Packaging and Characterization of a NbTiN Superconducting Nanowire for the Design of an Optimal Nanowire Meander Structure
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The Superconducting Nanowire single-photon detector (SNSPD) made with niobium-titanium nitride (NbTiN) thin films fabricated on oxidized silicon substrates are highly promising nanodevices. The SNSPD is an immensely capable infrared single photon detector. When cooled down with liquid helium the device exhibits high detection efficiency, low dark-counts in spite of fast response times, and low timing jitter. For good single-photon sensitivity at telecom wavelengths, picosecod timing resolutions (<100 ps), and high counting frequencies, the SNSPD is first choice compared to the alternatives like indium gallium arsenide avalanche diode detectors, silicon single photon avalanche diode detectors, and other superconducting single photon technology like, transition edge detectors. These SNSPD characteristics make it ideal for long distance quantum key distribution (QKD). Although the exploitation of the constructive interference that occurs at the SiO2/Si interface can boost detector efficiency, efficient packaging and fiber coupling is often a limiting factor in the overall system quantum efficiency (SQE). In an attempt to maximize SQE, we use a controlled expansion alloy for optimal performance at cryogenic temperatures. This nickel-iron alloy has a high relative permeability and, therefore, attenuates electromagnetic interference. Our report focuses on the theoretical and experimental methods used in the characterization of an un-patterned NbTiN superconducting nanowire. We use simulations based on the gathered experimental data to design an optimal SNSPD meander and calculate its expected SQE. We also use simulations to analyze the speed and value added to secure key rates in long distance QKD schemes up to 400 km with down-conversion sources with positively spectrally correlated and decorrelated photon pairs at 1550 nm wavelength.