Superconducting nanowire single-photon detectors (SNSPDs) are renowned for their high detection efficiency, low dark count rate, and fast timing resolution, which have emerged as an essential device in quantum technologies, including quantum computing and communication, which also significantly benefits classical applications such as deep-space communication, neuromorphic computing, and light detection and ranging (LiDAR). However, designing and customizing SNSPD into versatile functionalities to satisfy these intensive requirements poses significant challenges, including thin-film deposition, nanofabrication, electrical integration, and light delivery strategies. While prior work has typically optimized individual steps – such as fabrication or characterization – there is a lack of reported systematic, integrated frameworks that span all stages from device design to performance benchmarking. This gap hinders broader adoption of the technology across scientific and industrial communities and limits the rigorous exploration of advanced detection concepts. This thesis aims to establish an end-to-end workflow for the customizable fabrication and comprehensive characterization of superconducting nanowire single-photon detectors, and to demonstrate that such a platform is essential for the investigation of diverse single-photon detection architectures. It begins with the optimization of NbTiN thin-film deposition utilizing an ultrahigh vacuum sputtering system. By systematically identifying and optimizing key deposition parameters, we achieve NbTiN films with targeted superconducting properties along with high uniformity and reproducibility. This fast and reliable workflow establishes a solid foundation for detector fabrication and provides instructions for flexible control over film properties. Furthermore, effective nanofabrication protocols for the high-quality SNSPDs were elaborated, along with the specialized cryogenic packaging techniques for reliable electrical and optical integration. The achieved devices demonstrated performance metrics comparable to commercial systems. The implementation of this full fabrication—characterization cycle also enabled flexible exploration of advanced detector architectures. For example, we investigated superconducting microbridge single-photon detectors (SMSPDs) as a complementary platform, demonstrating a fivefold enhancement in signal-to-noise ratio and a tenfold reduction in recovery time relative to conventional SNSPDs. These results provide quantitative insight into detector design trade-offs. Within this framework, photon-number-resolving (PNR) detection using SNSPDs was further investigated as an essential function for advanced quantum and classical applications. We presented innovative detector architectures—interleaved multi-pixel arrays and multilayer coherent absorption structures—to overcome limitations inherent in conventional multiplexing methods, such as nonuniform detection, temporal degradation, and crosstalk. An interleaved multi-pixel SNSPD array with up to four-photon resolution was achieved with uniform detection, as well as a multilayer architecture based on coherent absorption with uniform and near-unity absorption efficiency. Importantly, a theoretical comparison between these two architectures was conducted to evaluate their absorption efficiency and PNR performance, offering insights into structural optimization and device scalability for advanced PNR detectors. Overall, this thesis establishes a fully in-house SNSPD fabrication and characterization capability, providing a robust foundation for the development, customization, and evaluation of high-performance superconducting single-photon detectors. This platform enables reproducible design optimization, quantitative performance benchmarking, and systematic exploration of detector architectures. By integrating fabrication, characterization, and architectural innovation within a consistent workflow, it facilitates both fundamental investigation and application-driven advancement of single-photon detection technologies in our laboratory and the broader research community in Singapore.
Superconducting nanowire single-photon detectors (SNSPDs) are renowned for their high detection efficiency, low dark count rate, and fast timing resolution, which have emerged as an essential device in quantum technologies, including quantum computing and communication, which also significantly benefits classical applications such as deep-space communication, neuromorphic computing, and light detection and ranging (LiDAR). However, designing and customizing SNSPD into versatile functionalities to satisfy these intensive requirements poses significant challenges, including thin-film deposition, nanofabrication, electrical integration, and light delivery strategies. While prior work has typically optimized individual steps – such as fabrication or characterization – there is a lack of reported systematic, integrated frameworks that span all stages from device design to performance benchmarking. This gap hinders broader adoption of the technology across scientific and industrial communities and limits the rigorous exploration of advanced detection concepts. This thesis aims to establish an end-to-end workflow for the customizable fabrication and comprehensive characterization of superconducting nanowire single-photon detectors, and to demonstrate that such a platform is essential for the investigation of diverse single-photon detection architectures. It begins with the optimization of NbTiN thin-film deposition utilizing an ultrahigh vacuum sputtering system. By systematically identifying and optimizing key deposition parameters, we achieve NbTiN films with targeted superconducting properties along with high uniformity and reproducibility. This fast and reliable workflow establishes a solid foundation for detector fabrication and provides instructions for flexible control over film properties. Furthermore, effective nanofabrication protocols for the high-quality SNSPDs were elaborated, along with the specialized cryogenic packaging techniques for reliable electrical and optical integration. The achieved devices demonstrated performance metrics comparable to commercial systems. The implementation of this full fabrication—characterization cycle also enabled flexible exploration of advanced detector architectures. For example, we investigated superconducting microbridge single-photon detectors (SMSPDs) as a complementary platform, demonstrating a fivefold enhancement in signal-to-noise ratio and a tenfold reduction in recovery time relative to conventional SNSPDs. These results provide quantitative insight into detector design trade-offs. Within this framework, photon-number-resolving (PNR) detection using SNSPDs was further investigated as an essential function for advanced quantum and classical applications. We presented innovative detector architectures—interleaved multi-pixel arrays and multilayer coherent absorption structures—to overcome limitations inherent in conventional multiplexing methods, such as nonuniform detection, temporal degradation, and crosstalk. An interleaved multi-pixel SNSPD array with up to four-photon resolution was achieved with uniform detection, as well as a multilayer architecture based on coherent absorption with uniform and near-unity absorption efficiency. Importantly, a theoretical comparison between these two architectures was conducted to evaluate their absorption efficiency and PNR performance, offering insights into structural optimization and device scalability for advanced PNR detectors. Overall, this thesis establishes a fully in-house SNSPD fabrication and characterization capability, providing a robust foundation for the development, customization, and evaluation of high-performance superconducting single-photon detectors. This platform enables reproducible design optimization, quantitative performance benchmarking, and systematic exploration of detector architectures. By integrating fabrication, characterization, and architectural innovation within a consistent workflow, it facilitates both fundamental investigation and application-driven advancement of single-photon detection technologies in our laboratory and the broader research community in Singapore.
Superconducting Nanowire Single-Photon Detectors (SNSPDs) are the premier single-photon counting technology in the near-infrared regime. Operating via a photon-induced phase transition from the superconducting to the normal state, SNSPDs act as highly sensitive digital detectors that now play a critical role in quantum communication, deep-space optical communications, and direct dark matter detection. As the demand for these detectors scales, there is a pressing need to extend their performance capabilities toward longer wavelengths, larger active areas, and higher operating temperatures. However, advancing these metrics requires navigating complex physical trade-offs inherent to the detection mechanism. This thesis explores the material optimizations, novel detector architectures, and fundamental device physics required to push SNSPD technology beyond its current limits. The first thrust of this work focuses on extending SNSPD sensitivity into the mid-infrared (MIR) and far-infrared regimes. By developing a low-Tc (sub-1 K) WSi film engineered to reduce the characteristic detection energy, the threshold for long-wavelength detection is significantly lowered. Coupled with a novel detector architecture utilizing impedance-matching tapers and superconducting nanowire avalanche photon detectors (SNAPs), sensitivity is initially demonstrated at wavelengths up to 29um using a blackbody thermal source. Expanding beyond this limit, the implementation of quantum cascade lasers enables the demonstration of single-photon counting at wavelengths up to 88um, marking a major improvement over the current state of the art. To address the practical MIR detection challenges of low signal-to-noise ratio and restricted fill factors, this work also demonstrates a 1 mm x 1mm array of superconducting microwire single-photon detectors (SMSPDs) sensitive out to 3.7um, establishing a pathway for future antenna-coupled detectors. The second thrust investigates the development of micrometer-scale wires to simultaneously increase active area and operating temperature. Moving to microwire geometries raises fundamental questions regarding intrinsic detection mechanisms. Using a scanning lens setup, this work demonstrates the first direct observation of transverse coordinate effects in microwires, where the precise location of photon incidence across the wire width significantly alters the detector's slew rate and timing performance. To counteract this, new rectification schemes are introduced to produce a more uniform detection landscape. Specifically, the integration of a superconducting rail alongside the main detector is shown to successfully mitigate the transverse coordinate effect, improve timing performance, and increase the viable operating temperature of the detector. The third thrust explores the use of unconventional superconductors, namely MgB2 to achieve an appreciably higher-temperature single-photon detection pathway. This section details the distinct challenges associated with the growth and fabrication of detectors using air-sensitive materials and evaluates their long-term viability for the SNSPD platform. Ultimately, this thesis highlights the critical material and architectural parameters required to advance the field of single-photon detection. By elucidating the metrics and mechanisms that govern ideal detector performance, this work aims to broaden the understanding and future application of SNSPDs across diverse scientific disciplines. All the data presented here is my own unless otherwise mentioned.
The outstanding performance of superconducting nanowire single-photon detectors (SNSPDs) has expanded their application areas from quantum technologies to astronomy, space communication, imaging, and LiDAR. As a result, there has been a surge in demand for these devices, that commercial products cannot readily meet. Consequently, more research and development efforts are being directed towards establishing in-house SNSPD manufacturing, leveraging existing nano-fabrication capabilities that can be customized and fine-tuned for specific needs. We report on the implementation of an end-to-end workflow for SNSPD fabrication and characterization, from superconducting film growth to meander nanowire fabrication and their integration with electrical readout circuits and optical testbeds. An essential aspect of this research involved identifying the key parameters of our workflow and developing reliable procedures for their optimization. As an outcome, the ab initio development of SNSPD technology yielded devices with characteristics comparable to commercial devices at a wavelength of 1550 nm, making them well-suited for telecommunication and integrated quantum systems. This report aims to provide useful insights to those entering the field and accelerate the establishment of superconducting detector technology and its application across various domains.
Superconducting single-photon detectors (SSPDs) have developed into a mature device technology and excel due outstanding performance metrics, in particular high detection efficiency combined with high time resolution and low dark count rate for a wide wavelength range from the visible to the mid-infrared. In addition to commercially available systems with devices coupled to optical fibers, SSPDs can be integrated with photonic circuits using scalable nanofabrication technologies. Here, we will present recent progress on SSPDs based on NbTiN thin films and their integration on different photonic material platforms. Our process for NbTiN growth at room temperature will be described, using magnetron reactive co-sputtering to achieve high-quality superconducting layers down to thicknesses of few nanometres. Optimized SSPD devices are realized by tuning the superconducting properties of NbTiN thin films, adjusting the material composition and nanocrystalline structure. The realization of different types of detectors and geometries will be shown, including nanofabrication techniques for achieving fully suspended nanowire structures. Furthermore, we will discuss challenges and prospects for scaling-up SSPD device technology as well as detector systems. Multiplexing schemes such as dispersion engineering of superconducting transmission lines will be highlighted as powerful approach to address multiple detectors and reduce the number of required feedthroughs and electrical lines in the cryostat. Eventually, exemplary applications of SSPDs for photon counting in quantum optics and light detection and ranging (LIDAR) will be outlined.
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.
Superconducting nanowire single-photon detectors (SNSPDs) have abstracted widespread attention in the fields of quantum information and quantum photonics due to their high detection efficiency and low dark count rates (DCR). However, single-pixel SNSPDs exist the notable limitations in detection speed and photon number resolution (PNR) capabilities, which pose challenges for practical applications. In this study, we propose a design of a series four-pixel SNSPD that incorporates a SPICE circuit simulation model to achieve PNR and spatial resolution of the four photons in an ideal state, and then determine the optical cavity structure of the detector at the highest light absorption of up to 97% through optical simulation. This design provides a theoretical foundation for the subsequent fabrication of large-area SNSPD with PNR and spatial resolution, significantly improving the flexibility and adaptability of SNSPDs to meet diverse detection requirements. Furthermore, our work also lays a research foundation for multi-photon detection in quantum communication and quantum computing.
Single photon detection advanced rapidly in the last decade. The superconducting nanowire single-photon detector (SNSPD) is an emerging technology, which offers low dark count rates, broadband sensitivity from visible to mid-infrared wavelengths and very high quantum efficiency. All these features are appealing for future emerging applications in remote gas sensing, quantum key distribution and quantum information processing. By enclosing the detector in an optical cavity, tuned to the operating wavelength, the total system detection of the SNSPD is improved. Based on the cavity design, the detector absorption is boosted at the desired experimental wavelength. Devices have been fabricated and tested, in this thesis, with enhanced detection efficiency at near-infrared, infrared and mid-infrared wavelengths. NbTiN detectors fabricated on Ta2O5 / SiO2 distributed Bragg reflectors (DBR) for front-side coupling exhibit a system detection efficiency of ≈ 90% at λ = 940 nm for the best device measured. SNSPDs were also fabricated in NbTiN on metallic mirrors for back-side coupling for 1550 and 2333 nm experimental wavelengths. The first design exhibited a system detection efficiency of ≈ 84.5%, at λ = 1550 nm for the best device measured. The second exhibited low system detection efficiency since the device suffered from unexpectedly high dark count rate mainly due to blackbody radiation at 2.2 K. However, it was possible to measure the timing jitter of this device at 350 mK indicating a FWHM of 84 ps. Another technique to improve the total system detection efficiency of the SNSPD, is by fabricating a hairpin-shaped nanowire atop a single mode waveguide. In this design, photons travel through the waveguide and via evanescent coupling are absorbed in the nanowire. These SNSPD devices have been designed to thread along single mode Si ridge waveguides using a hairpin-shaped nanowire structure, which offers enhanced coupling and absorption efficiency via evanescent coupling. These ultra-thin superconducting MoSi detectors are precisely patterned on top of a single mode Si waveguide (600 nm) with alignment error less than 40 nm. Low temperature electrical and nano-optical characterization was carried out using a two-stage pulse tube (PT) coldhead, using an additional 4He / 3He sorption pump, low vibration characterization of SNSPD devices at 350 mK was carried out. Characterised devices exhibit low dark count rate <5 counts per second and timing jitter of ≈ 83 ps full width half maximum (FWHM) at wavelength (λ) = 1550 nm and at critical current ≈ 30 µA.