36 research outputs found
Problems With the Vortex-Boson Mapping in 1+1 Dimensions
Using the well known boson mapping, we relate the transverse magnetic
susceptibility of a system of flux vortices in 1+1 dimensions to an
appropriately defined conductivity of a one-dimensional boson system. The tilt
response for a system free of disorder is calculated directly, and it is found
that a subtle order of limits is required to avoid deceptive results.Comment: 4 Pages (REVTeX 3.0). Postscript file for this paper is available on
the World Wide Web at http://cmtw.harvard.edu/~simon/
An Ultra-Low Noise Telecom Wavelength Free Running Single Photon Detector Using Negative Feedback Avalanche Diode
It is challenging to implement genuine free running single photon detectors
for the 1550 nm wavelength range with simultaneously high detection efficiency
(DE), low dark noise, and good time resolution. We report a novel read out
system for the signals from a negative feedback avalanche diode (NFAD) which
allows useful operation of these devices at a temperature of 193 K and results
in very low dark counts (~100 CPS), good time jitter (~30 ps), and good DE
(~10%). We characterized two NFADs with a time correlation method using photons
generated from weak coherent pulses (WCP) and photon pairs produced by
spontaneous parametric down conversion (SPDC). The inferred detector
efficiencies for both types of photon sources agree with each other. The best
noise equivalent power of the device is estimated to be 8.1 x 10^(-18) W
Hz^(-1/2), more than 10 times better than typical InP/InGaAs SPADs show in free
running mode. The afterpulsing probability was found to be less than 0.1% per
ns at the optimized operating point. In addition, we studied the performance of
an entanglement-based quantum key distribution (QKD) using these detectors and
develop a model for the quantum bit error rate (QBER) that incorporates the
afterpulsing coefficients. We verified experimentally that using these NFADs it
is feasible to implement QKD over 400 km of telecom fibre. Our NFAD photon
detector system is very simple, and is well suited for single-photon
applications where ultra-low noise and free-running operation is required, and
some afterpulsing can be tolerated.Comment: 28 pages, 16 figures, and 1 tabl
Resistance of Josephson Junction Arrays at Low Temperatures
We study motion of vortices in arrays of Josephson junctions at zero
temperature where it is controlled by quantum tunneling from one plaquette to
another. The tunneling process is characterized by a finite time and can be
slow compared to the superconducting gap (so that ). The
dissipation which accompanies this process arises from rare processes when a
vortex excites a quasiparticle above the gap while tunneling through a single
junction. We find that the dissipation is significant even in the case , in particular it is not exponentially small in this parameter. We
use the calculated energy dissipation for the single vortex jump to estimate
the physical resistance of the whole array.Comment: 24 pages, LaTeX references added, to appear in PR
Anisotropic vortex pinning in superconductors with a square array of rectangular submicron holes
We investigate vortex pinning in thin superconducting films with a square
array of rectangular submicron holes ("antidots"). Two types of antidots are
considered: antidots fully perforating the superconducting film, and "blind
antidots", holes that perforate the film only up to a certain depth. In both
systems, we observe a distinct anisotropy in the pinning properties, reflected
in the critical current Ic, depending on the direction of the applied
electrical current: parallel to the long side of the antidots or perpendicular
to it. Although the mechanism responsible for the effect is very different in
the two systems, they both show a higher critical current and a sharper
IV-transition when the current is applied along the long side of the
rectangular antidots
Spatio-temporal dynamics and plastic flow of vortices in superconductors with periodic arrays of pinning sites
We present simulations of flux-gradient-driven superconducting rigid vortices
interacting with square and triangular arrays of columnar pinning sites in an
increasing external magnetic field. These simulations allow us to
quantitatively relate spatio-temporal microscopic information of the vortex
lattice with typically measured macroscopic quantities, such as the
magnetization . The flux lattice does not become completely commensurate
with the pinning sites throughout the sample at the magnetization matching
peaks, but forms a commensurate lattice in a region close to the edge of the
sample. Matching fields related to unstable vortex configurations do not
produce peaks in . We observe a variety of evolving complex flux
profiles, including flat terraces or plateaus separated by winding
current-carrying strings and, near the peaks in , plateaus only in
certain regions, which move through the sample as the field increases
Linear and nonlinear excitations in two stacks of parallel arrays of long Josephson junctions
Quantum Interference in Superconducting Wire Networks and Josephson Junction Arrays: Analytical Approach based on Multiple-Loop Aharonov-Bohm Feynman Path-Integrals
We investigate analytically and numerically the mean-field
superconducting-normal phase boundaries of two-dimensional superconducting wire
networks and Josephson junction arrays immersed in a transverse magnetic field.
The geometries we consider include square, honeycomb, triangular, and kagome'
lattices. Our approach is based on an analytical study of multiple-loop
Aharonov-Bohm effects: the quantum interference between different electron
closed paths where each one of them encloses a net magnetic flux. Specifically,
we compute exactly the sums of magnetic phase factors, i.e., the lattice path
integrals, on all closed lattice paths of different lengths. A very large
number, e.g., up to for the square lattice, exact lattice path
integrals are obtained. Analytic results of these lattice path integrals then
enable us to obtain the resistive transition temperature as a continuous
function of the field. In particular, we can analyze measurable effects on the
superconducting transition temperature, , as a function of the magnetic
filed , originating from electron trajectories over loops of various
lengths. In addition to systematically deriving previously observed features,
and understanding the physical origin of the dips in as a result of
multiple-loop quantum interference effects, we also find novel results. In
particular, we explicitly derive the self-similarity in the phase diagram of
square networks. Our approach allows us to analyze the complex structure
present in the phase boundaries from the viewpoint of quantum interference
effects due to the electron motion on the underlying lattices.Comment: 18 PRB-type pages, plus 8 large figure
High Speed and High Efficiency Travelling Wave Single-Photon Detectors Embedded in Nanophotonic Circuits
Ultrafast, high quantum efficiency single photon detectors are among the most
sought-after elements in modern quantum optics and quantum communication. High
photon detection efficiency is essential for scalable measurement-based quantum
computation, quantum key distribution, and loophole-free Bell experiments.
However, imperfect modal matching and finite photon absorption rates have
usually limited the maximum attainable detection efficiency of single photon
detectors. Here we demonstrate a superconducting nanowire detector atop
nanophotonic waveguides which allows us to drastically increase the absorption
length for incoming photons. When operating the detectors close to the critical
current we achieve high on-chip single photon detection efficiency up to 91% at
telecom wavelengths, with uncertainty dictated by the variation of the
waveguide photon flux. We also observe remarkably low dark count rates without
significant compromise of detection efficiency. Furthermore, our detectors are
fully embedded in a scalable silicon photonic circuit and provide ultrashort
timing jitter of 18ps. Exploiting this high temporal resolution we demonstrate
ballistic photon transport in silicon ring resonators. The direct
implementation of such a detector with high quantum efficiency, high detection
speed and low jitter time on chip overcomes a major barrier in integrated
quantum photonics
Superconducting single photon detectors integrated with diamond nanophotonic circuits
Photonic quantum technologies promise to repeat the success of integrated
nanophotonic circuits in non-classical applications. Using linear optical
elements, quantum optical computations can be performed with integrated optical
circuits and thus allow for overcoming existing limitations in terms of
scalability. Besides passive optical devices for realizing photonic quantum
gates, active elements such as single photon sources and single photon
detectors are essential ingredients for future optical quantum circuits.
Material systems which allow for the monolithic integration of all components
are particularly attractive, including III-V semiconductors, silicon and also
diamond. Here we demonstrate nanophotonic integrated circuits made from high
quality polycrystalline diamond thin films in combination with on-chip single
photon detectors. Using superconducting nanowires coupled evanescently to
travelling waves we achieve high detection efficiencies up to 66 % combined
with low dark count rates and timing resolution of 190 ps. Our devices are
fully scalable and hold promise for functional diamond photonic quantum
devices.Comment: 28 pages, 5 figure