on both an 8 GHz and a 10 GHz grid. These results suggest that a low-loss WSS with fine channel resolution can be realized in a scalable manner using the silicon photonics platform

Large, Wafer-Thin Optical Apertures Leveraging Photonic Integrated Circuits to Replace Telescopes for Communications

To aid in driving down the size, weight, and power (SWaP) of space-based optical communications terminals, we present a large-aperture telescope-replacement technology that reshapes a beam from a single-mode fiber to ~5 cm and larger apertures on a silicon wafer by using photonic integrated circuit (PIC) components. We achieve multi-centimeter apertures by sacrificing wide-angle steering in favor of good beam quality and manageable controls. Light from a single-mode fiber is coupled to a silicon chip consisting of low-loss silicon nitride waveguides for signal distribution to large phase-controlled emitters. Our demonstrations of beam phasing across a 1.8-cm-diameter, 16-emitter phased array show excellent agreement with simulations. We have designed and simulated a 4.7 cm, 64-emitter array and have begun fabrication as of 2023. This architecture removes the need for beam expansion optics, free-space propagation for beam expansion, and the support structure and housing used in traditional telescope assemblies. Its low size and weight make it compatible with current and future beam steering mechanisms, and its reduced loading provides added potential for size and weight reductions in those subsystems. We believe the architecture can eventually be expanded to larger apertures of 10 cm or more without significantly increasing thickness

A Reconfigurable Quantum Local Area Network Over Deployed Fiber

Practical quantum networking architectures are crucial for scaling the connection of quantum resources. Yet quantum network testbeds have thus far underutilized the full capabilities of modern lightwave communications, such as flexible-grid bandwidth allocation. In this work, we implement flex-grid entanglement distribution in a deployed network for the first time, connecting nodes in three distinct campus buildings time-synchronized via the Global Positioning System (GPS). We quantify the quality of the distributed polarization entanglement via log-negativity, which offers a generic metric of link performance in entangled bits per second. After demonstrating successful entanglement distribution for two allocations of our eight dynamically reconfigurable channels, we demonstrate remote state preparation -- the first realization on deployed fiber -- showcasing one possible quantum protocol enabled by the distributed entanglement network. Our results realize an advanced paradigm for managing entanglement resources in quantum networks of ever-increasing complexity and service demands

2022 Roadmap on integrated quantum photonics

AbstractIntegrated photonics will play a key role in quantum systems as they grow from few-qubit prototypes to tens of thousands of qubits. The underlying optical quantum technologies can only be realized through the integration of these components onto quantum photonic integrated circuits (QPICs) with accompanying electronics. In the last decade, remarkable advances in quantum photonic integration have enabled table-top experiments to be scaled down to prototype chips with improvements in efficiency, robustness, and key performance metrics. These advances have enabled integrated quantum photonic technologies combining up to 650 optical and electrical components onto a single chip that are capable of programmable quantum information processing, chip-to-chip networking, hybrid quantum system integration, and high-speed communications. In this roadmap article, we highlight the status, current and future challenges, and emerging technologies in several key research areas in integrated quantum photonics, including photonic platforms, quantum and classical light sources, quantum frequency conversion, integrated detectors, and applications in computing, communications, and sensing. With advances in materials, photonic design architectures, fabrication and integration processes, packaging, and testing and benchmarking, in the next decade we can expect a transition from single- and few-function prototypes to large-scale integration of multi-functional and reconfigurable devices that will have a transformative impact on quantum information science and engineering

Spectral Multiplexing and Information Processing for Quantum Networks

Modern fiber-optic networks leverage massive parallelization of communications channels in the spectral domain, as well as low-noise recovery of optical signals, to achieve high rates of information transfer. However, quantum information imposes additional constraints on optical transport networks – the no-cloning theorem forbids use of signal regeneration and many network protocols are premised on operations like Bell state measurements that prize spectral indistinguishability. Consequently, a key challenge for quantum networks is identifying a path to high-rate and high-fidelity quantum state transport. To bridge this gap between the capabilities of classical and quantum networks, we developed techniques that harness spectral multiplexing of quantum channels, as well as that support frequency encoding. In relation to the former, we demonstrated reconfigurable connectivity over arbitrary subgraphs in a multi-user quantum network. In particular, through flexible provisioning of the pair source bandwidth, we adjusted the rate at which entanglement was distributed over any user-to-user link. To facilitate networking protocols compatible with both spectral multiplexing and frequency encoding, we synthesized a Bell state analyzer based on mixing outcomes that populate different spectral modes, in contrast to conventional approaches that are based on mixing outcomes that populate different spatial paths. This advance breaks the tradeoff between the fidelity of remote entanglement and the spectral distinguishability of photons participating in a joint measurement. Finally, we take steps toward field deployment by developing photonic integrated circuits to migrate the aforementioned functionality to a chip-scale platform while also achieving the low loss transmission and high-fidelity operation needed for practical quantum networks