24,009 research outputs found
Efficient two-step entanglement concentration for arbitrary W states
We present two two-step practical entanglement concentration protocols (ECPs)
for concentrating an arbitrary three-particle less-entangled W state into a
maximally entangled W state assisted with single photons. The first protocol
uses the linear optics and the second protocol adopts the cross-Kerr
nonlinearity to perform the protocol. In the first protocol, based on the
post-selection principle, three parties say Alice, Bob and Charlie in different
distant locations can obtain the maximally entangled W state from the arbitrary
less-entangled W state with a certain success probability. In the second
protocol, it dose not require the parties to posses the sophisticated
single-photon detectors and the concentrated photon pair can be retained after
performing this protocol successfully. Moreover, the second protocol can be
repeated to get a higher success probability. Both protocols may be useful in
practical quantum information applications.Comment: 10 pages, 4 figure
Exact solutions of semilinear radial wave equations in n dimensions
Exact solutions are derived for an n-dimensional radial wave equation with a
general power nonlinearity. The method, which is applicable more generally to
other nonlinear PDEs, involves an ansatz technique to solve a first-order PDE
system of group-invariant variables given by group foliations of the wave
equation, using the one-dimensional admitted point symmetry groups. (These
groups comprise scalings and time translations, admitted for any nonlinearity
power, in addition to space-time inversions admitted for a particular conformal
nonlinearity power). This is shown to yield not only group-invariant solutions
as derived by standard symmetry reduction, but also other exact solutions of a
more general form. In particular, solutions with interesting analytical
behavior connected with blow ups as well as static monopoles are obtained.Comment: 29 pages, 1 figure. Published version with minor correction
Tunable heat pump by modulating the coupling to the leads
We follow the nonequilibrium Green's function formalism to study
time-dependent thermal transport in a linear chain system consisting of two
semi-infinite leads connected together by a coupling that is harmonically
modulated in time. The modulation is driven by an external agent that can
absorb and emit energy. We determine the energy current flowing out of the
leads exactly by solving numerically the Dyson equation for the contour-ordered
Green's function. The amplitude of the modulated coupling is of the same order
as the interparticle coupling within each lead. When the leads have the same
temperature, our numerical results show that modulating the coupling between
the leads may direct energy to either flow into the leads simultaneously or
flow out of the leads simultaneously, depending on the values of the driving
frequency and temperature. A special combination of values of the driving
frequency and temperature exists wherein no net energy flows into or out of the
leads, even for long times. When one of the leads is warmer than the other, net
energy flows out of the warmer lead. For the cooler lead, however, the
direction of the energy current flow depends on the values of the driving
frequency and temperature. In addition, we find transient effects to become
more pronounced for higher values of the driving frequency.Comment: 10 pages; version 2 accepted for publication in PR
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