363 research outputs found
Impedance model for the polarization-dependent optical absorption of superconducting single-photon detectors
We measured the single-photon detection efficiency of NbN superconducting
single photon detectors as a function of the polarization state of the incident
light for different wavelengths in the range from 488 nm to 1550 nm. The
polarization contrast varies from ~5% at 488 nm to ~30% at 1550 nm, in good
agreement with numerical calculations. We use an optical-impedance model to
describe the absorption for polarization parallel to the wires of the detector.
For lossy NbN films, the absorption can be kept constant by keeping the product
of layer thickness and filling factor constant. As a consequence, we find that
the maximum possible absorption is independent of filling factor. By
illuminating the detector through the substrate, an absorption efficiency of
~70% can be reached for a detector on Si or GaAs, without the need for an
optical cavity.Comment: 15 pages, 5 figures, submitted to Journal of Applied Physic
Quantum Nature of Light Measured With a Single Detector
We realized the most fundamental quantum optical experiment to prove the
non-classical character of light: Only a single quantum emitter and a single
superconducting nanowire detector were used. A particular appeal of our
experiment is its elegance and simplicity. Yet its results unambiguously
enforce a quantum theory for light. Previous experiments relied on more complex
setups, such as the Hanbury-Brown-Twiss configuration, where a beam splitter
directs light to two photodetectors, giving the false impression that the beam
splitter is required. Our work results in a major simplification of the widely
used photon-correlation techniques with applications ranging from quantum
information processing to single-molecule detection.Comment: 7 page
Energy levels and lifetimes of Nd IV, Pm IV, Sm IV, and Eu IV
To address the shortage of experimental data for electron spectra of
triply-ionized rare earth elements we have calculated energy levels and
lifetimes of 4f{n+1} and 4f{n}5d configurations of Nd IV (n=2), Pm IV (n=3), Sm
IV (n=4), and Eu IV (n=5) using Hartree-Fock and configuration interaction
methods. To control the accuracy of our calculations we also performed similar
calculations for Pr III, Nd III and Sm III, for which experimental data are
available. The results are important, in particular, for physics of magnetic
garnets.Comment: 4 pages 1 tabl
Efficient and robust fiber coupling of superconducting single photon detectors
We applied a recently developed fiber coupling technique to superconducting
single photon detectors (SSPDs). As the detector area of SSPDs has to be kept
as small as possible, coupling to an optical fiber has been either inefficient
or unreliable. Etching through the silicon substrate allows fabrication of a
circularly shaped chip which self aligns to the core of a ferrule terminated
fiber in a fiber sleeve. In situ alignment at cryogenic temperatures is
unnecessary and no thermal stress during cooldown, causing misalignment, is
induced. We measured the quantum efficiency of these devices with an attenuated
tunable broadband source. The combination of a lithographically defined chip
and high precision standard telecommunication components yields near unity
coupling efficiency and a system detection efficiency of 34% at a wavelength of
1200 nm. This quantum efficiency measurement is confirmed by an absolute
efficiency measurement using correlated photon pairs (with = 1064 nm)
produced by spontaneous parametric down-conversion. The efficiency obtained via
this method agrees well with the efficiency measured with the attenuated
tunable broadband source
Correlated photon-pair generation in a periodically poled MgO doped stoichiometric lithium tantalate reverse proton exchanged waveguide
We demonstrate photon-pair generation in a reverse proton exchanged waveguide
fabricated on a periodically poled magnesium doped stoichiometric lithium
tantalate substrate. Detected pairs are generated via a cascaded second order
nonlinear process where a pump laser at wavelength of 1.55 m is first
doubled in frequency by second harmonic generation and subsequently
downconverted around the same spectral region. Pairs are detected at a rate of
42 per second with a coincidence to accidental ratio of 0.7. This cascaded pair
generation process is similar to four-wave-mixing where two pump photons
annihilate and create a correlated photon pair
The bdbDC operon of Bacillus subtilis encodes thiol-disulfide oxidoreductases required for competence development
The development of genetic competence in the Gram-positive eubacterium Bacillus subtilis is a complex postexponential process. Here we describe a new bicistronic operon, bdbDC, required for competence development, which was identified by the B. subtilis Systematic Gene Function Analysis program. Inactivation of either the bdbC or bdbD genes of this operon results in the loss of transformability without affecting recombination or the synthesis of ComK, the competence transcription factor. BdbC and BdbD are orthologs of enzymes known to be involved in extracytoplasmic disulfide bond formation. Consistent with this, BdbC and BdbD are needed for the secretion of theEscherichia coli disulfide bond-containing alkaline phosphatase, PhoA, by B. subtilis. Similarly, the amount of the disulfide bond-containing competence protein ComGC is severely reduced in bdbC or bdbD mutants. In contrast, the amounts of the competence proteins ComGA and ComEA remain unaffected by bdbDC mutations. Taken together, these observations imply that in the absence of either BdbC or BdbD, ComGC is unstable and that BdbC and BdbD catalyze the formation of disulfide bonds that are essential for the DNA binding and uptake machinery
Photon Pair Generation in Silicon Micro-Ring Resonator with Reverse Bias Enhancement
Photon sources are fundamental components for any quantum photonic
technology. The ability to generate high count-rate and low-noise correlated
photon pairs via spontaneous parametric down-conversion using bulk crystals has
been the cornerstone of modern quantum optics. However, future practical
quantum technologies will require a scalable integration approach, and
waveguide-based photon sources with high-count rate and low-noise
characteristics will be an essential part of chip-based quantum technologies.
Here, we demonstrate photon pair generation through spontaneous four-wave
mixing in a silicon micro-ring resonator, reporting a maximum
coincidence-to-accidental (CAR) ratio of 602 (+-) 37, and a maximum photon pair
generation rate of 123 MHz (+-) 11 KHz. To overcome free-carrier related
performance degradations we have investigated reverse biased p-i-n structures,
demonstrating an improvement in the pair generation rate by a factor of up to
2, with negligible impact on CAR.Comment: 5 pages, 3 figure
A gamma- and X-ray detector for cryogenic, high magnetic field applications
As part of an experiment to measure the spectrum of photons emitted in
beta-decay of the free neutron, we developed and operated a detector consisting
of 12 bismuth germanate (BGO) crystals coupled to avalanche photodiodes (APDs).
The detector was operated near liquid nitrogen temperature in the bore of a
superconducting magnet and registered photons with energies from 5 keV to 1000
keV. To enlarge the detection range, we also directly detected soft X-rays with
energies between 0.2 keV and 20 keV with three large area APDs. The
construction and operation of the detector is presented, as well as information
on operation of APDs at cryogenic temperatures
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