Bulaevskii, L. N., Graf, M. J., & Kogan, V. G. (2012). Vortex-assisted photon counts and their magnetic field dependence in single-photon superconducting detectors. Phys. Rev. B, 85(1), 9.
Abstract: We argue that photon counts in a superconducting nanowire single-photon detector (SNSPD) are caused by the transition from a current-biased metastable superconducting state to the normal state. Such a transition is triggered by vortices crossing the thin and narrow superconducting strip from one edge to another due to the Lorentz force. Detector counts in SNSPDs may be caused by three processes: (a) a single incident photon with sufficient energy to break enough Cooper pairs to create a normal-state belt across the entire width of the strip (direct photon count), (b) thermally induced single-vortex crossing in the absence of photons (dark count), which at high-bias currents releases the energy sufficient to trigger the transition to the normal state in a belt across the whole width of the strip, and (c) a single incident photon of insufficient energy to create a normal-state belt but initiating a subsequent single-vortex crossing, which provides the rest of the energy needed to create the normal-state belt (vortex-assisted single-photon count). We derive the current dependence of the rate of vortex-assisted photon counts. The resulting photon count rate has a plateau at high currents close to the critical current and drops as a power law with high exponent at lower currents. While the magnetic field perpendicular to the film plane does not affect the formation of hot spots by photons, it causes the rate of vortex crossings (with or without photons) to increase. We show that by applying a magnetic field one may characterize the energy barrier for vortex crossings and identify the origin of dark counts and vortex-assisted photon counts.
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Inderbitzin, K., Engel, A., Schilling, A., Il'in, K., & Siegel, M. (2012). An ultra-fast superconducting Nb nanowire single-photon detector for soft x-rays. Appl. Phys. Lett., 101.
Abstract: Although superconducting nanowire single-photon detectors (SNSPDs) are well studied regarding the
detection of infrared/optical photons and keV-molecules, no studies on continuous x-ray photon
counting by thick-film detectors have been reported so far. We fabricated a 100 nm thick niobium
x-ray SNSPD (an X-SNSPD) and studied its detection capability of photons with keV-energies in
continuous mode. The detector is capable to detect photons even at reduced bias currents of 0.4%,
which is in sharp contrast to optical thin-film SNSPDs. No dark counts were recorded in extended
measurement periods. Strikingly, the signal amplitude distribution depends significantly on the photon
energy spectrum.VC
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Steudle, G. A., Schietinger, S., Höckel, D., Dorenbos, S. N., Zadeh, I. E., Zwiller, V., et al. (2012). Measuring the quantum nature of light with a single source and a single detector. Phys. Rev. A, 86(5), 053814.
Abstract: An elementary experiment in optics consists of a light source and a detector. Yet, if the source generates nonclassical correlations such an experiment is capable of unambiguously demonstrating the quantum nature of light. We realized such an experiment with a defect center in diamond and a superconducting detector. Previous experiments relied on more complex setups, such as the Hanbury Brown and Twiss configuration, where a beam splitter directs light to two photodetectors, creating the false impression that the beam splitter is a fundamentally required element. As an additional benefit, our results provide a simplification of the widely used photon-correlation techniques.
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Minaeva, O., Fraine, A., Korneev, A., Divochiy, A., Goltsman, G., & Sergienko, A. (2012). High resolution optical time-domain reflectometry using superconducting single-photon detectors. In Frontiers in Opt. 2012/Laser Sci. XXVIII (Fw3a.39). Optical Society of America.
Abstract: We discuss the advantages and limitations of single-photon optical time-domain reflectometry with superconducting single-photon detectors. The higher two-point resolution can be achieved due to superior timing performance of SSPDs in comparison with InGaAs APDs.
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Henrich, D., Dorner, S., Hofherr, M., Il'in, K., Semenov, A., Heintze, E., et al. (2012). Broadening of hot-spot response spectrum of superconducting NbN nanowire single-photon detector with reduced nitrogen content. J. Appl. Phys., 112.
Abstract: The spectral detection efficiency and the dark count rate of superconducting nanowire
single-photon detectors (SNSPD) have been studied systematically on detectors made from thin
NbN films with different chemical compositions. Reduction of the nitrogen content in the 4 nm
thick NbN films results in a decrease of the dark count rates more than two orders of magnitude
and in a red shift of the cut-off wavelength of the hot-spot SNSPD response. The observed
phenomena are explained by an improvement of uniformity of NbN films that has been confirmed
by a decrease of resistivity and an increase of the ratio of the measured critical current to the
depairing current. The latter factor is considered as the most crucial for both the cut-off
wavelength and the dark count rates of SNSPD. Based on our results we propose a set of criteria
for material properties to optimize SNSPD in the infrared spectral region. VC 2012 American
Institute of Physics. [http://dx.doi.org/10.1063/1.4757625]
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