We investigate the source of the large variations in the observed detection efficiencies of superconducting nanowire single-photon detectors between many nominally identical devices. Through both electrical and optical measurements, we infer that these variations arise from “constrictions:” highly localized regions of the nanowires where the effective cross-sectional area for superconducting current is reduced. These constrictions limit the bias-current density to well below its critical value over the remainder of the wire, and thus prevent the detection efficiency from reaching the high values that occur in these devices when they are biased near the critical current density.
We investigate the source of the large variations in the observed detection efficiencies of superconducting nanowire single-photon detectors between many nominally identical devices. Through both electrical and optical measurements, we infer that these variations arise from “constrictions:” highly localized regions of the nanowires where the effective cross-sectional area for superconducting current is reduced. These constrictions limit the bias-current density to well below its critical value over the remainder of the wire, and thus prevent the detection efficiency from reaching the high values that occur in these devices when they are biased near the critical current density.
Nanowire superconducting single-photon detectors (SSPDs) are promising devices for single-photon detection applications at telecom wavelengths, in terms of high efficiency, low dark count rate and short dead time. They consist of a long, 100 nm wide, superconducting niobium nitride nanowire folded into a meander pattern and biased close to the critical current. Photons incident on the SSPD locally break the superconductivity which results in a resistive barrier across the nanowire. The voltage drop across this barrier is measured. The devices presented in this thesis are fabricated on a gallium arsenide substrate: a material system that allows integration of single-photon sources and detectors on a single chip. Such quantum photonic integrated circuits (QPICs) are of major importance for scalable quantum computing.Measurements on nominally identical SSPDs show large variations in critical current and efficiency of the devices . This is a huge problem for applications where arrays of SSPDs are needed such as linear optical quantum computing. The authors of [1] attribute the observations to geometrical constrictions: regions in the nanowire where the cross section is reduced. This reduces the critical current of the wire and the SSPD cannot be biased efficiently anymore. In this thesis, it is shown that the continuously distributed inhomogeneity of the nanowire dimension or the crystal structure, instead of localized constrictions, is the origin of the limited detection efficiency and poor uniformity of our SSPDs.To investigate the effect of inhomogeneities, SSPDs with lengths in the range from 100 nm to 15 µm are fabricated and characterized electrically and optically. The two main observations from the electrical characterization are a decrease of the critical current with increasing length and a decrease of the spread in critical current values with increasing length. These measurements also indicate that the length scale of the inhomogeneities is 100 nm or smaller. The main result of the optical characterization is a decrease of the detection efficiency with increasing wire length.Simple assumptions on how the inhomogeneities influence the critical current of the wires allowed us to create a numerical model that quantitatively reproduces the measured variations of the critical current. This result is used to model also the detection efficiency of the SSPDs. This model qualitatively reproduces the general trends in the efficiency measurements, although some deviations are observed for the longer wires. Nanowire superconducting single-photon detectors (SSPDs) are promising devices for single-photon detection applications at telecom wavelengths, in terms of high efficiency, low dark count rate and short dead time. They consist of a long, 100 nm wide, superconducting niobium nitride nanowire folded into a meander pattern and biased close to the critical current. Photons incident on the SSPD locally break the superconductivity which results in a resistive barrier across the nanowire. The voltage drop across this barrier is measured. The devices presented in this thesis are fabricated on a gallium arsenide substrate: a material system that allows integration of single-photon sources and detectors on a single chip. Such quantum photonic integrated circuits (QPICs) are of major importance for scalable quantum computing.Measurements on nominally identical SSPDs show large variations in critical current and efficiency of the devices . This is a huge problem for applications where arrays of SSPDs are needed such as linear optical quantum computing. The authors of [1] attribute the observations to geometrical constrictions: regions in the nanowire where the cross section is reduced. This reduces the critical current of the wire and the SSPD cannot be biased efficiently anymore. In this thesis, it is shown that the continuously distributed inhomogeneity of the nanowire dimension or the crystal structure, instead of localized constrictions, is the origin of the limited detection efficiency and poor uniformity of our SSPDs.To investigate the effect of inhomogeneities, SSPDs with lengths in the range from 100 nm to 15 µm are fabricated and characterized electrically and optically. The two main observations from the electrical characterization are a decrease of the critical current with increasing length and a decrease of the spread in critical current values with increasing length. These measurements also indicate that the length scale of the inhomogeneities is 100 nm or smaller. The main result of the optical characterization is a decrease of the detection efficiency with increasing wire length.Simple assumptions on how the inhomogeneities influence the critical current of the wires allowed us to create a numerical model that quantitatively reproduces the measured variations of the critical current. This result is used to model also the detection efficiency of the SSPDs. This model qualitatively reproduces the general trends in the efficiency measurements, although some deviations are observed for the longer wires.
Statistical studies on the performance of different superconducting nanowire single-photon detectors (SNSPDs) on one chip suggested that random constrictions existed in the nanowire that were barely registered by scanning electron microscopy. With the aid of advanced e-beam lithography, artificial geometric constrictions were fabricated on SNSPDs as well as single nanowires. In this way, we studied the influence of artificial constrictions on SNSPDs in a straight forward manner. By introducing artificial constrictions with different wire widths in single nanowires, we concluded that the dark counts of SNSPDs originate from a single constriction. Further introducing artificial constrictions in SNSPDs, we studied the relationship between detection efficiency and kinetic inductance and the bias current, confirming the hypothesis that constrictions exist in SNSPDs.
We report on the photoresponse mapping of nanowire superconducting single-photon detectors using a focal spot significantly smaller than the device area (10 μm x 10 μm). Using a solid immersion lens we achieve a spot size of 320 nm full-width half maximum onto the device at 470 nm wavelength. We compare the response maps of two devices: the higher detection efficiency device gives a uniform response whereas the lower detection efficiency device is limited by a single defect or constriction. A second optical setup is used to simultaneously image and measure the photoresponse of the lower detection efficiency device, allowing the constriction location to be pinpointed.
Superconducting nanowires have been used to detect single photons for more than 20 years. The basic principle is that photons can perturb superconducting Cooper pairs and thereby create a resistive hotspot, which produces a current-crowding effect. This current-crowding effect reduces the superconducting energy gap near the hotspot, which results in the nanowire changing from a superconducting to a normal state near the hotspot. The resulting high impedance causes the current to be squeezed into a high frequency coaxial wire connected in parallel with it, which finally produces a measurable voltage pulse. Thanks to the maturity of current micro- and nano- fabrication techniques, high quality superconducting thin films and nano-wires have been realized in a variety of materials such as NbN, WSi, MoSi and others. These techniques include magnetron sputtering, physical vapor deposition, electron beam lithography, optical lithography, reactive ion etching, wafer cutting, etc. Measurements of superconducting single-photon detectors require cryogenic refrigeration systems, ranging from the initial liquid helium cold tank, to modern dry dilution refrigerators. On one hand, a superconducting device needs to be cooled down below its critical temperature to work. On the other hand, photon-detection events generate hot spots, which release heat to the device substrate, which the refrigerator needs to efficiently take away. Research on superconducting nanowire single-photon detectors (SNSPDs) has made significant advancements over the past two decades, including high detection efficiency, low dark counts, and broad wavelength range capabilities. Beyond improvements in traditional NbN, WSi and MoSi superconducting nanowires, there have been numerous efforts to develop high-temperature superconducting nanowires, such as MgB2 and Bi2Sr2CaCu2O8 (BSCCO), due to the operational limitations imposed by low-temperature superconducting materials. Typically, single-photon detectors require operation temperatures below the critical temperature. High-temperature superconducting nanowire single-photon detectors have currently reported to exhibit the highest operating temperature around 20 K in BSCCO, which is much lower than its critical temperature of 80 K, showing that large-gap superconductors have a comparably low sensitivity to low-energy photons. In recent years since 2018, several groups have reported that micrometer-wide superconducting wires can also be used for single-photon detection, unlike the conventional nanometer-wide superconducting wires. In general, as long as the bias current is uniformly distributed in superconducting wires and approaches the critical Cooper pair-breaking current, the hot spots generated by photon absorption can induce vortices and anti-vortices, and their motion under the action of the Lorentz force generates resistance and voltage, which ultimately produces a normal state region. An increase of the wire width to the micron scale offers more freedom to design new detectors based on current nano- and microfabrication techniques. In this doctoral dissertation, I will first introduce the basic Ginzburg-Landau theory of superconductivity and the measurement theory of superconducting circuits related to single-photon detection. I will then provide a detailed description of the equipment construction and sample fabrication techniques used in the experiment. Finally, I will present my two main studies: in one project I examined the photon-detection performance of superconducting micro-wires with a multi-pinhole structure, the correlation between dark counts and magnetic vortices, as well as the properties of critical current in magnetic fields. In a subsequent study I conducted numerical simulations using the time dependent Ginzburg-Landau equation, to analyze the dynamics of vortices in superconducting micro-strips with regular large-area perforations by pinholes as a function of magnetic field and bias current. The thus obtained predictions were then successfully confirmed in a detailed experimental study on a series of microwires.
Superconducting nanowire single-photon detectors combine high detection efficiency, low noise, and excellent timing resolution, making them a leading platform for photon-counting applications. However, despite decades of materials and fabrication research, detector performance has never been shown to match theoretical performance expectations. Here, we demonstrate in situ tuning of a detector from its typical, suboptimal operation, to a regime limited only by material quality, allowing the device to reach its intrinsic performance limit. Our approach is based on current-biased superconducting “rails” placed on either side of the detector that redistribute current across its width to achieve peak performance. This technique reduces the dark count rate by 10 orders of magnitude. Further, we show operation at this intrinsic performance limit for devices up to 0.1 mm wide and also demonstrate near-unity internal detection efficiency at a wavelength of 4 µm for a 20 µm wide detector—a factor of 20 wider than the current state of the art. This work enables future detectors to overcome the Pearl limit for device width, paving the way for arbitrarily large detectors.