151 research outputs found
A -Vertex Kernel for Maximum Internal Spanning Tree
We consider the parameterized version of the maximum internal spanning tree
problem, which, given an -vertex graph and a parameter , asks for a
spanning tree with at least internal vertices. Fomin et al. [J. Comput.
System Sci., 79:1-6] crafted a very ingenious reduction rule, and showed that a
simple application of this rule is sufficient to yield a -vertex kernel.
Here we propose a novel way to use the same reduction rule, resulting in an
improved -vertex kernel. Our algorithm applies first a greedy procedure
consisting of a sequence of local exchange operations, which ends with a
local-optimal spanning tree, and then uses this special tree to find a
reducible structure. As a corollary of our kernel, we obtain a deterministic
algorithm for the problem running in time
Zeeman energy and spin relaxation in a one-electron quantum dot
We have measured the relaxation time, T1, of the spin of a single electron
confined in a semiconductor quantum dot (a proposed quantum bit). In a magnetic
field, applied parallel to the two-dimensional electron gas in which the
quantum dot is defined, Zeeman splitting of the orbital states is directly
observed by measurements of electron transport through the dot. By applying
short voltage pulses, we can populate the excited spin state with one electron
and monitor relaxation of the spin. We find a lower bound on T1 of 50
microseconds at 7.5 T, only limited by our signal-to-noise ratio. A continuous
measurement of the charge on the dot has no observable effect on the spin
relaxation.Comment: Replaced with the version published in Phys. Rev. Let
Ground-state characterization of Nb charge-phase Josephson qubits
We present investigations of Josephson charge-phase qubits inductively
coupled to a radio-frequency driven tank-circuit enabling the readout of the
states by measuring the Josephson inductance of the qubit. The circuits
including junctions with linear dimensions of 60 nm and 80 nm are fabricated
from Nb trilayer and allowing the determination of relevant sample parameters
at liquid helium temperature. The observed partial suppression of the
circulating supercurrent at 4.2 K is explained in the framework of a quantum
statistical model. We have probed the ground-state properties of qubit
structures with different ratios of the Josephson coupling to Coulomb charging
energy at 20 mK, demonstrating both the magnetic control of phase and the
electrostatic control of charge on the qubit island.Comment: 8 pages, 8 figure
Strong quantum memory at resonant Fermi edges revealed by shot noise
Studies of non-equilibrium current fluctuations enable assessing correlations
involved in quantum transport through nanoscale conductors. They provide
additional information to the mean current on charge statistics and the
presence of coherence, dissipation, disorder, or entanglement. Shot noise,
being a temporal integral of the current autocorrelation function, reveals
dynamical information. In particular, it detects presence of non-Markovian
dynamics, i.e., memory, within open systems, which has been subject of many
current theoretical studies. We report on low-temperature shot noise
measurements of electronic transport through InAs quantum dots in the
Fermi-edge singularity regime and show that it exhibits strong memory effects
caused by quantum correlations between the dot and fermionic reservoirs. Our
work, apart from addressing noise in archetypical strongly correlated system of
prime interest, discloses generic quantum dynamical mechanism occurring at
interacting resonant Fermi edges.Comment: 6 pages, 3 figure
Shot noise in resonant tunneling through a zero-dimensional state with a complex energy spectrum
We investigate the noise properties of a GaAs/AlGaAs resonant tunneling
structure at bias voltages where the current characteristic is determined by
single electron tunneling. We discuss the suppression of the shot noise in the
framework of a coupled two-state system. For large bias voltages we observed
super-Poissonian shot noise up to values of the Fano factor .Comment: 4 pages, 4 figures, accepted for Phys. Rev.
Spectral Intensity Bioaerosol Sensor (SIBS): an instrument for spectrally resolved fluorescence detection of single particles in real time
Primary biological aerosol particles (PBAPs) in the atmosphere are highly relevant
for the Earth system, climate, and public health. The analysis of PBAPs,
however, remains challenging due to their high diversity and large
spatiotemporal variability. For real-time PBAP analysis, light-induced
fluorescence (LIF) instruments have been developed and widely used in
laboratory and ambient studies. The interpretation of fluorescence data from
these instruments, however, is often limited by a lack of spectroscopic
information. This study introduces an instrument – the Spectral Intensity
Bioaerosol Sensor (SIBS; Droplet Measurement Technologies (DMT), Longmont,
CO, USA) – that resolves fluorescence spectra for single particles and thus
promises to expand the scope of fluorescent PBAP quantification and
classification.
The SIBS shares key design components with the latest versions of the
Wideband Integrated Bioaerosol Sensor (WIBS) and the findings presented here
are also relevant for the widely deployed WIBS-4A and WIBS-NEO as well as
other LIF instruments. The key features of the SIBS and the findings of this
study can be summarized as follows.
Particle sizing yields reproducible linear responses for particles in the
range of 300 nm to 20 µm. The lower sizing limit is significantly
smaller than for earlier commercial LIF instruments (e.g., WIBS-4A and the
Ultraviolet Aerodynamic Particle Sizer; UV-APS), expanding the analytical
scope into the accumulation-mode size range.
Fluorescence spectra are recorded for two excitation wavelengths (λex=285 and 370 nm) and a wide range of emission wavelengths
(λmean=302–721 nm) with a resolution of 16
detection channels, which is higher than for most other commercially
available LIF bioaerosol sensors.
Fluorescence spectra obtained for 16 reference compounds confirm that the
SIBS provides sufficient spectral resolution to distinguish major modes of
molecular fluorescence. For example, the SIBS resolves the spectral
difference between bacteriochlorophyll and chlorophyll a and b.
A spectral correction of the instrument-specific detector response is
essential to use the full fluorescence emission range.
Asymmetry factor (AF) data were assessed and were found to provide only
limited analytical information.
In test measurements with ambient air, the SIBS worked reliably and yielded
characteristically different spectra for single particles in the coarse mode
with an overall fluorescent particle fraction of ∼4 %
(3σ threshold), which is consistent with earlier studies in
comparable environments.</ul
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