A growing number of technologies employ quantum properties in order to produce solutions that surpass the performance of conventional devices, or to execute operations that are fundamentally impossible with classical systems alone. In the field of optical quantum information science, photons are utilized to encode, communicate and manipulate information, making them vitally important. While photon production always constitutes the first step in any optical experiment, in the field of quantum information science, the recording of data through the process of photon detection is an equally crucial final step. This thesis deals with both the single photons generation (based on diamond color defects) and their detection, utilizing a novel type of superconducting detectors. In particular, part one of this thesis is devoted to the construction of custom designed microscope setup, and the development of laboratory experiments, to enable the generation of single photons as well as the investigation of the optical and spin properties of diamond color centers. Confocal microscopy is used for this purpose, as it allows for the identification and addressing of individual color centers that emit only single photons. This microscope also feature an integrated self-built microwave and magnetic hardware setup, which allows for a wide range of spin environment spectroscopy studies. Single photon emission is demonstrated through both photon anti-bunching and Rabi oscillations at room temperature. The second part of the thesis offers an exploration of superconducting single photon detectors through experiment. Since electronics are an essential part of these detectors, the possibility of using a novel alternative scheme based on capacitive readout combined with fast gating to enable simplified readout is demonstrated. This scheme overcomes the limitations of conventional readout schemes, which require large bandwidth amplification and complex counting electronics. Besides photon detection, the capabilities of these detectors are also expanded to include high-energy particles in the MeV energy range, and the detectors are demonstrated to not only detect single α- and β-particles, but to do so with near unity efficiency. Finally, a multipurpose testing station for superconducting detectors is demonstrated with a central objective of optimizing the coupling efficiency of light to the active area of the detector, as well as to allow for a fast exchange of the optical fiber, thereby facilitating an efficient characterization of the detector. The optimization of this coupling efficiency was demonstrated through proof-of-principle experiments.
A growing number of technologies employ quantum properties in order to produce solutions that surpass the performance of conventional devices, or to execute operations that are fundamentally impossible with classical systems alone. In the field of optical quantum information science, photons are utilized to encode, communicate and manipulate information, making them vitally important. While photon production always constitutes the first step in any optical experiment, in the field of quantum information science, the recording of data through the process of photon detection is an equally crucial final step. This thesis deals with both the single photons generation (based on diamond color defects) and their detection, utilizing a novel type of superconducting detectors. In particular, part one of this thesis is devoted to the construction of custom designed microscope setup, and the development of laboratory experiments, to enable the generation of single photons as well as the investigation of the optical and spin properties of diamond color centers. Confocal microscopy is used for this purpose, as it allows for the identification and addressing of individual color centers that emit only single photons. This microscope also feature an integrated self-built microwave and magnetic hardware setup, which allows for a wide range of spin environment spectroscopy studies. Single photon emission is demonstrated through both photon anti-bunching and Rabi oscillations at room temperature. The second part of the thesis offers an exploration of superconducting single photon detectors through experiment. Since electronics are an essential part of these detectors, the possibility of using a novel alternative scheme based on capacitive readout combined with fast gating to enable simplified readout is demonstrated. This scheme overcomes the limitations of conventional readout schemes, which require large bandwidth amplification and complex counting electronics. Besides photon detection, the capabilities of these detectors are also expanded to include high-energy particles in the MeV energy range, and the detectors are demonstrated to not only detect single α- and β-particles, but to do so with near unity efficiency. Finally, a multipurpose testing station for superconducting detectors is demonstrated with a central objective of optimizing the coupling efficiency of light to the active area of the detector, as well as to allow for a fast exchange of the optical fiber, thereby facilitating an efficient characterization of the detector. The optimization of this coupling efficiency was demonstrated through proof-of-principle experiments.
This thesis is about the development of a detector for single photons, particles of light. New techniques are being developed that require high performance single photon detection, such as quantum cryptography, single molecule detection, optical radar, ballistic imaging, circuit testing and fluorescence spectroscopy. Superconducting single photon detectors (SSPDs) are sensitive to single photons from the ultraviolet to the near infrared. In this thesis steps has been taken towards improving this type of detectors and implementing them in experiments. We have fabricated SSPDs in the Van Leeuwenhoek Laboratory at the TU Delft from NbTiN on an oxidized silicon substrate and we show world record system detection efficiencies at telecommunication wavelengths. In addtition, we have adjusted the geometry to get rid of the polarization dependence of the quantum efficiency. SSPDs fabricated from a new material show enhanced efficiency at longer wavelengths. Different read out schemes can scale a single pixel to an array of detectors. We have proven by implementing the SSPDs in a Hanbury-Brown Twiss setup that nanowire quantum dots emit single photons. We also have demonstrated that SSPDs are sensitive to single surface plasmon polaritons and single electrons.
Single photon detectors are a vital part of many emerging technologies which harness the quantum properties of light to benefit the fields of communication, computation and sensing. Superconducting nanowire single photon detectors (SNSPDs) offer high detection efficiency, low dark count rates, low timing jitter, and infrared sensitivity that are required by the most demanding single photon counting applications. This thesis presents SNSPDs fabricated and tested at the University of Glasgow that are integrated with optical structures which enable enhanced detection efficiency and integration with waveguide circuit technology. The monolithic integration of waveguide circuit components presents a route towards realisation of an optical quantum information processor that has the stability and scalability to perform the demanding tasks of quantum computation. A novel process is introduced for incorporating superconducting detectors with single mode gallium arsenide waveguides and quantum dot single photon sources. Together these elements would enable the generation of quantum states of light which could be manipulated and detected on a single chip. Detectors are patterned in NbTiN thin superconducting films on to suspended nanobeam waveguides with better than 50 nm alignment accuracy. Low temperature electrical and optical testing confirms the detectors’ single photon sensitivity under direct illumination as well as to waveguide coupled light. Measured detectors were found to have internal registering efficiencies of 6.8 ± 2.4%. Enhancing absorption of photons into thin superconducting films is vital to the creation of high efficiency superconducting single photon detectors. Fabricating an SNSPD on a dielectric mirror creates a partial cavity that can be tailored to enhance detection of light at specific wavelengths. Devices have been fabricated and tested in this thesis with enhanced detection efficiency at infrared and visible wavelengths for quantum cryptography, remote sensing and life science applications. Detectors fabricated in NbTiN on GaAs/AlGaAs Bragg mirrors exhibited a system detection efficiency of 1.5% at 1500 nm wavelength for the best device measured. SNSPDs were also fabricated in NbN on aperiodic dielectric mirrors with a range of different bandwidths. A peak system detection efficiency of 82.7% at 808 nm wavelength was recorded.
The physics of single photons is fascinating, by manipulating them we can observe and probe quantum effects. Doing so requires the fabrication and utilization of single photon sources, of which many types have been developed including quantum dots, trapped atoms and ions, and color centers. On the other end of the experiments, single photon detectors play a role of utmost importance, and while several types of detectors exist, superconducting nanowire single photon detectors are now the state-of-the-art technology. By offering near unity detection efficiency from the ultra-violet to the mid-infrared light spectrum, with negligible noise and excellent time resolution, they made possible many experiments that were previously technologically unfeasible. The same appealing characteristics have found a use in applications outside of the quantum optics framework, with notably light detection and ranging, biomedical imaging or CMOS circuits testing.In this thesis a controlled growth method for tailoring the characteristics of niobium titanium nitride in the framework of superconducting nanowire single photon detectors was developed. Reactive co-sputter deposition of niobium titanium nitride was shown to be a versatile method, both in terms of the degree of control over the material composition, and in the choice of substrates that it allows. Unity internal detection efficiency of detectors at telecom wavelengths was achieved by optimizing the niobium content in the material. The influence of lattice matching on the critical temperatures of films deposited at room temperature was investigated. The fabrication of superconducting nanowire single photon detectors on aluminum nitride-on-sapphire, on lithium niobate nano-waveguides, on gallium arsenide, and the integration on SiN waveguides was achieved. The material was used to fabricate detectors with optimized response for any linear polarization of the incoming photons by using a fractal architecture. Another method was proposed to achieve the same results by encapsulating meandering detectors in a high index dielectric material, resulting in a decrease of the permittivity mismatch between the nanowire material and its surrounding and therefore optimizing the efficiency for both orthogonal linear polarizations.Finally, detectors were fabricated from films developed in this work, and were operated to enable the implementation of polarization-based entanglement distribution in optical fibers in a real-conditions scenario, over a record distance of 96 km. This paves the way for the development of quantum communication networks using existing optical fiber links.
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.
For applications in Quantum Information and Quantum Key Distribution an on-demand source of single photons is desirable because absolute security is of utmost importance. Photons are quantum systems; hence encoding information onto them offers a secure alternative to classical cryptography as a measurement cannot be performed on photons without altering their properties. The Nitrogen Vacancy (NV) centre in diamond is a good source of such photons. It is photo-stable and its location in diamond offers robustness. It has zero phonon line at 637 nm and its relative short luminescence life-time of about 12 ns makes it suitable for generating single photons. This thesis covers two aspects: Firstly the characterization of defects in diamonds and subsequent selection of diamonds suitable for use in the single photon setup and secondly, the development of the experimental setup for single photon generation. This thesis sets out to describe the development of a laboratory based single photon source using the NV centre in diamond. For this purpose a suite of diamond samples were selected and subjected to various spectroscopic tests in order to characterize and classify the samples, especially the presence of the NV centres and their concentrations. The characterization of the defects was done through the use of the following spectroscopic techniques: UltravioletVisible-Near infrared spectroscopy, Infrared spectroscopy, Electron Spin Resonance and Photoluminescence. These techniques enabled us to understand the types and origins of crystal defects that were present in the diamond samples used in this study and to use this to select diamonds that are most suitable for use in generation of single photons. The experimental setup for single photon generation using the NV centre is based on a confocal microscope arrangement. Single NV centres were identified by measuring the second order autocorrelation function of the fluorescence light emitted by the sample when illuminated with a laser. This measurement was done using a Hanbury-Brown Twiss (HBT) interferometer.
This dissertation explores the role of optimized resources in facilitating efficient optical quantum information processing and sensing. Three key applications are considered: quantum interferometry, detector characterization, and multi-photon processing. For each application, we examine the underlying quantum resource states and quantum technology resources. Careful attention is paid to how these resources can be optimized to maximize the efficiency of the overarching application in terms of resources consumed, whether photonic, temporal, or infrastructural. Such optimization is an important step in fully realizing the potential of quantum science as it emerges from the laboratory and into the mainstream. For quantum interferometry, we investigate the traditional trade-offs between two-photon quantum interference and single-photon classical interference in terms of measurement resolution and resilience against optical loss and background. We show how we can overcome these trade-offs by replacing conventional single photons with highly non-degenerate energy-entangled photons, which are more optimized for achieving high information content on a per-photon basis. In combination with a specialized dual-wavelength interferometer, our high per-photon information efficiency allows us to achieve nanometer-scale resolution in a matter of seconds with only O(10^4) photon pairs. We then show how this entanglement-enhanced quantum sensing modality can allow for new studies in challenging regimes, such as characterizing lossy thin films or detecting distant vibrations. Then, for detector characterization, we consider the instrumental role of photon detectors in various optical quantum science protocols. As these protocols expand into spectral bands with growing technological applications, such as the mid-infrared, there is a developing need to characterize suitable detectors, with their efficiencies being one of the most important metrics. Typical methods for efficiency characterization, whether classical or quantum, can face the challenge of being infrastructure-intensive, with some requiring resources such as independent reference standards or large bulk-optics apparatuses. To help realize a resource-efficient characterization system for mid-infrared detectors, we have developed a compact and portable solution involving highly non-degenerate photon pairs produced by a fiber-coupled waveguide device. We evaluate the performance of early prototypes and identify next steps for improved device engineering. Lastly, for multi-photon processing, we focus on the single-photon source, given its key role in preparing the required photonic states. From a scalability perspective, efficient single-photon generation is essential for performing many large-scale multi-photon protocols within useful timescales. To meet this requirement, we explore how multiplexing techniques allow us to pair heralded single photons with photon routing, storage, and counting technologies to realize an efficient single-photon source. We then discuss our efforts to implement and integrate optimized versions of these technologies to address limitations faced by earlier implementations with regard to preparing pure single photons at a high rate. This discussion includes an in-depth analysis of our work related to realizing an efficient photon-number-resolving detector.