2,830 research outputs found
Towards optimized suppression of dephasing in systems subject to pulse timing constraints
We investigate the effectiveness of different dynamical decoupling protocols
for storage of a single qubit in the presence of a purely dephasing bosonic
bath, with emphasis on comparing quantum coherence preservation under uniform
vs. non-uniform delay times between pulses. In the limit of instantaneous
bit-flip pulses, this is accomplished by establishing a new representation of
the controlled qubit evolution, where the resulting decoherence behaviour is
directly expressed in terms of the free evolution. Simple analytical
expressions are given to approximate the long- and short- term coherence
behaviour for both ohmic and supra-ohmic environments. We focus on systems with
physical constraints on achievable time delays, with emphasis on pure dephasing
of excitonic qubits in quantum dots. Our analysis shows that little advantage
of high-level decoupling schemes based on concatenated or optimal design is to
be expected if operational constraints prevent pulses to be applied
sufficiently fast. In such constrained scenarios, we demonstrate how simple
modifications of repeated periodic echo protocols can offer significantly
improved coherence preservation in realistic parameter regimes.Comment: 13 figures,1 tabl
The Timing of Nine Globular Cluster Pulsars
We have used the Robert C. Byrd Green Bank Telescope to time nine previously
known pulsars without published timing solutions in the globular clusters M62,
NGC 6544, and NGC 6624. We have full timing solutions that measure the spin,
astrometric, and (where applicable) binary parameters for six of these pulsars.
The remaining three pulsars (reported here for the first time) were not
detected enough to establish solutions. We also report our timing solutions for
five pulsars with previously published solutions, and find good agreement with
past authors, except for PSR J1701-3006B in M62. Gas in this system is probably
responsible for the discrepancy in orbital parameters, and we have been able to
measure a change in the orbital period over the course of our observations.
Among the pulsars with new solutions we find several binary pulsars with very
low mass companions (members of the so-called "black widow" class) and we are
able to place constraints on the mass-to-light ratio in two clusters. We
confirm that one of the pulsars in NGC 6624 is indeed a member of the rare
class of non-recycled pulsars found in globular clusters. We also have measured
the orbital precession and Shapiro delay for a relativistic binary in NGC 6544.
If we assume that the orbital precession can be described entirely by general
relativity, which is likely, we are able to measure the total system mass
(2.57190(73) M_sun) and companion mass (1.2064(20) M_sun), from which we derive
the orbital inclination [sin(i) = 0.9956(14)] and the pulsar mass (1.3655(21)
M_sun), the most precise such measurement ever obtained for a millisecond
pulsar. The companion is the most massive known around a fully recycled pulsar.Comment: Published in ApJ; 33 pages, 5 figures, 7 table
GMRT Discovery of A Millisecond Pulsar in a Very Eccentric Binary System
We report the discovery of the binary millisecond pulsar J0514-4002A, which
is the first known pulsar in the globular cluster NGC 1851 and the first pulsar
discovered using the Giant Metrewave Radio Telescope (GMRT). The pulsar has a
rotational period of 4.99 ms, an orbital period of 18.8 days, and the most
eccentric pulsar orbit yet measured (e = 0.89). The companion has a minimum
mass of 0.9 M_sun and its nature is presently unclear. After accreting matter
from a low-mass companion star which spun it up to a (few) millisecond spin
period, the pulsar eventually exchanged the low-mass star for its more massive
present companion. This is exactly the same process that could form a system
containing a millisecond pulsar and a black hole; the discovery of NGC 1851A
demonstrates that such systems might exist in the Universe, provided that
stellar mass black holes exist in globular clusters.Comment: 12 pages (referee format), 3 figures, accepted for publication in
Astrophysical Journal Letter
Massive Cosmologies
We explore the cosmological solutions of a recently proposed extension of
General Relativity with a Lorentz-invariant mass term. We show that the same
constraint that removes the Boulware-Deser ghost in this theory also prohibits
the existence of homogeneous and isotropic cosmological solutions.
Nevertheless, within domains of the size of inverse graviton mass we find
approximately homogeneous and isotropic solutions that can well describe the
past and present of the Universe. At energy densities above a certain crossover
value, these solutions approximate the standard FRW evolution with great
accuracy. As the Universe evolves and density drops below the crossover value
the inhomogeneities become more and more pronounced. In the low density regime
each domain of the size of the inverse graviton mass has essentially non-FRW
cosmology. This scenario imposes an upper bound on the graviton mass, which we
roughly estimate to be an order of magnitude below the present-day value of the
Hubble parameter. The bound becomes especially restrictive if one utilizes an
exact self-accelerated solution that this theory offers. Although the above are
robust predictions of massive gravity with an explicit mass term, we point out
that if the mass parameter emerges from some additional scalar field
condensation, the constraint no longer forbids the homogeneous and isotropic
cosmologies. In the latter case, there will exist an extra light scalar field
at cosmological scales, which is screened by the Vainshtein mechanism at
shorter distances.Comment: 21 page
Potential distribution in deformed ZnO nanowires
AbstractThe potential distribution in a deformed ZnO nanowire relies upon its piezoelectric and semiconductive properties. Here we systematically investigate the influence of different parameters on the equilibrium potential distribution. In particular we calculate the electric potential distribution when thermodynamic equilibrium among free charge carriers is achieved for nanowires under different doping concentrations (n or p type), different applied forces, and different geometric configurations. We show that doping concentration is the parameter that mostly affects the magnitude and distribution of the piezoelectric potential
Equilibrium properties of highly asymmetric star-polymer mixtures
We employ effective interaction potentials to study the equilibrium structure
and phase behavior of highly asymmetric mixtures of star polymers. We consider
in particular the influence of the addition of a component with a small number
of arms and a small size on a concentrated solution of large stars with a high
functionality. By employing liquid integral equation theories we examine the
evolution of the correlation functions of the big stars upon addition of the
small ones, finding a loss of structure that can be attributed to a weakening
of the repulsions between the large stars due to the presence of the small
ones. We analyze this phenomenon be means of a generalized depletion mechanism
which is supported by computer simulations. By applying thermodynamic
perturbation theory we draw the phase diagram of the asymmetric mixture,
finding that the addition of small stars melts the crystal formed by the big
ones. A systematic comparison between the two- and effective one-component
descriptions of the mixture that corroborates the reliability of the
generalized depletion picture is also carried out.Comment: 26 pages, 9 figures, submitted to Phys. Rev.
Detection of ionized gas in the globular cluster 47 Tucanae
We report the detection of ionized intracluster gas in the globular cluster
47 Tucanae. Pulsars in this cluster with a negative period derivative, which
must lie in the distant half of the cluster, have significantly higher measured
integrated electron column densities than the pulsars with a positive period
derivative. We derive the plasma density within the central few pc of the
cluster using two different methods which yield consistent values. Our best
estimate of n_e = (0.067+-0.015)/cm^3 is about 100 times the free electron
density of the ISM in the vicinity of 47 Tucanae, and the ionized gas is
probably the dominant component of the intracluster medium.Comment: 5 pages, 3 included figures, accepted for publication by ApJ Letter
Single-spin polaron memory effect
The single-spin memory effect is considered within a minimal polaron model
describing a single-level quantum dot interacting with a vibron and weakly
coupled to ferromagnetic leads. We show that in the case of strong
electron-vibron and Coulomb interactions the rate of spontaneous quantum
switching between two spin states is suppressed at zero bias voltage, but can
be tuned through a wide range of finite switching timescales upon changing the
bias. We further find that such junctions exhibit hysteretic behavior enabling
controlled switching of a spin state. Spin lifetime, current and spin
polarization are calculated as a function of the bias voltage by the master
equation method. We also propose to use a third tunneling contact to control
and readout the spin state.Comment: LaTeX, 4 pages, 6 figures, submitte
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