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.
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.
The dark count is one of the key physical issues for superconducting nanowire single-photon detectors (SNSPDs) that limits various applications for optical quantum information and classical optics. When the bias current approaches the switching current of SNSPDs, the dark count is actually dominated by the intrinsic dark counts (iDCs). However, the origin of iDCs and its relation to constrictions remains unclear for practical SNSPDs. We herein systematically characterize the iDCs of the SNSPDs with and without artificial geometric constrictions by applying the differential readout method. For these devices with constrictions, we have observed distinct Gaussian distributions in the temporal distribution of iDCs, in which the time difference between the distributions is consistent with the geometric distance between constrictions, and the rates of iDCs produced by each constriction are in good agreement with constrictions' widths. With respect to practical SNSPDs, surprisingly, we also observe several Gaussian distributions in the temporal domain and it shows no significant dependence on the devices’ sizes, demonstrating that the iDCs of SNSPDs are mainly dominated by a few specific constrictions.
We fabricated superconducting nanowire single-photon detectors (SNSPDs) made of NbTiN and NbN thin films in the same fabrication batch. Various parameters, such as critical temperature, resistivity, switching current, and kinetic inductance, were compared between the two types of SNSPDs. These data indicate that NbTiN SNSPD surpasses NbN SNSPD in many aspects, including a shorter recovery time due to smaller kinetic inductance, a lower timing jitter due to higher switching current, and better uniformity. Based on the measurement of 28 out of 121 NbTiN SNSPDs on a 2-in Si wafer, 71% of detectors show a system detection efficiency of over 50% at a dark count rate of 100 Hz, with the maximum efficiency being 77.5%.
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.
Detecting single photons is essential for applications such as dark matter detection, quantum science and technology, and biomedical imaging. Superconducting nanowire single-photon detectors (SNSPDs) excel in this task due to their near-unity detection efficiency, subhertz dark count rates, and picosecond timing jitter. However, a local increase of current density (current crowding) in the bends of meander-shaped SNSPDs limits these performance metrics. By locally irradiating the SNSPD’s straight segments with helium ions while leaving the bends unirradiated, we realize current crowding–free SNSPDs with simultaneously enhanced sensitivity: After irradiation with 800 ions nm −2 , locally irradiated SNSPDs showed a relative saturation plateau width of 37%, while fully irradiated SNSPDs reached only 10%. This larger relative plateau width allows operation at lower relative bias currents, thereby reducing the dark count rate while still detecting single photons efficiently. We achieve an internal detection efficiency of 94% with 7 mHz dark count rate near the onset of saturating detection efficiency for a wavelength of 780 nm.
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.
Single-photon detection is extremely important for a number of different applications such as quantum cryptography, CMOS testing and even biomedical research. Most of the applications of single-photon detector require high efficiency combined with high time resolution, high count rates and low dark counts. Superconducting nanowire single-photon detectors has emerged provides this combination unlike any other available single-photon detectors. Some of the applications of superconducting nanowire single-photon detectors (SNSPD's) require larger area SNSPD's without affecting it's performance. For this purpose, multi-pixel SNSPD's would provide a solution. To this end, prototype 4- and 16-pixel detectors have been characterised in terms of a number of performance parameters such as critical current, count rate behaviour and timing jitter. Aside from this, simulations have been performed on detection statistics and relative illumination of the individual pixels of a multi-pixel SNSPD. Results show that for 4-pixel detectors, all desired aspects have been achieved, but not yet combined in one detector. The 16-pixel detectors showed excellent critical currents and count rate behaviour but, most likely due to the fact that they were prototype chips with little to no protection, had several not functioning pixels.