KEYWORDS: Single photon detectors, Superconductors, Quantum communications, Quantum optics, Metrology, Signal to noise ratio, Photon counting, Nanowires, Current controlled current source
Superconducting nanowire single-photon detectors (SNSPDs) have shown to be promising in applications such as quantum communication and computation, quantum optics, imaging, metrology and sensing. They offer the advantages of a low dark count rate, high efficiency, a broadband response, a short time jitter, a high repetition rate, and no need for gated-mode operation. Several SNSPD designs have been proposed in literature. Here, we discuss the so-called parallel nanowires configurations. They were introduced with the aim of improving some SNSPD property like detection efficiency, speed, signal-to-noise ratio, or photon number resolution. Although apparently similar, the various parallel designs are not the same. There is no one design that can improve the mentioned properties all together. In fact, each design presents its own characteristics with specific advantages and drawbacks. In this work, we will discuss the various designs outlining peculiarities and possible improvements.
The phenomenon of dark counts in nanostripes of different superconductor systems such as high-temperature superconducting YBa2Cu3O7-x and superconductor/ferromagnet hybrids consisting of either NbN/NiCu or YBa2Cu3O7- x/L0.7Sr0.3MnO3 bilayers have been investigated. For NbN/NiCu the rate of dark-count transients have been reduced with respect to pure NbN nanostripes and the events were dominated by a single vortex entry from the edge of the stripe. In the case of nanostripes based on YBa2Cu3O7-x, we have found that thermal activation of vortices was also, apparently, responsible for triggering dark-count signals.
Nanowires of Y-Ba-Cu-O, with the thickness of 50 nm and the width ranging from 90 nm to 500 nm have been successfully grown on lanthanum aluminate substrates for photon detection experiments. The nanowires were up to 10- μm long and formed a meander structure, covering the area of up to 30×10 μm2 with a fill factor of 50%. The samples were excited using optical laser pulses at a 1550 nm wavelength and resulting photoresponse signals were measured as a function of both temperature and normalized bias current. Presence of two, distinct regimes in the photoresponse temperature dependence has been clearly evidenced, suggesting different physical mechanisms of the signal formation. Presented experimental results shed new light on prospects of implementation of high-temperature superconducting oxides in photon detection and counting.
Nanostripes of hybrid superconductor/ferromagnetic (S/F) NbN/NiCu bilayers and pure superconducting NbN nanostripes have been investigated in dark count experiments. Presence of a ferromagnetic layer influences the superconducting properties of the S/F bilayer, such as the critical current density and the transient photoresponse. The observed significant decrease of the dark-count rate is discussed in terms of vortex-related fluctuation models to shed more light in the intriguing question of the basic mechanism responsible for dark counts in superconducting nanostripe single photon detectors.
Transport properties of NbN/NiCu superconductor/ferromagnet (S/F) nanostripes fabricated in both in single-wire and series-parallel, meander-type configurations are presented down to T = 4.2 K. In particular, the enhancement of the superconducting critical current has been observed at smaller widths, apparently, due to an extra pinning mechanism, arising from clustering of ferromagnetic atoms inside the thin S layer. Moreover, we observed a number of characteristic voltage steps on the nanostripe current-voltage characteristics and their nature was investigated as a function of temperature. An explanation in terms of active phase-slip phenomena has been proposed based of the time-dependent Ginzburg-Landau theory and led to an estimation of the inelastic electron-phonon relaxation time τe-ph ~ 1 ps, in agreement with the τopt = 1.2±0.3 ps value, measured by the femtosecond transient optical reflectivity spectroscopy method on the same bilayer. Transient optical properties of our superconducting S/F nano-bilayers have been also investigated and compared to those obtained for pure NbN nanostripe reference samples. Finally, electrical photoresponse signals of S/F heterostructures exposed to ultraweak pulsed (width 400 ps, repetition rate ~100 MHz) laser radiation at 850 nm wavelength exhibited the falling time of voltage responses directly dependent on the NiCu overlayer. We have also noticed that the presence of the top F layer and the resulting proximity effect reduced frequency of dark counts in our samples.
KEYWORDS: Inductance, Switches, Superconductors, Single photon detectors, Sensors, Signal to noise ratio, Switching, Solids, Scanning electron microscopy, Nanowires
Ultrathin NbN nanowires is the material of choice for superconducting single photon detectors (SSPD) due to the good
efficiency, dark count rate and timing jitter at 1550 nm wavelength obtained. These performance parameters are achieved
using nanowires a few nanometers thick and 100 nm wide patterned into a meander shape in order to achieve area
coverage. The meander shape effectively makes the SSPD of a single very long nanowire in turn giving it a significant
inductance which limits the maximum count rate of the detector. Recently, we demonstrated how one can exploit a
cascade switch to the normal state of nanowires connected in parallel to significantly reduce the SSPD inductance and
increase the signal amplitude. Here we present how one can configure SSPDs that uses multiple cascade switches to the
normal state. We show how this principle can be used to expand the SSPD coverage area with a very limited increase in
detector inductance with area. Finally we discuss our first results obtained with SSPD based on the multiple cascade
switch principle, showing correct operation.
KEYWORDS: Switches, Inductance, Superconductors, Sensors, Photodetectors, Single photon detectors, Signal to noise ratio, Signal processing, Electron beam lithography, Nanowires
Ultrathin NbN nanowires is the material of choice for superconducting single photon detectors (SSPD) due to the good
efficiency, dark count rate and timing jitter at 1550 nm wavelength obtained. These performance parameters are achieved
using nanowires a few nanometers thick and 100 nm wide patterned into a meander shape in order to achieve area
coverage. The meander shape effectively makes the SSPD of a single very long nanowire in turn giving it a significant
inductance which limits the maximum count rate of the detector. Recently, we demonstrated how one can exploit a
cascade switch to the normal state of nanowires connected in parallel to significantly reduce the SSPD inductance and
increase the signal amplitude. Here we present how one can configure SSPDs that uses multiple cascade switches to the
normal state. We show how this principle can be used to expand the SSPD coverage area with a very limited increase in
detector inductance with area. Finally we discuss our first results obtained with SSPD based on the multiple cascade
switch principle, showing correct operation, increased operational bias range and increased signal pulse amplitudes.
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