2,503 research outputs found
Mesoporous matrices for quantum computation with improved response through redundance
We present a solid state implementation of quantum computation, which improves previously proposed optically driven schemes. Our proposal is based on vertical arrays of quantum dots embedded in a mesoporous material which can be fabricated with present technology. The redundant encoding typical of the chosen hardware protects the computation against gate errors and the effects of measurement induced noise. The system parameters required for quantum computation applications are calculated for II-VI and III-V materials and found to be within the experimental range. The proposed hardware may help minimize errors due to polydispersity of dot sizes, which is at present one of the main problems in relation to quantum dot-based quantum computation. (c) 2007 American Institute of Physics
Towards optimized suppression of dephasing in systems subject to pulse timing constraints
We investigate the effectiveness of different dynamical decoupling protocols
for storage of a single qubit in the presence of a purely dephasing bosonic
bath, with emphasis on comparing quantum coherence preservation under uniform
vs. non-uniform delay times between pulses. In the limit of instantaneous
bit-flip pulses, this is accomplished by establishing a new representation of
the controlled qubit evolution, where the resulting decoherence behaviour is
directly expressed in terms of the free evolution. Simple analytical
expressions are given to approximate the long- and short- term coherence
behaviour for both ohmic and supra-ohmic environments. We focus on systems with
physical constraints on achievable time delays, with emphasis on pure dephasing
of excitonic qubits in quantum dots. Our analysis shows that little advantage
of high-level decoupling schemes based on concatenated or optimal design is to
be expected if operational constraints prevent pulses to be applied
sufficiently fast. In such constrained scenarios, we demonstrate how simple
modifications of repeated periodic echo protocols can offer significantly
improved coherence preservation in realistic parameter regimes.Comment: 13 figures,1 tabl
Coulomb interaction effects in spin-polarized transport
We study the effect of the electron-electron interaction on the transport of
spin polarized currents in metals and doped semiconductors in the diffusive
regime. In addition to well-known screening effects, we identify two additional
effects, which depend on many-body correlations and exchange and reduce the
spin diffusion constant. The first is the "spin Coulomb drag" - an intrinsic
friction mechanism which operates whenever the average velocities of up-spin
and down-spin electrons differ. The second arises from the decrease in the
longitudinal spin stiffness of an interacting electron gas relative to a
noninteracting one. Both effects are studied in detail for both degenerate and
non-degenerate carriers in metals and semiconductors, and various limiting
cases are worked out analytically. The behavior of the spin diffusion constant
at and below a ferromagnetic transition temperature is also discussed.Comment: 9 figure
Photoluminescence dispersion as a probe of structural inhomogeneity in silica
We report time-resolved photoluminescence spectra of point defects in
amorphous silicon dioxide (silica), in particular the decay kinetics of the
emission signals of extrinsic Oxygen Deficient Centres of the second type from
singlet and directly-excited triplet states are measured and used as a probe of
structural inhomogeneity. Luminescence activity in sapphire
(-AlO) is studied as well and used as a model system to compare
the optical properties of defects in silica with those of defects embedded in a
crystalline matrix. Only for defects in silica, we observe a variation of the
decay lifetimes with emission energy and a time dependence of the first moment
of the emission bands. These features are analyzed within a theoretical model
with explicit hypothesis about the effect introduced by the disorder of
vitreous systems. Separate estimations of the homogenous and inhomogeneous
contributions to the measured emission linewidth are obtained: it is found that
inhomogeneous effects strongly condition both the triplet and singlet
luminescence activities of oxygen deficient centres in silica, although the
degree of inhomogeneity of the triplet emission turns out to be lower than that
of the singlet emission. Inhomogeneous effects appear to be negligible in
sapphire
Detection of ionized gas in the globular cluster 47 Tucanae
We report the detection of ionized intracluster gas in the globular cluster
47 Tucanae. Pulsars in this cluster with a negative period derivative, which
must lie in the distant half of the cluster, have significantly higher measured
integrated electron column densities than the pulsars with a positive period
derivative. We derive the plasma density within the central few pc of the
cluster using two different methods which yield consistent values. Our best
estimate of n_e = (0.067+-0.015)/cm^3 is about 100 times the free electron
density of the ISM in the vicinity of 47 Tucanae, and the ionized gas is
probably the dominant component of the intracluster medium.Comment: 5 pages, 3 included figures, accepted for publication by ApJ Letter
Timing of Millisecond Pulsars in NGC 6752: Evidence for a High Mass-to-Light Ratio in the Cluster Core
Using pulse timing observations we have obtained precise parameters,
including positions with about 20 mas accuracy, of five millisecond pulsars in
NGC 6752. Three of them, located relatively close to the cluster center, have
line-of-sight accelerations larger than the maximum value predicted by the
central mass density derived from optical observation, providing dynamical
evidence for a central mass-to-light ratio >~ 10, much higher than for any
other globular cluster. It is likely that the other two millisecond pulsars
have been ejected out of the core to their present locations at 1.4 and 3.3
half-mass radii, respectively, suggesting unusual non-thermal dynamics in the
cluster core.Comment: Accepted by ApJ Letter. 5 pages, 2 figures, 1 tabl
Label-free electrochemical immunosensor as a reliable point-of-care device for the detection of Interleukin-6 in serum samples from patients with psoriasis
interleukin-6 (IL-6) plays a crucial role in autoimmunity and chronic inflammation. this study aims to develop a low-cost, simple-to-manufacture, and user-friendly label-free electrochemical point-of-care device for the rapid detection of IL-6 in patients with psoriasis. precisely, a sandwich-based format immunosensor was developed using two primary antibodies (mAb-IL6 clone-5 and clone-7) and screen-printed electrodes modified with an inexpensive recycling electrochemical enhancing material, called biochar. mAb-IL6 clone-5 was used as a covalently immobilized capture bioreceptor on modified electrodes, and mAb-IL6 clone-7 was used to recognize the immunocomplex (Anti-IL6 clone-5 and IL-6) and form the sandwich. cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to conduct electrochemical characterization of the layer-by-layer assembly of the immunosensor, while square wave voltammetry (SWV) was used to perform the sensing. the developed immunosensor demonstrated robust analytical performance in buffer solution, with a wide linear range (LR) by varying from 2 to 250 pg/mL, a good limit of detection (LOD) of 0.78 pg/mL and reproducibility (RSD<7%). In addition, a spectrophotometric ELISA kit was employed to validate the results obtained with the label-free device by analyzing twenty-five serum samples from control and patients affected by psoriasis. a strong correlation in terms of pg/mL concentration of IL-6 was found comparing the two methods, with the advantage for our label-free biosensor of an ease use and a quicker detection time. based on IL-6 levels, the proposed immunosensor is a dependable, non-invasive screening device capable of predicting disease onset, progression, and treatment efficacy
Dynamical Formation of Millisecond Pulsars in Globular Clusters
The cumulative luminosity distribution functions (CLFs) of radio millisecond
pulsars (MSPs) in globular clusters (GCs) and in the Galactic field at a
frequency of 1.4 GHz have been examined. Assuming a functional form, where is the number of MSPs and is the luminosity at 1.4 GHz, it
is found that the CLFs significantly differ with a steeper slope, , in GCs than in the Galactic field (), suggesting a
different formation or evolutionary history of MSPs in these two regions of the
Galaxy. To probe the production mechanism of MSPs in clusters, a search of the
possible relationships between the MSP population and cluster properties was
carried out. The results of an investigation of 9 GCs indicate positive
correlations between the MSP population and the stellar encounter rate and
metallicity. This provides additional evidence suggesting that stellar
dynamical interactions are important in the formation of the MSP population in
GCs.Comment: 18 pages, 3 figures, 4 tables, accepted for publication in Ap
PSR J1016-5857: a young radio pulsar with possible supernova remnant, X-ray, and gamma-ray associations
We report the discovery of a young and energetic pulsar in the Parkes
multibeam survey of the Galactic plane. PSR J1016-5857 has a rotation period of
107 ms and period derivative of 8e-14, implying a characteristic age of 21 kyr
and spin-down luminosity of 2.6e36 erg/s. The pulsar is located just outside,
and possibly interacting with, the shell supernova remnant G284.3-1.8. Archival
X-ray data show a source near the pulsar position which is consistent with
emission from a pulsar wind nebula. The pulsar is also located inside the error
box of the unidentified EGRET source 3EG J1013-5915, for which it represents a
plausible counterpart.Comment: 5 pages, 3 included figures, accepted for publication by ApJ Letter
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