1,006 research outputs found
Demonstrating multipartite entanglement of single-particle W states: linear optical schemes
We present two linear optical schemes using nonideal photodetectors to
demonstrate inseparability of W-type N-partite entangled states containing only
a single photon. First, we show that the pairwise entanglement of arbitrary two
modes chosen from N optical modes can be detected using the method proposed by
Nha and Kim [Phys. Rev. A 74, 012317 (2006)], thereby suggesting the full
inseparability among N parties. In particular, this scheme is found to succeed
for any nonzero quantum efficiency of photodetectors. Second, we consider a
quantum teleportation network using linear optics without auxiliary modes. The
conditional teleportation can be optimized by a suitable choice of the
transmittance of the beam splitter in the Bell measurement. Specifically, we
identify the conditions under which maximum fidelity larger than classical
bound 2/3 is achieved only in cooperation with other parties. We also
investigate the case of on-off photodetectors that cannot discriminate the
number of detected photons.Comment: 5.5 pages, 2 figures, published version with slight modification
Quantum state engineering by a coherent superposition of photon subtraction and addition
We study a coherent superposition of field annihilation and creation operator
acting on continuous variable systems and propose its application for quantum
state engineering. Specifically, it is investigated how the superposed
operation transforms a classical state to a nonclassical one, together with
emerging nonclassical effects. We also propose an experimental scheme to
implement this elementary coherent operation and discuss its usefulness to
produce an arbitrary superposition of number states involving up to two
photons.Comment: published version, 7 pages, 8 figure
Entropic Uncertainty Relations via Direct-Sum Majorization Relation for Generalized Measurements
We derive an entropic uncertainty relation for generalized
positive-operator-valued measure (POVM) measurements via a direct-sum
majorization relation using Schur concavity of entropic quantities in a
finite-dimensional Hilbert space. Our approach provides a significant
improvement of the uncertainty bound compared with previous majorization-based
approaches [S. Friendland, V. Gheorghiu and G. Gour, Phys. Rev. Lett. 111,
230401 (2013); A. E. Rastegin and K. \.Zyczkowski, J. Phys. A, 49, 355301
(2016)], particularly by extending the direct-sum majorization relation first
introduced in [\L. Rudnicki, Z. Pucha{\l}a and K. \.{Z}yczkowski, Phys. Rev. A
89, 052115 (2014)]. We illustrate the usefulness of our uncertainty relations
by considering a pair of qubit observables in a two-dimensional system and
randomly chosen unsharp observables in a three-dimensional system. We also
demonstrate that our bound tends to be stronger than the generalized
Maassen--Uffink bound with an increase in the unsharpness effect. Furthermore,
we extend our approach to the case of multiple POVM measurements, thus making
it possible to establish entropic uncertainty relations involving more than two
observables
Selective interactions in trapped ions: state reconstruction and quantum logic
We propose the implementation of selective interactions of atom-motion
subspaces in trapped ions. These interactions yield resonant exchange of
population inside a selected subspace, leaving the others in a highly
dispersive regime. Selectivity allows us to generate motional Fock (and other
nonclassical) states with high purity out of a wide class of initial states,
and becomes an unconventional cooling mechanism when the ground state is
chosen. Individual population of number states can be distinctively measured,
as well as the motional Wigner function. Furthermore, a protocol for
implementing quantum logic through a suitable control of selective subspaces is
presented.Comment: 4 revtex4 pages and 2 eps figures. Submitted for publicatio
Entanglement condition via su(2) and su(1,1) algebra using Schr{\"o}dinger-Robertson uncertainty relation
The Schr{\"o}dinger-Robertson inequality generally provides a stronger bound
on the product of uncertainties for two noncommuting observables than the
Heisenberg uncertainty relation, and as such, it can yield a stricter
separability condition in conjunction with partial transposition. In this
paper, using the Schr{\"o}dinger-Robertson uncertainty relation, the
separability condition previously derived from the su(2) and the su(1,1)
algebra is made stricter and refined to a form invariant with respect to local
phase shifts. Furthermore, a linear optical scheme is proposed to test this
invariant separability condition.Comment: published version, 3.5 pages, 1 figur
Optimal continuous-variable teleportation under energy constraint
Quantum teleportation is one of the crucial protocols in quantum information
processing. It is important to accomplish an efficient teleportation under
practical conditions, aiming at a higher fidelity desirably using fewer
resources. The continuous-variable (CV) version of quantum teleportation was
first proposed using a Gaussian state as a quantum resource, while other
attempts were also made to improve performance by applying non-Gaussian
operations. We investigate the CV teleportation to find its ultimate fidelity
under energy constraint identifying an optimal quantum state. For this purpose,
we present a formalism to evaluate teleportation fidelity as an expectation
value of an operator. Using this formalism, we prove that the optimal state
must be a form of photon-number entangled states. We further show that Gaussian
states are near-optimal while non-Gaussian states make a slight improvement and
therefore are rigorously optimal, particularly in the low-energy regime.Comment: 8 pages, 4 figures, published versio
Improved BPSO for optimal PMU placement
Optimal phasor measurement unit (PMU) placement involves the process of minimizing the number of PMU needed while ensuring entire power system network completely observable. This paper presents the improved binary particle swarm (IBPSO) method that converges faster and also manage to maximize the measurement redundancy compared to the existing BPSO method. This method is applied to IEEE-30 bus system for the case of considering zero-injection bus and its effectiveness is verified by the simulation results done by using MATLAB software
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