1,074 research outputs found
Polarization rotation, reference frames and Mach's principle
Polarization of light rotates in a gravitational field. The accrued phase is
operationally meaningful only with respect to a local polarization basis. In
stationary space-times, we construct local reference frames that allow us to
isolate the Machian gravimagnetic effect from the geodetic (mass) contribution
to the rotation. The Machian effect is supplemented by the geometric term that
arises from the choice of standard polarizations. The phase accrued along a
close trajectory is gauge-independent and is zero in the Schwarzschild
space-time. The geometric term may give a dominant contribution to the phase.
We calculate polarization rotation for several trajectories and find it to be
more significant than is usually believed, pointing to its possible role as a
future gravity probe.Comment: 4 pages. Final versio
Discord and quantum computational resources
Discordant states appear in a large number of quantum phenomena and seem to
be a good indicator of divergence from classicality. While there is evidence
that they are essential for a quantum algorithm to have an advantage over a
classical one, their precise role is unclear. We examine the role of discord in
quantum algorithms using the paradigmatic framework of `restricted distributed
quantum gates' and show that manipulating discordant states using local
operations has an associated cost in terms of entanglement and communication
resources. Changing discord reduces the total correlations and reversible
operations on discordant states usually require non-local resources. Discord
alone is, however, not enough to determine the need for entanglement. A more
general type of similar quantities, which we call K-discord, is introduced as a
further constraint on the kinds of operations that can be performed without
entanglement resources.Comment: Closer to published versio
Unified Brane Gravity: Cosmological Dark Matter from Scale Dependent Newton Constant
We analyze, within the framework of unified brane gravity, the weak-field
perturbations caused by the presence of matter on a 3-brane. Although deviating
from the Randall-Sundrum approach, the masslessness of the graviton is still
preserved. In particular, the four-dimensional Newton force law is recovered,
but serendipitously, the corresponding Newton constant is shown to be
necessarily lower than the one which governs FRW cosmology. This has the
potential to puzzle out cosmological dark matter. A subsequent conjecture
concerning galactic dark matter follows.Comment: 6 pages, to be published in Phys. Rev.
Photorefractive properties of Ce- and Ca-doped Sr0.6Ba0.4Nb2O6
We present the results of experimental study of the absorption coefficient, two-beam photorefractive coupling constant, and photorefractive response time of a doubly Ce- and Ca-doped Sr0.6Ba0.4Nb2O6. This crystal displays enhanced photorefractive response at near infrared wavelengths when compared to Ce-doped SBN:60. The temperature dependence of the coupling constant over the range from –30 to 40 °C has also been studied
The clash of symmetries in a Randall-Sundrum-like spacetime
We present a toy model that exhibits clash-of-symmetries style Higgs field
kink configurations in a Randall-Sundrum-like spacetime. The model has two
complex scalar fields Phi_{1,2}, with a sextic potential obeying global
U(1)xU(1) and discrete Phi_1 Phi_2 interchange symmetries. The scalar
fields are coupled to 4+1 dimensional gravity endowed with a bulk cosmological
constant. We show that the coupled Einstein-Higgs field equations have an
interesting analytic solution provided the sextic potential adopts a particular
form. The 4+1 metric is shown to be that of a smoothed-out Randall-Sundrum type
of spacetime. The thin-brane Randall-Sundrum limit, whereby the Higgs field
kinks become step functions, is carefully defined in terms of the fundamental
parameters in the action. The ``clash of symmetries'' feature, defined in
previous papers, is manifested here through the fact that both of the U(1)
symmetries are spontaneously broken at all non-asymptotic points in the extra
dimension . One of the U(1)'s is asymptotically restored as w --> -infinity,
with the other U(1) restored as w --> +infinity. The spontaneously broken
discrete symmetry ensures topological stability. In the gauged version of this
model we find new flat-space solutions, but in the warped metric case we have
been unable to find any solutions with nonzero gauge fields.Comment: 15 pages, 5 figures; minor changes including added references and an
updated figure; to appear in Phys Rev
High Resolution Polar Kerr Effect Measurements of Sr2RuO4: Evidence for Broken Time Reversal Symmetry in the Superconducting State
Polar Kerr effect in the spin-triplet superconductor Sr2RuO4 was measured
with high precision using a Sagnac interferometer with a zero-area Sagnac loop.
We observed non-zero Kerr rotations as big as 65 nanorad appearing below Tc in
large domains. Our results imply a broken time reversal symmetry state in the
superconducting state of Sr2RuO4, similar to He3-A
Quantum discord and local demons
Quantum discord was proposed as a measure of the "quantumness" of
correlations. There are at least three different discord-like quantities, two
of which determine the difference between the efficiencies of a Szilard's
engine under different sets of restrictions. The three discord measures vanish
simulataneosly. We introduce an easy way to test for zero discord, relate it to
the Cerf-Adami conditional entropy and show that there is no relation between
the discord and the local disitnguishability.Comment: 7 pages, RevTeX. Some minor changes after comments from colleagues,
some references added. Similar to published versio
Clock transition by continuous dynamical decoupling of a three-level system
We present a novel continuous dynamical decoupling scheme for the
construction of a robust qubit in a three-level system. By means of a clock
transition adjustment, we first show how robustness to environmental noise is
achieved, while eliminating drive-noise, to first-order. We demonstrate this
scheme with the spin sub-levels of the NV-centre's electronic ground state. By
applying drive fields with moderate Rabi frequencies, the drive noise is
eliminated and an improvement of 2 orders of magnitude in the coherence time is
obtained compared to the pure dephasing time. We then show how the clock
transition adjustment can be tuned to eliminate also the second-order effect of
the environmental noise with moderate drive fields. A further improvement of
more than 1 order of magnitude in the coherence time is expected and confirmed
by simulations. Hence, our scheme prolongs the coherence time towards the
lifetime-limit using a relatively simple experimental setup.Comment: 7 pages, 5 figure
Photon polarization and geometric phase in general relativity
Rotation of polarization in an external gravitational field is one of the
effects of general relativity that can serve as a basis for its precision
tests. A careful analysis of reference frames is crucial for a proper
evaluation of this effect. We introduce an operationally-motivated local
reference frame that allows for a particularly simple description. We present a
solution of null geodesics in Kerr space-time that is organized around a new
expansion parameter, allowing a better control of the series, and use it to
calculate the resulting polarization rotation. While this rotation depends on
the reference-frame convention, we demonstrate a gauge-independent geometric
phase for closed paths in general space-times.Comment: 17 pages; published version of a companion paper to arXiv:1107.127
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