35,008 research outputs found
Enhancement of Entanglement Percolation in Quantum Networks via Lattice Transformations
We study strategies for establishing long-distance entanglement in quantum
networks. Specifically, we consider networks consisting of regular lattices of
nodes, in which the nearest neighbors share a pure, but non-maximally entangled
pair of qubits. We look for strategies that use local operations and classical
communication. We compare the classical entanglement percolation protocol, in
which every network connection is converted with a certain probability to a
singlet, with protocols in which classical entanglement percolation is preceded
by measurements designed to transform the lattice structure in a way that
enhances entanglement percolation. We analyze five examples of such comparisons
between protocols and point out certain rules and regularities in their
performance as a function of degree of entanglement and choice of operations.Comment: 12 pages, 17 figures, revtex4. changes from v3: minor stylistic
changes for journal reviewer, minor changes to figures for journal edito
Intermediate quantum maps for quantum computation
We study quantum maps displaying spectral statistics intermediate between
Poisson and Wigner-Dyson. It is shown that they can be simulated on a quantum
computer with a small number of gates, and efficiently yield information about
fidelity decay or spectral statistics. We study their matrix elements and
entanglement production, and show that they converge with time to distributions
which differ from random matrix predictions. A randomized version of these maps
can be implemented even more economically, and yields pseudorandom operators
with original properties, enabling for example to produce fractal random
vectors. These algorithms are within reach of present-day quantum computers.Comment: 4 pages, 4 figures, research done at
http://www.quantware.ups-tlse.fr
Completely positive covariant two-qubit quantum processes and optimal quantum NOT operations for entangled qubit pairs
The structure of all completely positive quantum operations is investigated
which transform pure two-qubit input states of a given degree of entanglement
in a covariant way. Special cases thereof are quantum NOT operations which
transform entangled pure two-qubit input states of a given degree of
entanglement into orthogonal states in an optimal way. Based on our general
analysis all covariant optimal two-qubit quantum NOT operations are determined.
In particular, it is demonstrated that only in the case of maximally entangled
input states these quantum NOT operations can be performed perfectly.Comment: 14 pages, 2 figure
Eta Carinae across the 2003.5 Minimum: Analysis in the visible and near infrared spectral region
We present an analysis of the visible through near infrared spectrum of Eta
Carinae and its ejecta obtained during the "Eta Carinae Campaign with the UVES
at the ESO VLT". This is a part of larger effort to present a complete Eta
Carinae spectrum, and extends the previously presented analyses with the
HST/STIS in the UV (1240-3159 A) to 10,430 A. The spectrum in the mid and near
UV is characterized by the ejecta absorption. At longer wavelengths, stellar
wind features from the central source and narrow emission lines from the
Weigelt condensations dominate the spectrum. However, narrow absorption lines
from the circumstellar shells are present. This paper provides a description of
the spectrum between 3060 and 10,430 A, including line identifications of the
ejecta absorption spectrum, the emission spectrum from the Weigelt
condensations and the P-Cygni stellar wind features. The high spectral
resolving power of VLT/UVES enables equivalent width measurements of atomic and
molecular absorption lines for elements with no transitions at the shorter
wavelengths. However, the ground based seeing and contributions of nebular
scattered radiation prevent direct comparison of measured equivalent widths in
the VLT/UVES and HST/STIS spectra. Fortunately, HST/STIS and VLT/UVES have a
small overlap in wavelength coverage which allows us to compare and adjust for
the difference in scattered radiation entering the instruments' apertures. This
paper provides a complete online VLT/UVES spectrum with line identifications
and a spectral comparison between HST/STIS and VLT/UVES between 3060 and 3160
A.Comment: 13 pages, 11 figures + atlas. The paper accepted for the ApJS and is
accompanied with an atlas in the online edition pape
Entanglement evolution in finite dimensions
We provide a relation which describes how the entanglement of two d-level
systems evolves as either system undergoes an arbitrary physical process. The
dynamics of the entanglement turns out to be of a simple form, and is fully
captured by a single quantity.Comment: 4 pages, 1 figure; new title and introduction, added references, some
makeup; published versio
Quantum state engineering, purification, and number resolved photon detection with high finesse optical cavities
We propose and analyze a multi-functional setup consisting of high finesse
optical cavities, beam splitters, and phase shifters. The basic scheme projects
arbitrary photonic two-mode input states onto the subspace spanned by the
product of Fock states |n>|n> with n=0,1,2,.... This protocol does not only
provide the possibility to conditionally generate highly entangled photon
number states as resource for quantum information protocols but also allows one
to test and hence purify this type of quantum states in a communication
scenario, which is of great practical importance. The scheme is especially
attractive as a generalization to many modes allows for distribution and
purification of entanglement in networks. In an alternative working mode, the
setup allows of quantum non demolition number resolved photodetection in the
optical domain.Comment: 14 pages, 10 figure
Quantum Operation Time Reversal
The dynamics of an open quantum system can be described by a quantum
operation, a linear, complete positive map of operators. Here, I exhibit a
compact expression for the time reversal of a quantum operation, which is
closely analogous to the time reversal of a classical Markov transition matrix.
Since open quantum dynamics are stochastic, and not, in general, deterministic,
the time reversal is not, in general, an inversion of the dynamics. Rather, the
system relaxes towards equilibrium in both the forward and reverse time
directions. The probability of a quantum trajectory and the conjugate, time
reversed trajectory are related by the heat exchanged with the environment.Comment: 4 page
Landau-Zener Interference in Multilevel Superconducting Flux Qubits Driven by Large Amplitude Fields
We proposed an analytical model to analyze the Landau-Zener interference in a
multilevel superconducting flux qubit driven by large amplitude external
fields. Our analytical results agree remarkably with those of the experiment
[Nature 455, 51 (2008)]. Moreover, we studied the effect of driving-frequency
and dephasing rate on the interference. The dephasing generally destroys the
interference while increasing frequency rebuilds the interference at large
dephasing rate. At certain driving frequency and dephasing rate, the
interference shows some anomalous features as observed in recent experiments.Comment: 7 pages, 6 figure
Operator-Schmidt decompositions and the Fourier transform, with applications to the operator-Schmidt numbers of unitaries
The operator-Schmidt decomposition is useful in quantum information theory
for quantifying the nonlocality of bipartite unitary operations. We construct a
family of unitary operators on C^n tensor C^n whose operator-Schmidt
decompositions are computed using the discrete Fourier transform. As a
corollary, we produce unitaries on C^3 tensor C^3 with operator-Schmidt number
S for every S in {1,...,9}. This corollary was unexpected, since it
contradicted reasonable conjectures of Nielsen et al [Phys. Rev. A 67 (2003)
052301] based on intuition from a striking result in the two-qubit case. By the
results of Dur, Vidal, and Cirac [Phys. Rev. Lett. 89 (2002) 057901
quant-ph/0112124], who also considered the two-qubit case, our result implies
that there are nine equivalence classes of unitaries on C^3 tensor C^3 which
are probabilistically interconvertible by (stochastic) local operations and
classical communication. As another corollary, a prescription is produced for
constructing maximally-entangled operators from biunimodular functions.
Reversing tact, we state a generalized operator-Schmidt decomposition of the
quantum Fourier transform considered as an operator C^M_1 tensor C^M_2 -->
C^N_1 tensor C^N_2, with M_1 x M_2 = N_1 x N_2. This decomposition shows (by
Nielsen's bound) that the communication cost of the QFT remains maximal when a
net transfer of qudits is permitted. In an appendix, a canonical procedure is
given for removing basis-dependence for results and proofs depending on the
"magic basis" introduced in [S. Hill and W. Wootters, "Entanglement of a pair
of quantum bits," Phys Rev. Lett 78 (1997) 5022-5025, quant-ph/9703041 (and
quant-ph/9709029)].Comment: More formal version of my talk at the Simons Conference on Quantum
and Reversible Computation at Stony Brook May 31, 2003. The talk slides and
audio are available at
http://www.physics.sunysb.edu/itp/conf/simons-qcomputation.html. Fixed typos
and minor cosmetic
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