348 research outputs found
Nano-optical observation of cascade switching in a parallel superconducting nanowire single photon detector
The device physics of parallel-wire superconducting nanowire single photon
detectors is based on a cascade process. Using nano-optical techniques and a
parallel wire device with spatially-separate pixels we explicitly demonstrate
the single- and multi-photon triggering regimes. We develop a model for
describing efficiency of a detector operating in the arm-trigger regime. We
investigate the timing response of the detector when illuminating a single
pixel and two pixels. We see a change in the active area of the detector
between the two regimes and find the two-pixel trigger regime to have a faster
timing response than the one-pixel regime.Comment: 11 pages, 2 figure
Practical long-distance quantum key distribution system using decoy levels
Quantum key distribution (QKD) has the potential for widespread real-world
applications. To date no secure long-distance experiment has demonstrated the
truly practical operation needed to move QKD from the laboratory to the real
world due largely to limitations in synchronization and poor detector
performance. Here we report results obtained using a fully automated, robust
QKD system based on the Bennett Brassard 1984 protocol (BB84) with low-noise
superconducting nanowire single-photon detectors (SNSPDs) and decoy levels.
Secret key is produced with unconditional security over a record 144.3 km of
optical fibre, an increase of more than a factor of five compared to the
previous record for unconditionally secure key generation in a practical QKD
system.Comment: 9 page
Superconducting nanowire single-photon detectors: physics and applications
Single-photon detectors based on superconducting nanowires (SSPDs or SNSPDs)
have rapidly emerged as a highly promising photon-counting technology for
infrared wavelengths. These devices offer high efficiency, low dark counts and
excellent timing resolution. In this review, we consider the basic SNSPD
operating principle and models of device behaviour. We give an overview of the
evolution of SNSPD device design and the improvements in performance which have
been achieved. We also evaluate device limitations and noise mechanisms. We
survey practical refrigeration technologies and optical coupling schemes for
SNSPDs. Finally we summarize promising application areas, ranging from quantum
cryptography to remote sensing. Our goal is to capture a detailed snapshot of
an emerging superconducting detector technology on the threshold of maturity.Comment: 27 pages, 5 figures, Review article preprint versio
Investigation of Dark Counts in Innovative Materials for Superconducting Nanowire Single-photon Detector Applications
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
Fano fluctuations in superconducting-nanowire single-photon detectors
Because of their universal nature, Fano fluctuations are expected to influence the response of superconducting-nanowire single-photon detectors (SNSPDs). We predict that photon counting rate (PCR) as a function of bias current (IB) in SNSPDs is described by an integral over a transverse coordinate-dependent complementary error function. Fano fluctuations in the amount of energy deposited into the electronic system contribute to the finite width of this error function ΔIB. The local response of an SNSPD can also affect this width: the location of the initial photon absorption site across the width of the wire can impact the probability of vortex-antivortex unbinding and vortex entry from the edges. In narrow-nanowire SNSPDs, the local responses are uniform, and Fano fluctuations dominate ΔIB. We demonstrate good agreement between theory and experiments for a series of bath temperatures and photon energies in narrow-wire WSi SNSPDs. In a wide-nanowire device, the strong local dependence will introduce a finite width to the PCR curve, but with sharp cusps. We show how Fano fluctuations can smooth these features to produce theoretical curves that better match experimental data. We also show that the time-resolved hotspot relaxation curves predicted by Fano fluctuations match the previously measured Lorentzian shapes (except for their tails) over the entire range of bias currents investigated experimentally
Superconducting nanowire single-photon detectors for advanced photon-counting applications
The ability to detect infrared photons is increasingly important in many
elds of scienti c endeavour, including astronomy, the life sciences and
quantum information science. Improvements in detector performance
are urgently required. The Superconducting Nanowire Single-Photon
Detector (SNSPD/SSPD) is an emerging single-photon detector technology
o ering broadband sensitivity, negligible dark counts and picosecond
timing resolution. SNSPDs have the potential to outperform conventional
semiconductor-based photon-counting technologies, provided the
di culties of low temperature operation can be overcome. This thesis
describes how these important challenges have been addressed, enabling
the SNSPDs to be used in new applications. A multichannel SNSPD
system based on a closed-cycle refrigerator has been constructed and
tested. E cient optical coupling has been achieved via carefully aligned
optical bre. Fibre-coupled SNSPDs based on (i) NbN on MgO substrates
and (ii) NbTiN on oxidised Si substrates have been studied. The
latter give enhanced performance at telecom wavelengths, exploiting the
re
ection from the Si=SiO2 interface. Currently, the detector system
houses four NbTiN SNSPDs with average detection e ciency >20% at
1310 nm wavelength. We have employed SNSPDs in the characterisation
of quantum waveguide circuits, opening the pathway to operating this
promising platform for optical quantum computing for the first time at
telecom wavelengths
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