15,571 research outputs found
On the Key-Uncertainty of Quantum Ciphers and the Computational Security of One-way Quantum Transmission
We consider the scenario where Alice wants to send a secret (classical)
-bit message to Bob using a classical key, and where only one-way
transmission from Alice to Bob is possible. In this case, quantum communication
cannot help to obtain perfect secrecy with key length smaller then . We
study the question of whether there might still be fundamental differences
between the case where quantum as opposed to classical communication is used.
In this direction, we show that there exist ciphers with perfect security
producing quantum ciphertext where, even if an adversary knows the plaintext
and applies an optimal measurement on the ciphertext, his Shannon uncertainty
about the key used is almost maximal. This is in contrast to the classical case
where the adversary always learns bits of information on the key in a known
plaintext attack. We also show that there is a limit to how different the
classical and quantum cases can be: the most probable key, given matching
plain- and ciphertexts, has the same probability in both the quantum and the
classical cases. We suggest an application of our results in the case where
only a short secret key is available and the message is much longer.Comment: 19 pages, 2 figures. This is a revised version of an earlier version
that appeared in the proc. of Eucrocrypt'04:LNCS3027, 200
Foreground removal from WMAP 5yr temperature maps using an MLP neural network
One of the main obstacles for extracting the cosmic microwave background
(CMB) signal from observations in the mm/sub-mm range is the foreground
contamination by emission from Galactic component: mainly synchrotron,
free-free, and thermal dust emission. The statistical nature of the intrinsic
CMB signal makes it essential to minimize the systematic errors in the CMB
temperature determinations. The feasibility of using simple neural networks to
extract the CMB signal from detailed simulated data has already been
demonstrated. Here, simple neural networks are applied to the WMAP 5yr
temperature data without using any auxiliary data. A simple \emph{multilayer
perceptron} neural network with two hidden layers provides temperature
estimates over more than 75 per cent of the sky with random errors
significantly below those previously extracted from these data. Also, the
systematic errors, i.e.\ errors correlated with the Galactic foregrounds, are
very small. With these results the neural network method is well prepared for
dealing with the high - quality CMB data from the ESA Planck Surveyor
satellite.Comment: 6 pages, 13 figure
Fighting Decoherence by Feedback-controlled Dissipation
Repeated closed-loop control operations acting as piecewise-constant
Liouville superoperators conditioned on the outcomes of regularly performed
measurements may effectively be described by a fixed-point iteration for the
density matrix. Even when all Liouville superoperators point to the completely
mixed state, feedback of the measurement result may lead to a pure state, which
can be interpreted as selective dampening of undesired states. Using a
microscopic model, we exemplify this for a single qubit, which can be purified
in an arbitrary single-qubit state by tuning the measurement direction and two
qubits that may be purified towards a Bell state by applying a special
continuous two-local measurement. The method does not require precise knowledge
of decoherence channels and works for large reservoir temperatures provided
measurement, processing, and control can be implemented in a continuous
fashion.Comment: to appear in PR
Deep rest-frame far-UV spectroscopy of the giant Lyman-alpha emitter 'Himiko'
We present deep 10h VLT/XSHOOTER spectroscopy for an extraordinarily luminous
and extended Lya emitter at z=6.595 referred to as Himiko and first discussed
by Ouchi et al. (2009), with the purpose of constraining the mechanisms
powering its strong emission. Complementary to the spectrum, we discuss NIR
imaging data from the CANDELS survey. We find neither for HeII nor any metal
line a significant excess, with 3 sigma upper limits of 6.8, 3.1, and
5.8x10^{-18} erg/s/cm^2 for CIV 1549, HeII 1640, CIII]
1909, respectively, assuming apertures with 200 km/s widths and offset
by -250 km/s w.r.t to the peak Lya redshift. These limits provide strong
evidence that an AGN is not a major contribution to Himiko's Lya flux. Strong
conclusions about the presence of PopIII star-formation or gravitational
cooling radiation are not possible based on the obtained HeII upper limit. Our
Lya spectrum confirms both spatial extent and flux (8.8+/-0.5x10^{-17}
erg/s/cm^2) of previous measurements. In addition, we can unambiguously exclude
any remaining chance of it being a lower redshift interloper by significantly
detecting a continuum redwards of Lya, while being undetected bluewards
A spiral-like disk of ionized gas in IC 1459: Signature of a merging collision
The authors report the discovery of a large (15 kpc diameter) H alpha + (NII) emission-line disk in the elliptical galaxy IC 1459, showing weak spiral structure. The line flux peaks strongly at the nucleus and is more concentrated than the stellar continuum. The major axis of the disk of ionized gas coincides with that of the stellar body of the galaxy. The mass of the ionized gas is estimated to be approx. 1 times 10 (exp 5) solar mass, less than 1 percent of the total mass of gas present in IC 1459. The total gas mass of 4 times 10(exp 7) solar mass has been estimated from the dust mass derived from a broad-band color index image and the Infrared Astronomy Satellite (IRAS) data. The authors speculate that the presence of dust and gas in IC 1459 is a signature of a merger event
Non-markovian dynamics of double quantum dot charge qubit with static bias
The dynamics of charge qubit in double quantum dot coupled to phonons is
investigated theoretically. The static bias is considered. By means of the
perturbation approach based on unitary transformations, the dynamical tunneling
current is obtained explicitly. The biased system displays broken symmetry and
a significantly larger coherence-incoherence transition critical point . We also analyzed the decoherence induced by piezoelectric coupling
phonons in detail. The results show that reducing the coupling between system
and bath make coherence frequency increase and coherence time prolong. To
maintain quantum coherence, applying static bias also is a good means.Comment: 13 pages, 5 figure
ISO far-infrared observations of rich galaxy clusters II. Sersic 159-03
The far-infrared emission from rich galaxy clusters is investigated. Maps
have been obtained by ISO at 60, 100, 135, and 200 microns using the PHT-C
camera. Ground based imaging and spectroscopy were also acquired. Here we
present the results for the cooling flow cluster Sersic 159-03. An infrared
source coincident with the dominant cD galaxy is found. Some off-center sources
are also present, but without any obvious counterparts.Comment: 6 pages, 4 postscript figures, accepted for publication in `Astronomy
and Astrophysics
Distinguishing two single-mode Gaussian states by homodyne detection: An information-theoretic approach
It is known that the quantum fidelity, as a measure of the closeness of two
quantum states, is operationally equivalent to the minimal overlap of the
probability distributions of the two states over all possible POVMs; the POVM
realizing the minimum is optimal. We consider the ability of homodyne detection
to distinguish two single-mode Gaussian states, and investigate to what extent
it is optimal in this information-theoretic sense. We completely identify the
conditions under which homodyne detection makes an optimal distinction between
two single-mode Gaussian states of the same mean, and show that if the Gaussian
states are pure, they are always optimally distinguished.Comment: 6 pages, 4 figures, published version with a detailed discussio
Deep far infrared ISOPHOT survey in "Selected Area 57", I. Observations and source counts
We present here the results of a deep survey in a 0.4 sq.deg. blank field in
Selected Area 57 conducted with the ISOPHOT instrument aboard ESAs Infrared
Space Observatory (ISO) at both 60 um and 90 um. The resulting sky maps have a
spatial resolution of 15 x 23 sq.arcsec. per pixel which is much higher than
the 90 x 90 sq.arcsec. pixels of the IRAS All Sky Survey. We describe the main
instrumental effects encountered in our data, outline our data reduction and
analysis scheme and present astrometry and photometry of the detected point
sources. With a formal signal to noise ratio of 6.75 we have source detection
limits of 90 mJy at 60 um and 50 mJy at 90 um. To these limits we find
cumulated number densities of 5+-3.5 per sq.deg. at 60 um and 14.8+-5.0 per
sq.deg.at 90 um. These number densities of sources are found to be lower than
previously reported results from ISO but the data do not allow us to
discriminate between no-evolution scenarios and various evolutionary models.Comment: 15 pages, 11 figures, accepted by Astronomy & Astrophysic
A practical scheme for error control using feedback
We describe a scheme for quantum error correction that employs feedback and
weak measurement rather than the standard tools of projective measurement and
fast controlled unitary gates. The advantage of this scheme over previous
protocols (for example Ahn et. al, PRA, 65, 042301 (2001)), is that it requires
little side processing while remaining robust to measurement inefficiency, and
is therefore considerably more practical. We evaluate the performance of our
scheme by simulating the correction of bit-flips. We also consider
implementation in a solid-state quantum computation architecture and estimate
the maximal error rate which could be corrected with current technology.Comment: 12 pages, 3 figures. Minor typographic change
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