1,657 research outputs found
A Quantitative Measure of Interference
We introduce an interference measure which allows to quantify the amount of
interference present in any physical process that maps an initial density
matrix to a final density matrix. In particular, the interference measure
enables one to monitor the amount of interference generated in each step of a
quantum algorithm. We show that a Hadamard gate acting on a single qubit is a
basic building block for interference generation and realizes one bit of
interference, an ``i-bit''. We use the interference measure to quantify
interference for various examples, including Grover's search algorithm and
Shor's factorization algorithm. We distinguish between ``potentially
available'' and ``actually used'' interference, and show that for both
algorithms the potentially available interference is exponentially large.
However, the amount of interference actually used in Grover's algorithm is only
about 3 i-bits and asymptotically independent of the number of qubits, while
Shor's algorithm indeed uses an exponential amount of interference.Comment: 13 pages of latex; research done at http://www.quantware.ups-tlse.fr
Macroscopic Quantum Tunneling of Ferromagnetic Domain Walls
Quantum tunneling of domain walls out of an impurity potential in a
mesoscopic ferromagnetic sample is investigated. Using improved expressions for
the domain wall mass and for the pinning potential, we find that the cross-over
temperature between thermal activation and quantum tunneling is of a different
functional form than found previously. In materials like Ni or YIG, the
crossover temperatures are around 5 mK. We also find that the WKB exponent is
typically two orders of magnitude larger than current estimates. The sources
for these discrepancies are discussed, and precise estimates for the transition
from three-dimensional to one-dimensional magnetic behavior of a wire are
given. The cross-over temperatures from thermal to quantum transitions and
tunneling rates are calculated for various materials and sample sizes.Comment: 10 pages, 2 postscript figures, REVTe
Master equation for collective spontaneous emission with quantized atomic motion
We derive a markovian master equation for the internal dynamics of an
ensemble of two-level atoms including all effects related to the quantization
of their motion. Our equation provides a unifying picture of the consequences
of recoil and indistinguishability of atoms beyond the Lamb-Dicke regime on
both their dissipative and conservative dynamics, and applies equally well to
distinguishable and indistinguishable atoms. We give general expressions for
the decay rates and the dipole-dipole shifts for any motional states, and we
find closed-form formulas for a number of relevant states (Gaussian states,
Fock states and thermal states). In particular, we show that dipole-dipole
interactions and cooperative photon emission can be modulated through the
external state of motion.Comment: 16 pages, 7 figures, minor correction
Recent Star Formation in Sextans A
We investigate the relationship between the spatial distributions of stellar
populations and of neutral and ionized gas in the Local Group dwarf irregular
galaxy Sextans A. This galaxy is currently experiencing a burst of localized
star formation, the trigger of which is unknown. We have resolved various
populations of stars via deep UBV(RI)_C imaging over an area with diameter \sim
5.'3. We have compared our photometry with theoretical isochrones appropriate
for Sextans A, in order to determine the ages of these populations. We have
mapped out the history of star formation, most accurately for times \lesssim
100 Myr. We find that star formation in Sextans A is correlated both in time
and space, especially for the most recent (\lesssim 12 Myr) times. The youngest
stars in the galaxy are forming primarily along the inner edge of the large H I
shell. Somewhat older populations, \lesssim 50 Myr, are found inward of the
youngest stars. Progressively older star formation, from \sim 50--100 Myr,
appears to have some spatially coherent structure and is more centrally
concentrated. The oldest stars we can accurately sample appear to have
approximately a uniform spatial distribution, which extends beyond a surface
brightness of \mu_B \simeq 25.9 mag arcsec^{-2} (or, a radius r \simeq 2.'3$).
Although other processes are also possible, our data provides support for a
mechanism of supernova-driven expansion of the neutral gas, resulting in cold
gas pileup and compression along the H I shell and sequential star formation in
recent times.Comment: 64 pages, 22 figures, to appear in A
Bloch oscillations of magnetic solitons in anisotropic spin-1/2 chains
We study the quantum dynamics of soliton-like domain walls in anisotropic
spin-1/2 chains in the presence of magnetic fields. In the absence of fields,
domain walls form a Bloch band of delocalized quantum states while a static
field applied along the easy axis localizes them into Wannier wave packets and
causes them to execute Bloch oscillations, i.e. the domain walls oscillate
along the chain with a finite Bloch frequency and amplitude. In the presence of
the field, the Bloch band, with a continuum of extended states, breaks up into
the Wannier-Zeeman ladder -- a discrete set of equally spaced energy levels. We
calculate the dynamical structure factor in the one-soliton sector at finite
frequency, wave vector, and temperature, and find sharp peaks at frequencies
which are integer multiples of the Bloch frequency. We further calculate the
uniform magnetic susceptibility and find that it too exhibits peaks at the
Bloch frequency. We identify several candidate materials where these Bloch
oscillations should be observable, for example, via neutron scattering
measurements. For the particular compound CoCl_2.2H_2O we estimate the Bloch
amplitude to be on the order of a few lattice constants, and the Bloch
frequency on the order of 100 GHz for magnetic fields in the Tesla range and at
temperatures of about 18 Kelvin.Comment: 31 single-spaced REVTeX pages, including 7 figures embedded with eps
The CHARA Array Angular Diameter of HR 8799 Favors Planetary Masses for Its Imaged Companions
HR 8799 is an hF0 mA5 gamma Doradus, lambda Bootis, Vega-type star best known
for hosting four directly imaged candidate planetary companions. Using the
CHARA Array interferometer, we measure HR 8799's limb-darkened angular diameter
to be 0.342 +/- 0.008 mas; this is the smallest interferometrically measured
stellar diameter to date, with an error of only 2%. By combining our
measurement with the star's parallax and photometry from the literature, we
greatly improve upon previous estimates of its fundamental parameters,
including stellar radius (1.44 +/- 0.06 R_Sun), effective temperature (7193 +/-
87 K, consistent with F0), luminosity (5.05 +/- 0.29 L_Sun), and the extent of
the habitable zone (1.62 AU to 3.32 AU). These improved stellar properties
permit much more precise comparisons with stellar evolutionary models, from
which a mass and age can be determined, once the metallicity of the star is
known. Considering the observational properties of other lambda Bootis stars
and the indirect evidence for youth of HR 8799, we argue that the internal
abundance, and what we refer to as the effective abundance, is most likely
near-solar. Finally, using the Yonsei-Yale evolutionary models with uniformly
scaled solar-like abundances, we estimate HR 8799's mass and age considering
two possibilities: 1.516 +0.038/-0.024 M_Sun and 33 +7/-13 Myr if the star is
contracting toward the zero age main-sequence or 1.513 +0.023/-0.024 M_Sun and
90 +381/-50 Myr if it is expanding from it. This improved estimate of HR 8799's
age with realistic uncertainties provides the best constraints to date on the
masses of its orbiting companions, and strongly suggests they are indeed
planets. They nevertheless all appear to orbit well outside the habitable zone
of this young star.Comment: Accepted for publication in ApJ; 37 pages, 6 tables, 13 figure
Boundary conditions, the critical conductance distribution, and one-parameter scaling
Published versio
Motor coordination: when two have to act as one
Trying to pass someone walking toward you in a narrow corridor is a familiar example of a two-person motor game that requires coordination. In this study, we investigate coordination in sensorimotor tasks that correspond to classic coordination games with multiple Nash equilibria, such as âchoosing sides,â âstag hunt,â âchicken,â and âbattle of sexesâ. In these tasks, subjects made reaching movements reflecting their continuously evolving âdecisionsâ while they received a continuous payoff in the form of a resistive force counteracting their movements. Successful coordination required two subjects to âchooseâ the same Nash equilibrium in this force-payoff landscape within a single reach. We found that on the majority of trials coordination was achieved. Compared to the proportion of trials in which miscoordination occurred, successful coordination was characterized by several distinct features: an increased mutual information between the playersâ movement endpoints, an increased joint entropy during the movements, and by differences in the timing of the playersâ responses. Moreover, we found that the probability of successful coordination depends on the playersâ initial distance from the Nash equilibria. Our results suggest that two-person coordination arises naturally in motor interactions and is facilitated by favorable initial positions, stereotypical motor pattern, and differences in response times
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