This dissertation explores the integration and interfacing of a variety of photonic devices with optical waveguides, aiming to minimize the optical losses and physical footprint of such systems. The specific investigations include interfacing solid-core and hollow-core optical fibers, development of a monolithic single-photon source based on a quantum dot embedded in a semiconductor nanowire aligned with a single-mode fiber, a proposal for a waveguide-integrated power limiter that can protect detectors in a quantum key distribution (QKD) network, and integrating superconducting-nanowire single photon detectors (SNSPD) with laser-written waveguides. \n \nHollow-core photonic-crystal fibers (HCPCFs), which can allow simultaneous tight confinement of both photons and atoms in their hollow core, offer a platform for enhancing interactions between light and atomic ensembles. However, interfacing HCPCFs with conventional solid core (SC) fibers presents a unique set of challenges including significant losses in the joint region due to partial melting and deformation of the photonic crystal during the conventional arc-fusion splicing process. We address this issue with a lithographically defined, vacuum-compatible on-chip structure acting as a mechanical splicer that allows efficient injection of light from a SC fiber to a HCPCF and vice versa, with maximum observed light injection efficiency from a single-mode SC fiber into a HCPCF of 90%. \n \nSingle photon sources (SPS) are often required in photonic quantum information applications. Among various candidates, quantum dots (QD) embedded in tapered semiconductor nanowires have demonstrated excellent progress so far to realize a SPS. However, the overall collection efficiency of the emitted photon into a single mode fiber remains as one of the major challenges, mainly due to loss in the intermediate optical components. We design a complete scheme for high efficiency light coupling from such quantum dot to a single mode fiber. We optimize the geometry of the tapered nanowire to achieve a low numerical aperture Gaussian output mode profile from the wire tip and use a lithographically defined structure to align the SMF collecting the emitted photons. A graded-index multimode fiber acting as a lens is spliced on the SMF end for high efficiency coupling of light from the nanowire into the fiber. \n \nExtremely sensitive photonic devices are often used in quantum optics applications for detection and manipulation of signals with very low power. To prevent malfunction or even damage to these sensitive devices by excess incident power, we propose an optical power limiter based on a pair of cavities integrated with optical waveguides. Such power limiter can be used to protect sensitive electro-optical devices and prevent certain types of attacks on QKD systems. \n \nLastly, a 2D photonic lattice formed by laser-written waveguides in bulk glass can be used as an analogue quantum simulator. We explore the integration of SNSPDs with laser-written waveguides with the goal of making this platform fully on-chip.
This dissertation explores the integration and interfacing of a variety of photonic devices with optical waveguides, aiming to minimize the optical losses and physical footprint of such systems. The specific investigations include interfacing solid-core and hollow-core optical fibers, development of a monolithic single-photon source based on a quantum dot embedded in a semiconductor nanowire aligned with a single-mode fiber, a proposal for a waveguide-integrated power limiter that can protect detectors in a quantum key distribution (QKD) network, and integrating superconducting-nanowire single photon detectors (SNSPD) with laser-written waveguides. \n \nHollow-core photonic-crystal fibers (HCPCFs), which can allow simultaneous tight confinement of both photons and atoms in their hollow core, offer a platform for enhancing interactions between light and atomic ensembles. However, interfacing HCPCFs with conventional solid core (SC) fibers presents a unique set of challenges including significant losses in the joint region due to partial melting and deformation of the photonic crystal during the conventional arc-fusion splicing process. We address this issue with a lithographically defined, vacuum-compatible on-chip structure acting as a mechanical splicer that allows efficient injection of light from a SC fiber to a HCPCF and vice versa, with maximum observed light injection efficiency from a single-mode SC fiber into a HCPCF of 90%. \n \nSingle photon sources (SPS) are often required in photonic quantum information applications. Among various candidates, quantum dots (QD) embedded in tapered semiconductor nanowires have demonstrated excellent progress so far to realize a SPS. However, the overall collection efficiency of the emitted photon into a single mode fiber remains as one of the major challenges, mainly due to loss in the intermediate optical components. We design a complete scheme for high efficiency light coupling from such quantum dot to a single mode fiber. We optimize the geometry of the tapered nanowire to achieve a low numerical aperture Gaussian output mode profile from the wire tip and use a lithographically defined structure to align the SMF collecting the emitted photons. A graded-index multimode fiber acting as a lens is spliced on the SMF end for high efficiency coupling of light from the nanowire into the fiber. \n \nExtremely sensitive photonic devices are often used in quantum optics applications for detection and manipulation of signals with very low power. To prevent malfunction or even damage to these sensitive devices by excess incident power, we propose an optical power limiter based on a pair of cavities integrated with optical waveguides. Such power limiter can be used to protect sensitive electro-optical devices and prevent certain types of attacks on QKD systems. \n \nLastly, a 2D photonic lattice formed by laser-written waveguides in bulk glass can be used as an analogue quantum simulator. We explore the integration of SNSPDs with laser-written waveguides with the goal of making this platform fully on-chip.
Advancements in integrated photonics have enabled the miniaturization and scaling of complex photonic circuits, providing stability and reliability beyond the capabilities of conventional bulk setups. A central component of quantum photonic circuits is the single-photon detector, responsible for quantum state measurement. Due to the stringent performance requirements of quantum applications, superconducting nanowire single-photon detectors (SNSPDs) have emerged as the preferred detection technology, offering superior efficiency, low noise, and high timing resolution. While tabletop SNSPDs are now mature, commercially available and contributed to significant progress in quantum computing and communication, their integration into photonic circuits is still in its infancy. Integrated SNSPDs have shown the potential to not only match but surpass tabletop detectors in several performance metrics. Nevertheless, demonstrations have largely focused on individual integrated detectors, with limited investigation into more complex integrated circuits and advanced functionalities. In this thesis, we present a novel integration platform based on photonic bound states in the continuum (BICs) to address a critical limitation of conventional SNSPD designs, where nanowires placed directly atop the waveguide core are constrained by the core geometry. The etchless nature of our BIC waveguides provides a flat surface that decouples the design of the electronic readout circuitry below from the optical circuit layer above, enabling innovative integrated-nanowire geometries and facilitating large-scale integration of SNSPDs with CMOS-compatible circuits. We established a complete fabrication and characterization workflow for integrating NbTiN-based superconducting nanowires with BIC waveguides. Our integrated detectors achieved state-of-the-art performance, including detection efficiencies exceeding 70%, intrinsic jitter below 70 ps, and ultrafast recovery times (~1.7 ns). These results pave the way toward GHz-rate quantum state measurements, essential for scalable quantum photonic processors. Leveraging the design flexibility of BIC waveguides, we introduce the concept of inline detection with SNSPDs which exploits their unique ability to precisely engineer absorption through nanowire length control. By cascading multiple partially absorbing SNSPDs, it is possible to significantly reduce the number of required optical components and miniaturize complex quantum measurement setups within an extremely compact footprint. Traditional integration geometries prevent such implementations due to significant scattering losses at nanowire terminations, which limit the feasibility of cascaded designs. On the other hand, the design flexibility provided by our BIC platform makes it ideal for development of inline detection schemes. As a proof-of-concept, we use inline detection to miniaturize two fundamental quantum optics tools: an integrated Hanbury Brown and Twiss (HBT) interferometer and a photon-number-resolving (PNR) detector. We replicated the functionalities of these setups using only two nanowires integrated along a single waveguide. These devices show extremely compact footprint, highlighting the scalability potential of our approach. We validated these devices through characterization of a single-photon source and the discrimination of collinear photon pairs. The experimental results aligned well with previous literature and theoretical predictions, confirming the reliability of our integration method. Overall, this work presents a novel approach to quantum measurements within integrated quantum photonic circuits. Inline detection represents a step forward in quantum integrated photonics, moving from individual device demonstrations toward functional quantum circuits. By introducing our BIC-based integration platform, we simplify detector design and fabrication, lowering barriers to the development of inline detection circuits and opening new opportunities in multiphoton quantum state measurements, single-photon spectroscopy, and multiphoton subtraction.
Quantum mechanics endows light with properties that have no classical analogue - superposition, entanglement, and non-classical photon statistics, that can be harnessed as resources for computing, communication, sensing, and simulation. After decades of foundational research, quantum states of light can now be generated, manipulated, and measured with sufficient precision that their non-classical properties are no longer merely curiosities, but engineerable resources. Integrated photonics offers a compelling path forward. By confining light in lithographically defined waveguides on a chip, it replaces fragile bulk optical assemblies with compact, phase-stable, and reproducible circuits. Semiconductor quantum dots embedded in photonic nanostructures provide bright, on-demand single-photon emission compatible with waveguide integration, while superconducting nanowire single-photon detectors provide the efficiency and timing precision that quantum optical measurements demand. A particularly powerful frontier is reconfigurability: programmable photonic circuits whose functionality is defined in software can serve as versatile quantum photonic processors without any physical modification. This dissertation presents work on engineering quantum light across the full chain of generation, control, and detection using integrated photonic platforms. It covers coherent control of nanowire quantum dot emitters, generation of multipartite entangled states on a silicon nitride chip, and the use of programmable silicon-on-insulator interferometers as reconfigurable quantum optical processors for simulating quantum systems.
Fast and efficient detection of single photons with high timing accuracy is a crucial requirement in most quantum optics experiments and enables novel sensing and imaging solutions. Superconducting nanowire single-photon detectors (SNSPD) achieve technology-leading performance in terms of detection efficiency, dark count rate, timing jitter, and detector dead times. However, conventional SNSPDs with high system detection efficiency typically rely on resonant enhancement of the absorption efficiency, thus only achieving attractive detector benchmarks over narrow spectral windows. Waveguide-integrated SNSPDs allow for leveraging the wideband material absorption in superconducting nanowires by absorbing light in a traveling-wave geometry but have been limited to low system detection efficiencies due to interface losses when coupling to optical fibers. Here, we show how high system detection efficiencies of 22%–73% are realized over a broad wavelength range from 532 nm to 1640 nm in a single waveguide-integrated SNSPD device. We accomplish efficient coupling between optical fibers and waveguide-integrated nanowire detectors by employing a 3D interface, produced in direct laser writing, that relies on total internal reflection for achieving a broad transmission bandwidth. We further find low timing jitter of 25.7 ps and detector decay times of 9.8 ns, allowing for single-photon counting with high repetition rates up to 100 MHz. Our work paves the way for an efficient single-photon detector solution that combines the spectral requirements of an extremely wide range of quantum optics experiments in a single device. The coupling approach and SNSPD-integration with nanophotonic circuits are further well-suited for realizing large-scale detector arrays.
Waveguide-Integrated Superconducting Nanowire Single-Photon Detectors (WI-SNSPDs) represent a transformative advancement in single-photon detection technology, combining the exceptional sensitivity of traditional SNSPDs with the scalability and integration potential of Photonic Integrated Circuits (PICs). This integration eliminates the need for optical cavities, significantly broadens spectral bandwidth, and reduces optical losses, enabling compact, high-throughput systems. SNSPDs achieve peak system detection efficiencies exceeding 99%, timing jitter as low as 3ps, and ultra-low dark count rates below 0.1 mHz. In combination with photonic integration these features make WI-SNSPDs ideal for time-sensitive quantum applications such as Quantum Key Distribution (QKD) and photonic quantum computing. Key challenge for a high System Detection Efficiency (SDE) of WI-SNSPDs is the coupling of light to the photonic circuit. Recent advancements in Direct Laser Written (DLW) spot size converters show very high potential, achieving coupling efficiencies exceeding 83% (up to 95% in simulation) with exceptional reproducibility potentially enabling very high System Detection Efficiencies (SDE). WI-SNSPDs also support transformative applications in imaging and microscopy, enabling high-resolution, non-invasive deep-tissue imaging with minimal optical noise due to their potential for multi-mode and extreme broadband sensitivity. Pixel Photonics’ industrial roadmap focuses on the scalability of WI-SNSPDs through wafer-scale production and seamless integration with PICs, supporting the development of co-integrated systems that combine detectors with photonic quantum processors. This integration eliminates fiber-chip links, paving the way for compact and scalable quantum computing architectures. Additionally, WI-SNSPDs facilitate the creation of quantum links in large-scale quantum networks by enabling efficient photon entanglement distribution. These advancements position WI-SNSPDs as a cornerstone technology for next-generation quantum computing, secure communication networks, advanced imaging, and hybrid photonic systems, driving innovation across research and industry.
Superconducting Nanowire Single Photon Detectors (SNSPD) have set leading performance benchmarks in terms of detection efficiency, low noise operation, speed, and timing accuracy. Besides scaling up the numbers of individually addressable nanowire detector channels, integration with nanophotonic functionalities as well as optical and electrical multiplexing will be key to advance single photon counting applications, such as quantum key distribution, optical quantum information processing and simulations of sampling problems. Here we show how integration of SNSPDs with nanophotonic circuit components gives rise to novel use cases. We combine highperformance waveguide-integrated SNSPDs with nano-electromechanical phase-shifters, high speed electro-optic modulators, sub-wavelength grating structures, directional coupler networks, on-chip delay lines and highly efficient and broadband 3D-interconnects to optical fibers, achieving crucial active and passive functionalities for integrated quantum photonics. We showcase the capabilities and practical relevance of photonic integrated SNSPD solutions for the example of high-rate quantum key distribution receivers.