117 research outputs found
Hypothesis testing for Gaussian states on bosonic lattices
The asymptotic state discrimination problem with simple hypotheses is
considered for a cubic lattice of bosons. A complete solution is provided for
the problems of the Chernoff and the Hoeffding bounds and Stein's lemma in the
case when both hypotheses are gauge-invariant Gaussian states with
translation-invariant quasi-free parts.Comment: 22 pages, submitted versio
Multifrequency nature of the 0.75 mHz feature in the X-ray light curves of the nova V4743 Sgr
We present timing analyses of eight X-ray light curves and one optical/UV
light curve of the nova V4743 Sgr (2002) taken by CHANDRA and XMM on days after
outburst: 50 (early hard emission phase), 180, 196, 302, 371, 526 (super soft
source, SSS, phase), and 742 and 1286 (quiescent emission phase). We have
studied the multifrequency nature and time evolution of the dominant peak at
~0.75 mHz using the standard Lomb-Scargle method and a 2-D sine fitting method.
We found a double structure of the peak and its overtone for days 180 and 196.
The two frequencies were closer together on day 196, suggesting that the
difference between the two peaks is gradually decreasing. For the later
observations, only a single frequency can be detected, which is likely due to
the exposure times being shorter than the beat period between the two peaks,
especially if they are moving closer together. The observations on days 742 and
1286 are long enough to detect two frequencies with the difference found for
day 196, but we confidently find only a single frequency. We found significant
changes in the oscillation frequency and amplitude. We have derived blackbody
temperatures from the SSS spectra, and the evolution of changes in frequency
and blackbody temperature suggests that the 0.75-mHz peak was modulated by
pulsations. Later, after nuclear burning had ceased, the signal stabilised at a
single frequency, although the X-ray frequency differs from the optical/UV
frequency obtained consistently from the OM onboard XMM and from ground-based
observations. We believe that the late frequency is the white dwarf rotation
and that the ratio of spin/orbit period strongly supports that the system is an
intermediate polar.Comment: 17 pages, 22 figures, 7 tables, accepted for publication in MNRA
A phenomenological model for the X-ray spectrum of Nova V2491 Cygni
The X-ray flux of Nova V2491 Cyg reached a maximum some forty days after
optical maximum. The X-ray spectrum at that time, obtained with the RGS of
XMM-Newton, shows deep, blue-shifted absorption by ions of a wide range of
ionization. We show that the deep absorption lines of the X-ray spectrum at
maximum, and nine days later, are well described by the following
phenomenological model with emission from a central blackbody and from a
collisionally ionized plasma (CIE). The blackbody spectrum (BB) is absorbed by
three main highly-ionized expanding shells; the CIE and BB are absorbed by cold
circumstellar and interstellar matter that includes dust. The outflow density
does not decrease monotonically with distance. The abundances of the shells
indicate that they were ejected from an O-Ne white dwarf. We show that the
variations on time scales of hours in the X-ray spectrum are caused by a
combination of variation in the central source and in the column density of the
ionized shells. Our phenomenological model gives the best description so far of
the supersoft X-ray spectrum of nova V2491 Cyg, but underpredicts, by a large
factor, the optical and ultraviolet flux. The X-ray part of the spectrum must
originate from a very different layer in the expanding envelope, presumably
much closer to the white dwarf than the layers responsible for the
optical/ultraviolet spectrum. This is confirmed by absence of any correlation
between the X-ray and UV/optical observed fluxes.Comment: 11 pages, 6 figure
Modeling CHANDRA Low Energy Transmission Grating Spectrometer Observations of Classical Novae with PHOENIX. I. V4743 Sagittarii
We use the PHOENIX code package to model the X-ray spectrum of Nova V4743
Sagittarii observed with the LETGS onboard the Chandra satellite on March 2003.
Our atmosphere models are 1D spherical, expanding, line blanketed, and in full
NLTE. To analyze nova atmospheres and related systems with an underlying
nuclear burning envelope at X-ray wavelengths, it was necessary to update the
code with new microphysics, as discussed in this paper. We demonstrate that the
X-ray emission is dominated by thermal bremsstrahlung and that the hard X-rays
are dominated by Fe and N absorption. The best fit to the observation is
provided at a temperature of T_eff = 5.8 x 10^5 K, with L_bol = 50 000 L_sun.
The models are calculated for solar abundances. It is shown that the models can
be used to determine abundances in the nova ejecta.Comment: 8 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
State tomography for two qubits using reduced densities
The optimal state determination (or tomography) is studied for a composite
system of two qubits when measurements can be performed on one of the qubits
and interactions of the two qubits can be implemented. The goal is to minimize
the number of interactions to be used. The algebraic method applied in the
paper leads to an extension of the concept of mutually unbiased measurements.Comment: 8 pages LATE
Variability and multi-periodic oscillations in the X-ray light curve of the classical nova V4743 Sgr
The classical nova V4743 Sgr was observed with XMM-Newton for about 10 hours
on April 4 2003, 6.5 months after optical maximum. At this time, this nova had
become the brightest supersoft X-ray source ever observed. We present the
results of a time series analysis performed on the X-ray light curve obtained
in this observation, and in a previous shorter observation done with Chandra 16
days earlier. Intense variability, with amplitude as large as 40% of the total
flux, was observed both times. Similarities can be found between the two
observations in the structure of the variations. Most of the variability is
well represented as a combination of oscillations at a set of discrete
frequencies lower than 1.7 mHz. At least five frequencies are constant over the
16 day time interval between the two observations. We suggest that a periods in
the power spectrum of both light curves at the frequency of 0.75 mHz and its
first harmonic are related to the spin period of the white dwarf in the system,
and that other observed frequencies are signatures of nonradial white dwarf
pulsations. A possible signal with a 24000 sec period is also found in the
XMM-Newton light curve: a cycle and a half are clearly identified. This period
is consistent with the 24278 s periodicity discovered in the optical light
curve of the source and thought to be the orbital period of the nova binary
system.Comment: In press in Monthly Notices of the Royal Astronomical Societ
Bipartite entanglement in systems of identical particles: the partial transposition criterion
We study bipartite entanglement in systems of N identical bosons distributed
in M different modes. For such systems, a definition of separability not
related to any a priori Hilbert space tensor product structure is needed and
can be given in terms of commuting subalgebras of observables. Using this
generalized notion of separability, we classify the states for which partial
transposition turns out to be a necessary and sufficient condition for
entanglement detection.Comment: LaTeX, 22 page
Unknown Quantum States: The Quantum de Finetti Representation
We present an elementary proof of the quantum de Finetti representation
theorem, a quantum analogue of de Finetti's classical theorem on exchangeable
probability assignments. This contrasts with the original proof of Hudson and
Moody [Z. Wahrschein. verw. Geb. 33, 343 (1976)], which relies on advanced
mathematics and does not share the same potential for generalization. The
classical de Finetti theorem provides an operational definition of the concept
of an unknown probability in Bayesian probability theory, where probabilities
are taken to be degrees of belief instead of objective states of nature. The
quantum de Finetti theorem, in a closely analogous fashion, deals with
exchangeable density-operator assignments and provides an operational
definition of the concept of an ``unknown quantum state'' in quantum-state
tomography. This result is especially important for information-based
interpretations of quantum mechanics, where quantum states, like probabilities,
are taken to be states of knowledge rather than states of nature. We further
demonstrate that the theorem fails for real Hilbert spaces and discuss the
significance of this point.Comment: 30 pages, 2 figure
Quantum Fluctuation Theorems
Recent advances in experimental techniques allow one to measure and control
systems at the level of single molecules and atoms. Here gaining information
about fluctuating thermodynamic quantities is crucial for understanding
nonequilibrium thermodynamic behavior of small systems. To achieve this aim,
stochastic thermodynamics offers a theoretical framework, and nonequilibrium
equalities such as Jarzynski equality and fluctuation theorems provide key
information about the fluctuating thermodynamic quantities. We review the
recent progress in quantum fluctuation theorems, including the studies of
Maxwell's demon which plays a crucial role in connecting thermodynamics with
information.Comment: As a chapter of: F. Binder, L. A. Correa, C. Gogolin, J. Anders, and
G. Adesso (eds.), "Thermodynamics in the quantum regime - Fundamental Aspects
and New Directions", (Springer International Publishing, 2018
Examples of conditional SIC-POVMs
The state of a quantum system is a density matrix with several parameters. The concern herein is how to recover the parameters. Several possibilities exist for the optimal recovery method, and we consider some special cases. We assume that a few parameters are known and that the others are to be recovered. The optimal positive-operator-valued measure (POVM) for recovering unknown parameters with an additional condition is called a conditional symmetric informationally complete POVM (SIC-POVM). In this paper, we study the existence or nonexistence of conditional SIC-POVMs. We provide a necessary condition for existence and some examples.ArticleQUANTUM INFORMATION PROCESSING. 14(10):3983-3999 (2015)journal articl
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