1,726 research outputs found
Geometric phase distributions for open quantum systems
In an open system, the geometric phase should be described by a distribution.
We show that a geometric phase distribution for open system dynamics is in
general ambiguous, but the imposition of reasonable physical constraints on the
environment and its coupling with the system yields a unique geometric phase
distribution that applies even for mixed states, non-unitary dynamics, and
non-cyclic evolutions.Comment: Some minor revisions, references update
Kinematic approach to the mixed state geometric phase in nonunitary evolution
A kinematic approach to the geometric phase for mixed quantal states in
nonunitary evolution is proposed. This phase is manifestly gauge invariant and
can be experimentally tested in interferometry. It leads to well-known results
when the evolution is unitary.Comment: Minor changes; journal reference adde
Qubit quantum channels: A characteristic function approach
A characterization of qubit quantum channels is introduced. In analogy to
what happens in the context of Bosonic channels we exploit the possibility of
representing the states of the system in terms of characteristic function. The
latter are functions of non-commuting variables (Grassmann variables) and are
defined in terms of generalized displacement operators. In this context we
introduce the set of Gaussian channels and show that they share similar
properties with the corresponding Bosonic counterpart.Comment: 10 pages (minor editing
Fidelity of Quantum Teleportation through Noisy Channels
We investigate quantum teleportation through noisy quantum channels by
solving analytically and numerically a master equation in the Lindblad form. We
calculate the fidelity as a function of decoherence rates and angles of a state
to be teleported. It is found that the average fidelity and the range of states
to be accurately teleported depend on types of noise acting on quantum
channels. If the quantum channels is subject to isotropic noise, the average
fidelity decays to 1/2, which is smaller than the best possible value 2/3
obtained only by the classical communication. On the other hand, if the noisy
quantum channel is modeled by a single Lindblad operator, the average fidelity
is always greater than 2/3.Comment: 6 pages, 5 figures, accepted for publication in Phys. Rev.
Quantum thermodynamics with missing reference frames: Decompositions of free energy into non-increasing components
If an absolute reference frame with respect to time, position, or orientation
is missing one can only implement quantum operations which are covariant with
respect to the corresponding unitary symmetry group G. Extending observations
of Vaccaro et al., I argue that the free energy of a quantum system with
G-invariant Hamiltonian then splits up into the Holevo information of the orbit
of the state under the action of G and the free energy of its orbit average.
These two kinds of free energy cannot be converted into each other. The first
component is subadditive and the second superadditive; in the limit of
infinitely many copies only the usual free energy matters.
Refined splittings of free energy into more than two independent
(non-increasing) terms can be defined by averaging over probability measures on
G that differ from the Haar measure.
Even in the presence of a reference frame, these results provide lower bounds
on the amount of free energy that is lost after applying a covariant channel.
If the channel properly decreases one of these quantities, it decreases the
free energy necessarily at least by the same amount, since it is unable to
convert the different forms of free energies into each other.Comment: 17 pages, latex, 1 figur
CO Distribution and Kinematics Along the Bar in the Strongly Barred Spiral NGC 7479
We report on the 2.5 arcsec (400 pc) resolution CO (J = 1 -> 0) observations
covering the whole length of the bar in the strongly barred late-type spiral
galaxy NGC 7479. CO emission is detected only along a dust lane that traverses
the whole length of the bar, including the nucleus. The emission is strongest
in the nucleus. The distribution of emission is clumpy along the bar outside
the nucleus, and consists of gas complexes that are unlikely to be
gravitationally bound. The CO kinematics within the bar consist of two separate
components. A kinematically distinct circumnuclear disk, < 500 pc in diameter,
is undergoing predominantly circular motion with a maximum rotational velocity
of 245 km/s at a radius of 1 arcsec (160 pc). The CO-emitting gas in the bar
outside the circumnuclear disk has substantial noncircular motions which are
consistent with a large radial velocity component, directed inwards. The CO
emission has a large velocity gradient across the bar dust lane, ranging from
0.5 to 1.9 km/s/pc after correcting for inclination, and the projected velocity
change across the dust lane is as high as 200 km/s. This sharp velocity
gradient is consistent with a shock front at the location of the bar dust lane.
A comparison of H-alpha and CO kinematics across the dust lane shows that
although the H-alpha emission is often observed both upstream and downstream
from the dust lane, the CO emission is observed only where the velocity
gradient is large. We also compare the observations with hydrodynamic models
and discuss star formation along the bar.Comment: 16 pages, including 10 figures. Accepted for publication in Ap
Generalized and weighted Strichartz estimates
In this paper, we explore the relations between different kinds of Strichartz
estimates and give new estimates in Euclidean space . In
particular, we prove the generalized and weighted Strichartz estimates for a
large class of dispersive operators including the Schr\"odinger and wave
equation. As a sample application of these new estimates, we are able to prove
the Strauss conjecture with low regularity for dimension 2 and 3.Comment: Final version, to appear in the Communications on Pure and Applied
Analysis. 33 pages. 2 more references adde
The Potential-Density Phase Shift Method for Determining the Corotation Radii in Spiral and Barred Galaxies
We have developed a new method for determining the corotation radii of
density waves in disk galaxies, which makes use of the radial distribution of
an azimuthal phase shift between the potential and density wave patterns. The
approach originated from improved theoretical understandings of the relation
between the morphology and kinematics of galaxies, and on the dynamical
interaction between density waves and the basic-state disk stars which results
in the secular evolution of disk galaxies. In this paper, we present the
rationales behind the method, and the first application of it to several
representative barred and grand-design spiral galaxies, using near-infrared
images to trace the mass distributions, as well as to calculate the potential
distributions used in the phase shift calculations. We compare our results with
those from other existing methods for locating the corotations, and show that
the new method both confirms the previously-established trends of bar-length
dependence on galaxy morphological types, as well as provides new insights into
the possible extent of bars in disk galaxies. Application of the method to a
larger sample and the preliminary analysis of which show that the phase shift
method is likely to be a generally-applicable, accurate, and essentially
model-independent method for determining the pattern speeds and corotation
radii of single or nested density wave patterns in galaxies. Other implications
of this work are: most of the nearby bright disk galaxies appear to possess
quasi-stationary spiral modes; that these density wave modes and the associated
basic state of the galactic disk slowly transform over time; and that
self-consistent N-particle systems contain physics not revealed by the passive
orbit analysis approaches.Comment: 48 pages, 16 figures. Accepted for publication in the Astronomical
Journa
A Dust-Penetrated Classification Scheme for Bars as Inferred from their Gravitational Force Fields
The division of galaxies into ``barred'' (SB) and ``normal'' (S) spirals is a
fundamental aspect of the Hubble galaxy classification system. This ``tuning
fork'' view was revised by de Vaucouleurs, whose classification volume
recognized apparent ``bar strength'' (SA, SAB, SB) as a continuous property of
galaxies called the ``family''. However, the SA, SAB, and SB families are
purely visual judgments that can have little bearing on the actual bar strength
in a given galaxy. Until very recently, published bar judgments were based
exclusively on blue light images, where internal extinction or star formation
can either mask a bar completely or give the false impression of a bar in a
nonbarred galaxy. Near-infrared camera arrays, which principally trace the old
stellar populations in both normal and barred galaxies, now facilitate a
quantification of bar strength in terms of their gravitational potentials and
force fields. In this paper, we show that the maximum value, Qb, of the ratio
of the tangential force to the mean radial force is a quantitative measure of
the strength of a bar. Qb does not measure bar ellipticity or bar shape, but
rather depends on the actual forcing due to the bar embedded in its disk. We
show that a wide range of true bar strengths characterizes the category ``SB'',
while de Vaucouleurs category ``SAB'' corresponds to a much narrower range of
bar strengths. We present Qb values for 36 galaxies, and we incorporate our bar
classes into a dust-penetrated classification system for spiral galaxies.Comment: Accepted for publication in the Astrophysical Journal (LaTex, 30
pages + 3 figures); Figs. 1 and 3 are in color and are also available at
http://bama.ua.edu/~rbuta/bars
Decoherence due to contacts in ballistic nanostructures
The active region of a ballistic nanostructure is an open quantum-mechanical
system, whose nonunitary evolution (decoherence) towards a nonequilibrium
steady state is determined by carrier injection from the contacts. The purpose
of this paper is to provide a simple theoretical description of the
contact-induced decoherence in ballistic nanostructures, which is established
within the framework of the open systems theory. The active region's evolution
in the presence of contacts is generally non-Markovian. However, if the
contacts' energy relaxation due to electron-electron scattering is sufficiently
fast, then the contacts can be considered memoryless on timescales coarsened
over their energy relaxation time, and the evolution of the current-limiting
active region can be considered Markovian. Therefore, we first derive a general
Markovian map in the presence of a memoryless environment, by coarse-graining
the exact short-time non-Markovian dynamics of an abstract open system over the
environment memory-loss time, and we give the requirements for the validity of
this map. We then introduce a model contact-active region interaction that
describes carrier injection from the contacts for a generic two-terminal
ballistic nanostructure. Starting from this model interaction and using the
Markovian dynamics derived by coarse-graining over the effective memory-loss
time of the contacts, we derive the formulas for the nonequilibrium
steady-state distribution functions of the forward and backward propagating
states in the nanostructure's active region. On the example of a double-barrier
tunneling structure, the present approach yields an I-V curve with all the
prominent resonant features. The relationship to the Landauer-B\"{u}ttiker
formalism is also discussed, as well as the inclusion of scattering.Comment: Published versio
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