56,733 research outputs found
No periodicity revealed for an "eclipsing" ultraluminous supersoft X-ray source in M81
Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds
are likely white dwarfs that steadily or cyclically burn accreted matter on
their surface, which are promising type Ia supernova progenitors. Observations
of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources
that are generally hotter and more luminous, including some ultraluminous
supersoft sources (ULSs) that are possibly intermediate mass black holes of a
few thousand solar masses. In this paper we report our X-ray spectral and
timing analysis for M81-ULS1, an ultraluminous supersoft source in the nearby
spiral galaxy M81. M81-ULS1 has been persistently supersoft in 17 Chandra ACIS
observations spanning six years, and its spectrum can be described by either a
eV blackbody for a white dwarf, or a
eV multicolor accretion disk for a
intermediate mass black hole. In two observations, the light curves exhibited
dramatic flux drop/rise on time scales of seconds, reminiscent of
eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive
search for periodicity in the reasonable range of 50 ksec to 50 days failed to
reveal an orbital period. The failure to reveal any periodicity is consistent
with the long period ( yrs) predicted for this system given the optical
identification of the secondary with an asymptotic giant star. Also, the
eclipse-like dramatic flux changes in hours are hard to explain under the white
dwarf model, but can in principle be explained by disk temperature changes
induced by accretion rate variations under the intermediate mass black hole
model.Comment: 19 pages, 7 figures, 1 table, to appear in ApJ
Preparation of cluster states and W states with superconducting- quantum-interference-device qubits in cavity QED
We propose schemes to create cluster states and W states by many
superconducting-quantum-interference-device (SQUID) qubits in cavities under
the influence of the cavity decay. Our schemes do not require auxiliary qubits,
and the excited levels are only virtually coupled throughout the scheme, which
could much reduce the experimental challenge. We consider the cavity decay in
our model and analytically demonstrate its detrimental influence on the
prepared entangled states.Comment: 6 pages, 3 figures, to appear in Phys. Rev.
Universal quantum computation with electronic qubits in decoherence-free subspace
We investigate how to carry out universal quantum computation
deterministically with free electrons in decoherence-free subspace by using
polarizing beam splitters, charge detectors, and single-spin rotations. Quantum
information in our case is encoded in spin degrees of freedom of the
electron-pairs which construct a decoherence-free subspace. We design building
blocks for two noncommutable single-logic-qubit gates and a logic controlled
phase gate, based on which a universal and scalable quantum information
processing robust to dephasing is available in a deterministic way.Comment: 14 pages, 3 figure
Simple scheme for two-qubit Grover search in cavity QED
Following the proposal by F. Yamaguchi et al.[Phys. Rev. A 66, 010302 (R)
(2002)], we present an alternative way to implement the two-qubit Grover search
algorithm in cavity QED. Compared with F. Yamaguchi et al.'s proposal, with a
strong resonant classical field added, our method is insensitive to both the
cavity decay and thermal field, and doesn't require that the cavity remain in
the vacuum state throughout the procedure. Moreover, the qubit definitions are
the same for both atoms, which makes the experiment easier. The strictly
numerical simulation shows that our proposal is good enough to demonstrate a
two-qubit Grover's search with high fidelity.Comment: manuscript 10 pages, 2 figures, to appear in Phys. Rev.
Matrix Product Representation of Locality Preserving Unitaries
The matrix product representation provides a useful formalism to study not
only entangled states, but also entangled operators in one dimension. In this
paper, we focus on unitary transformations and show that matrix product
operators that are unitary provides a necessary and sufficient representation
of 1D unitaries that preserve locality. That is, we show that matrix product
operators that are unitary are guaranteed to preserve locality by mapping local
operators to local operators while at the same time all locality preserving
unitaries can be represented in a matrix product way. Moreover, we show that
the matrix product representation gives a straight-forward way to extract the
GNVW index defined in Ref.\cite{Gross2012} for classifying 1D locality
preserving unitaries. The key to our discussion is a set of `fixed point'
conditions which characterize the form of the matrix product unitary operators
after blocking sites. Finally, we show that if the unitary condition is relaxed
and only required for certain system sizes, the matrix product operator
formalism allows more possibilities than locality preserving unitaries. In
particular, we give an example of a simple matrix product operator which is
unitary only for odd system sizes, does not preserve locality and carries a
`fractional' index as compared to their locality preserving counterparts.Comment: 14 page
Supersymmetry and the Anomalous Anomalous Magnetic Moment of the Muon
The recently reported measurement of the muon's anomalous magnetic moment
differs from the standard model prediction by 2.6 standard deviations. We
examine the implications of this discrepancy for supersymmetry. Deviations of
the reported magnitude are generic in supersymmetric theories. Based on the new
result, we derive model-independent upper bounds on the masses of observable
supersymmetric particles. We also examine several model frameworks. The sign of
the reported deviation is as predicted in many simple models, but disfavors
anomaly-mediated supersymmetry breaking.Comment: 4 pages, 4 figures, version to appear in Phys. Rev. Let
Subwavelength fractional Talbot effect in layered heterostructures of composite metamaterials
We demonstrate that under certain conditions, fractional Talbot revivals can
occur in heterostructures of composite metamaterials, such as multilayer
positive and negative index media, metallodielectric stacks, and
one-dimensional dielectric photonic crystals. Most importantly, without using
the paraxial approximation we obtain Talbot images for the feature sizes of
transverse patterns smaller than the illumination wavelength. A general
expression for the Talbot distance in such structures is derived, and the
conditions favorable for observing Talbot effects in layered heterostructures
is discussed.Comment: To be published in Phys. Rev.
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