895 research outputs found
Effects of noise on quantum error correction algorithms
It has recently been shown that there are efficient algorithms for quantum
computers to solve certain problems, such as prime factorization, which are
intractable to date on classical computers. The chances for practical
implementation, however, are limited by decoherence, in which the effect of an
external environment causes random errors in the quantum calculation. To combat
this problem, quantum error correction schemes have been proposed, in which a
single quantum bit (qubit) is ``encoded'' as a state of some larger number of
qubits, chosen to resist particular types of errors. Most such schemes are
vulnerable, however, to errors in the encoding and decoding itself. We examine
two such schemes, in which a single qubit is encoded in a state of qubits
while subject to dephasing or to arbitrary isotropic noise. Using both
analytical and numerical calculations, we argue that error correction remains
beneficial in the presence of weak noise, and that there is an optimal time
between error correction steps, determined by the strength of the interaction
with the environment and the parameters set by the encoding.Comment: 26 pages, LaTeX, 4 PS figures embedded. Reprints available from the
authors or http://eve.physics.ox.ac.uk/QChome.htm
The Flux Variability of Markarian 501 in Very High Energy Gamma Rays
The BL Lacertae object Markarian 501 was identified as a source of gamma-ray
emission at the Whipple Observatory in March 1995. Here we present a flux
variability analysis on several times-scales of the 233 hour data set
accumulated over 213 nights (from March 1995 to July 1998) with the Whipple
Observatory 10 m atmospheric Cherenkov imaging telescope. In 1995, with the
exception of a single night, the flux from Markarian 501 was constant on daily
and monthly time-scales and had an average flux of only 10% that of the Crab
Nebula, making it the weakest VHE source detected to date. In 1996, the average
flux was approximately twice the 1995 flux and showed significant
month-to-month variability. No significant day-scale variations were detected.
The average gamma-ray flux above ~350 GeV in the 1997 observing season rose to
1.4 times that of the Crab Nebula -- 14 times the 1995 discovery level --
allowing a search for variability on time-scales shorter than one day.
Significant hour-scale variability was present in the 1997 data, with the
shortest, observed on MJD 50607, having a doubling time of ~2 hours. In 1998
the average emission level decreased considerably from that of 1997 (to ~20% of
the Crab Nebula flux) but two significant flaring events were observed. Thus,
the emission from Markarian 501 shows large amplitude and rapid flux
variability at very high energies as does Markarian 421. It also shows large
mean flux level variations on year-to-year time-scales, behaviour which has not
been seen from Markarian 421 so far.Comment: 19 pages, 3 figures, to appear in ApJ, June 20, 1999, Vol. 518 #
Identifying the mechanisms underpinning recognition of structured sequences of action
© 2012 The Experimental Psychology SocietyWe present three experiments to identify the specific information sources that skilled participants use to make recognition judgements when presented with dynamic, structured stimuli. A group of less skilled
participants acted as controls. In all experiments, participants were presented with filmed stimuli containing structured action sequences. In a subsequent recognition phase, participants were presented with new and previously seen stimuli and were required to make judgements as to whether or not each sequence had been presented earlier (or were edited versions of earlier sequences). In Experiment 1,
skilled participants demonstrated superior sensitivity in recognition when viewing dynamic clips compared with static images and clips where the frames were presented in a nonsequential, randomized manner, implicating the importance of motion information when identifying familiar or unfamiliar sequences. In Experiment 2, we presented normal and mirror-reversed sequences in order to distort access to absolute motion information. Skilled participants demonstrated superior recognition sensitivity, but no significant differences were observed across viewing conditions, leading to the suggestion
that skilled participants are more likely to extract relative rather than absolute motion when making such judgements. In Experiment 3, we manipulated relative motion information by occluding several display
features for the duration of each film sequence. A significant decrement in performance was reported when centrally located features were occluded compared to those located in more peripheral positions.
Findings indicate that skilled participants are particularly sensitive to relative motion information when attempting to identify familiarity in dynamic, visual displays involving interaction between numerous features
Elevated MCP-1, TNF-α, Monocyte Concentration, and Dyslipidemia in Obese Mexican-American Children
Background: Obesity is an independent risk factor for chronic disease. The prevalence of obesity is especially high among Mexican-American children. Peripheral blood monocytes contribute to systemic inflammation and may mediate the relationship between obesity and chronic disease. Obesity affects monocytes and the circulating levels of cytokines/chemokines that influence monocyte behavior. Purpose: investigate alterations in blood monocytes and plasma cytokines/chemokine levels among healthy weight (zBMI †85th percentile; N=66), overweight (zBMI=85th-95th percentile; N=23), and obese (zBMI ℠95th percentile; N=39) Mexican-American children. Methods: Blood samples were analyzed for total monocyte concentration, pro-inflammatory monocyte concentration, and classic monocyte concentration via flow cytometry. Serum MCP-1, Fractalkine, IL-8, and TNF-α were measured using a Milliplex MagPix assay. Serum cholesterol, HDL, triglycerides, and glucose were measured using an enzymatic reagent kit. Results: Total monocyte concentration (P=0.012), classic monocyte concentration (P=0.045), MCP-1 (P=0.015), and TNF- α (P=0.002) were significantly greater in obese children compared to heathly weight children. Also, overweight and obese children had elevated triglycerides (P=0.001) and reduced HDL (P=0.033) compared to healthy weight children. Conclusion: Elevations in circulating monocytes, MCP-1, and TNF-α have been implicated in the development of obesity-related chronic disease in adults. Childhood obesity alters monocytes and circulating chemokines, putting children at a greater risk of developing obesity-related chronic diseases in adulthood. Further characterization of early immune alterations in childhood obesity may provide additional clinical insight into the assessment of obesity-related disease risk
TeV Observations of the Variability and Spectrum of Markarian 501
Markarian 501 is only the second extragalactic source to be detected with
high statistical certainty at TeV energies; it is similar in many ways to
Markarian 421. The Whipple Observatory gamma-ray telescope has been used to
observe the AGN Markarian 501 in 1996 and 1997, the years subsequent to its
initial detection. The apparent variability on the one-day time-scale observed
in TeV gamma rays in 1995 is confirmed and compared with the variability in
Markarian 421. Observations at X-ray and optical wavelengths from 1997 are also
presented.Comment: 4 pages, 2 figures, to appear in proceedings of 25th ICRC (Durban
Dynamical Stability and Quantum Chaos of Ions in a Linear Trap
The realization of a paradigm chaotic system, namely the harmonically driven
oscillator, in the quantum domain using cold trapped ions driven by lasers is
theoretically investigated. The simplest characteristics of regular and chaotic
dynamics are calculated. The possibilities of experimental realization are
discussed.Comment: 24 pages, 17 figures, submitted to Phys. Rev
Tev Observations of the Variability and Spectrum of Markarian 421
Markarian 421 was the first extragalactic source to be detected with high
statistical certainty at TeV energies. The Whipple Observatory gamma-ray
telescope has been used to observe the Active Galactic Nucleus, Markarian 421
in 1996 and 1997. The rapid variability observed in TeV gamma rays in previous
years is confirmed. Doubling times as short as 15 minutes are reported with
flux levels reaching 15 photons per minute. The TeV energy spectrum is derived
using two independent methods. The implications for the intergalactic infra-red
medium of an observed unbroken power law spectrum up to energies of 5 TeV is
discussed.Comment: 4 pages, 4 figures, to appear in proceedings of 25 ICRC (Durban
Quantum Revivals in Periodically Driven Systems close to nonlinear resonance
We calculate the quantum revival time for a wave-packet initially well
localized in a one-dimensional potential in the presence of an external
periodic modulating field. The dependence of the revival time on various
parameters of the driven system is shown analytically. As an example of
application of our approach, we compare the analytically obtained values of the
revival time for various modulation strengths with the numerically computed
ones in the case of a driven gravitational cavity. We show that they are in
very good agreement.Comment: 14 pages, 1 figur
Childhood Overweight is Associated with Increased Monocyte Concentration and Altered Subset Distribution
Childhood obesity rates have nearly tripled in the last 30 years. Obesity increases risk for chronic disease. While monocytes, cells of the innate immune system that are altered with obesity, are purported to play an integral role in the development of these chronic diseases, no research has focused on early phenotypic changes in monocytes of overweight children. Two monocytes subsets exist, classic and pro-inflammatory; alterations in number and distribution may be implicated in disease development in obesity. The purpose of this study was to examine the concentration and relative distribution of monocytes among ânormal weightâ (N=66) and âat risk for being overweight/overweightâ (N=56) Mexican American children. Blood samples were collected and analyzed for total monocyte concentration and monocyte subset concentration via flow cytometry. Total monocyte concentration, as well as the concentration of both classic and pro-inflammatory monocyte subsets was significantly greater in the âat risk for overweight/overweightâ children (P\u3e0.05). Understanding early alterations in monocyte populations will be the first step in the development of early diagnosis and treatment techniques
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