The quantum properties of electromagnetic radiation at single photon level promise to offer what are classically inaccessible. Single photon sources and detectors are therefore on demand for exploiting these properties in practical applications, including but not limited to quantum information processing and communication. In this thesis, I advance Superconducting Nanowire Single Photon Detectors (SNSPD) both in terms of models describing their operation, and their performance. I report on characterization, semi-empirical modeling, quantum-optical modeling and detector tomography. The results provide more accurate methods and formulations to characterize and mathematically describe the detectors, valuable findings from both application and device points of views. I also introduce the concept of Gated SNSPDs, show how to implement and how to characterize them. Through series of theoretical and experimental investigations, I show performance advantages of Gated SNSPDs in terms of dead time and dark count rate, important figures for many applications like quantum key distribution. The ultimate limitations of gated operation are also explored by physical modeling and simulation steps.

The quantum properties of electromagnetic radiation at single photon level promise to offer what are classically inaccessible. Single photon sources and detectors are therefore on demand for exploiting these properties in practical applications, including but not limited to quantum information processing and communication. In this thesis, I advance Superconducting Nanowire Single Photon Detectors (SNSPD) both in terms of models describing their operation, and their performance. I report on characterization, semi-empirical modeling, quantum-optical modeling and detector tomography. The results provide more accurate methods and formulations to characterize and mathematically describe the detectors, valuable findings from both application and device points of views. I also introduce the concept of Gated SNSPDs, show how to implement and how to characterize them. Through series of theoretical and experimental investigations, I show performance advantages of Gated SNSPDs in terms of dead time and dark count rate, important figures for many applications like quantum key distribution. The ultimate limitations of gated operation are also explored by physical modeling and simulation steps.

Magnesium diboride MgB2 is an interesting material that was discovered to be a superconductor in 2001. It has a remarkably high critical temperature of 39 K which is much greater than was previously thought possible for a phonon-mediated superconductor. MgB2 was also the first material found to exhibit multiple gap superconductivity. It has two energy gaps, the pi gap with a value of 2.3 meV, and the sigma gap with a value of 7.1 meV. Both the high critical temperature and the multiple large energy gaps make MgB2 an attractive candidate for superconducting devices. While the initial discovery of MgB2 was accompanied by much excitement, the enthusiasm has mostly disappeared due to the lack of progress made in implementing MgB2 in practical devices. The aim of this thesis is to attempt to reinvigorate interest in this remarkable material through a study of a variety of practical superconducting devices made with MgB2 thin films grown by hybrid physical-chemical vapor deposition (HPCVD). Two different methods of fabricating MgB2 Josephson junctions are explored. The first is a sandwich type trilayer configuration with a barrier made by magnetron sputtered MgO. Junctions of this sort have been previously studied and implemented in a variety of devices. While they do show some attractive properties, the on-chip spread in critical current due to barrier non-uniformity was too high to be considered a viable option for use in many-junction devices. By developing a fabrication scheme which utilizes electron beam lithography, modest improvements were made in the on-chip parameter spread, and miniaturization of junction size yielded some insight into the non-uniform barriers. The second approach of creating MgB2 Josephson junctions utilized a planar geometry with a normal metal barrier created by irradiating nano-sized strips of the material with a focused helium ion beam. The properties of these junctions are investigated for different irradiation doses. This new technique is capable of producing high quality junctions and furthermore the parameter spread is greatly reduced as compared to the sandwich type junctions. While more research is necessary in order to increase the IcRn products, these junctions show promise for use in many-junction devices such as RSFQ circuits. Prior to this work, the largest substrates that could be coated with HPCVD grown MgB2 were 2" in diameter. A new chamber was designed and constructed which demonstrated the ability to coat substrates as large as 4". This scaled-up system was used to grow MgB2 films on 1 x 10 cm flexible substrates. A method of fabrication was developed which could pattern these 10 cm long samples into ribbon cables consisting of many high frequency transmission lines. This technology can be utilized to increase the cooling efficiency of cryogenic systems used for RSFQ systems which require many connections between low temperature and room temperature electronics. Finally, a method of producing MgB2 films with thicknesses as low as 8 nm was developed. This is achieved by first growing thicker films and using a low angle ion milling step to gradually reduce the film thickness while still maintaining well connected high quality films. A procedure was developed for fabricating meandering nanowires in these films with widths as low as 100 nm for use as superconducting nanowire single photon detectors (SNSPDs). A study of the transport properties of these devices is first presented. Measurements show low values of kinetic inductance which is ideal for high count rates in SNSPDs. The kinetic inductance measurements also yielded the first measurements of the penetration depth of MgB2 films in the ultra-thin regime. Devices made from these ultra-thin films were found to be photon sensitive by measurements made by our collaborators. / Physics

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Superconducting single-photon detectors (SSPDs) are the leading detectors in terms of high-speed single-photon counting and high detection efficiency (DE). One factor that limits the DE is the critical current Ic, which is the maximum current before the superconductor switches to the normal state. Increasing device’s bias current towards the Ic can improve the DE. However, the device’s Ic is reduced due to constriction and current crowding at the edges of the wire. Typically, this is caused by fabrication defects. Locally suppressing superconductivity at these defects can potentially lessen the occurrence of current crowding. In this thesis, we used the beam from the helium ion microscope (HIM) and measured the Ic to observe the effects of locally irradiating specific areas on a SSPD wire. Due to the HIM’s small spot size and high collimation, we can control the superconducting gap precisely at the center and edges of the wire. Suppressing the edges can potentially reduce current crowding and increase the device’s critical current while suppressing the center can improve detection sensitivity for photons incident at that location. Our results showed that the irradiated devices had reduced Ic compared to unirradiated devices for both cases. We then extend this method of local suppression of superconductivity to explore an alternative method of fabricating SSPDs by directly writing the device on the superconducting thin film. This can enable the fabrication of devices without the use of lithography resist. In our experiment, we fabricated a 3 μm wire using optical lithography that was disconnected at the center and connected it by writing a single 1 μm wire with the He+ ion beam. We measured the Ic for samples with and without the 1 μm wire pattern and observed that the Ic decreased as we increased the ion dose. Overall, this work aims to contribute to the continuing investigation of the detection mechanism for SSPDs and the improvement of nanofabrication methods using the HIM.

Superconducting nanowire single-photon detectors are the most promising technology in quantum photon information. They offer high speed, high detection efficiency, low dark count rate as well as low timing jitter compared to other single photon detection solutions. Since the recent advances in photonic quantum computing, the drive for improvement of the implementation complexity, performance and scalability of quantum photon detection has increased. This presents challenges with the current device readout schemes and alternative solutions are required. One of the key parameters to improve the scalability of superconducting nanowire single-photon detectors, is reducing the power dissipation per pixel. This is especially important in cryogenic readouts, where the performance of electronic components changes compared to room temperature. Moreover, the performance of a cryogenic superconducting nanowire single-photon detector readout is dependent both on the device and readout electronics level characteristics, and both must be fine-tuned for desired performance. A solution to the scalability of superconducting nanowire single-photon detectors (SNSPDs) is the development of a readout scheme with minimized power dissipation. We propose a fully digital readout scheme interfaced with a superconducting nanowire single-photon detector (SNSPD), that allows photon detection and reset. For this purpose, a digital single-pixel SiGe Bi-CMOS readout is designed, simulated, and characterised. An improved readout scheme is proposed with an addition of a die resistor to allow a full reset of the detector. / Supraledande nanotrådsdetektorer baserade på enstaka fotoner är ett av de mest avancerade koncepten inom kvantfotoninformationsteknik. Syftet med att utveckla denna teknik är att förbättra egenskaper så som komplexiteten, prestandan och skalbarheten. En av de viktigaste parametrarna för att förbättra skalbarheten hos supraledande nanotrådsdetektorer med enstaka fotoner är att minska energiförbrukningen per pixel. Detta är särskilt viktigt i kryogena avläsningar, där prestandan hos elektroniska komponenter förändras jämfört med rumstemperatur. Dessutom, beror prestandan hos en kryogen supraledande nanotrådsdetektor både på komponenten och på avläsningselektroniken,och båda måste finjusteras för att uppnå önskad prestanda. En lösning på kalbarheten för supraledande nanotrådsdetektorer med enstaka fotoner (SNSPDs) är att realisera avläsning med minimerad effektförlust. Vi föreslår en helt digital avläsning som är kopplad till en supraledande enfoton nanotrådsdetektor (SNSPD), som gör det möjligt att detektera fotoner och att återställa detektorn efter avläsning. För detta ändamål, designades, simuleras och karakteriserades en digital avläsningkrets med en enda pixel. Ett förbättrat avläsningssystem föreslås genom att lägga till ett diskret motstånd för att möjliggöra en fullständig återställning av detektorn.

The recent advancement of superconducting nano devices has allowed for making a Superconducting Nanowire Single Photon Detector (SNSPD), whose extraordinary features have strongly motivated the research community to exploit it in many practical applications. In this thesis, an experimental setup for testing the SNSPD has been established. It contains an in-house packaging that meets the requirements of RF/microwave and optoelectronic characterizations. The quantum efficiency and detection efficiency measurements have confirmed that our approach is satisfactory. The dark count performance has reached the anticipated level. The factors affecting rise and fall times of the photoresponses are addressed. Based on the successful setup, the characterizations including dc, small signal ac measurements have been undertaken. The measurements are aimed at quantitatively investigating Cooper pair density in the superconducting nanowire. The experimental method involves a two-step, small signal S-parameter measurement either in the presence or absence of optical powers. The subsequent measurements by varying the temperature and dc bias current have achieved remarkable understanding on the physical properties of SNSPD nanowires. Then, the electrically induced nonlinearity is studied via the large signal RF and Microwave measurements. The experiments are a set of one-tone and two-tone measurements, in which either the RF driving power is varied at a fixed frequency, or vice versa. Two major nonlinear microwave circuit analysis methods, i.e. time-domain transient and hybrid-domain harmonic balance analysis, are employed. The simulation result reveals the optimized conditions of reaching the desired nonlinearity. Finally, we have successfully measured the optoelectronic mixing products in an electrically pumped optoelectronic mixer, which has identical structures as that of the SNSPD. The experiments confirm that this mixer is not only sensitive to the classical light intensities, but also to that of the single photon level. Meanwhile, the quantum conversion matrices is derived to interpret the quantum optoelectronic mixing effects.

The recent advancement of superconducting nano devices has allowed for making a Superconducting Nanowire Single Photon Detector (SNSPD), whose extraordinary features have strongly motivated the research community to exploit it in many practical applications. In this thesis, an experimental setup for testing the SNSPD has been established. It contains an in-house packaging that meets the requirements of RF/microwave and optoelectronic characterizations. The quantum efficiency and detection efficiency measurements have confirmed that our approach is satisfactory. The dark count performance has reached the anticipated level. The factors affecting rise and fall times of the photoresponses are addressed. Based on the successful setup, the characterizations including dc, small signal ac measurements have been undertaken. The measurements are aimed at quantitatively investigating Cooper pair density in the superconducting nanowire. The experimental method involves a two-step, small signal S-parameter measurement either in the presence or absence of optical powers. The subsequent measurements by varying the temperature and dc bias current have achieved remarkable understanding on the physical properties of SNSPD nanowires. Then, the electrically induced nonlinearity is studied via the large signal RF and Microwave measurements. The experiments are a set of one-tone and two-tone measurements, in which either the RF driving power is varied at a fixed frequency, or vice versa. Two major nonlinear microwave circuit analysis methods, i.e. time-domain transient and hybrid-domain harmonic balance analysis, are employed. The simulation result reveals the optimized conditions of reaching the desired nonlinearity. Finally, we have successfully measured the optoelectronic mixing products in an electrically pumped optoelectronic mixer, which has identical structures as that of the SNSPD. The experiments confirm that this mixer is not only sensitive to the classical light intensities, but also to that of the single photon level. Meanwhile, the quantum conversion matrices is derived to interpret the quantum optoelectronic mixing effects.

In this thesis, superconducting nanostructures for quantum detection of electromagnetic radiation are studied. In this regard, electrodynamics of topological excitations in 1D superconducting nanowires and 2D superconducting nanostrips is investigated. Topological excitations in superconducting nanowires and nanostrips lead to crucial deviation from the bulk properties. In 1D superconductors, topological excitations are phase slippages of the order parameter in which the magnitude of the order parameter locally drops to zero and the phase jumps by integer multiple of 2\pi. We investigate the effect of high-frequency field on 1D superconducting nanowires and derive the complex conductivity. Our study reveals that the rate of the quantum phase slips (QPSs) is exponentially enhanced under high-frequency irradiation. Based on this finding, we propose an energy-resolving terahertz radiation detector using superconducting nanowires. In superconducting nanostrips, topological fluctuations are the magnetic vortices. The motion of magnetic vortices result in dissipative processes that limit the efficiency of devices using superconducting nanostrips. It will be shown that in a multi-layer structure, the potential barrier for vortices to penetrate inside the structure is elevated. This results in significant reduction in dissipative process. In superconducting nanowire single photon detectors (SNSPDs), vortex motion results in dark counts and reduction of the critical current which results in low efficiency in these detectors. Based on this finding, we show that a multi-layer SNSPD is capable of approaching characteristics of an ideal single photon detector in terms of the dark count and quantum efficiency. It is shown that in a multi-layer SNSPD the photon coupling efficiency is dramatically enhanced due to the increase in the optical path of the incident photon.