887 research outputs found
Unconditional privacy over channels which cannot convey quantum information
By sending systems in specially prepared quantum states, two parties can
communicate without an eavesdropper being able to listen. The technique, called
quantum cryptography, enables one to verify that the state of the quantum
system has not been tampered with, and thus one can obtain privacy regardless
of the power of the eavesdropper. All previous protocols relied on the ability
to faithfully send quantum states. In fact, until recently, they could all be
reduced to a single protocol where security is ensured though sharing maximally
entangled states. Here we show this need not be the case -- one can obtain
verifiable privacy even through some channels which cannot be used to reliably
send quantum states.Comment: Related to quant-ph/0608195 and for a more general audienc
Measuring Multipartite Concurrence with a Single Factorizable Observable
We show that, for any composite system with an arbitrary number of
finite-dimensional subsystems, it is possible to directly measure the
multipartite concurrence of pure states by detecting only one single
factorizable observable, provided that two copies of the composite state are
available. This result can be immediately put into practice in trapped-ion and
entangled-photon experiments.Comment: 4 pages; no figures; published versio
On the geometric distance between quantum states with positive partial transposition and private states
We prove an analytic positive lower bound for the geometric distance between
entangled positive partial transpose (PPT) states of a broad class and any
private state that delivers one secure key bit. Our proof holds for any Hilbert
space of finite dimension. Although our result is proven for a specific class
of PPT states, we show that our bound nonetheless holds for all known entangled
PPT states with non-zero distillable key rates whether or not they are in our
special class.Comment: 16 page
Dynamics of quantum entanglement
A model of discrete dynamics of entanglement of bipartite quantum state is
considered. It involves a global unitary dynamics of the system and periodic
actions of local bistochastic or decaying channel. For initially pure states
the decay of entanglement is accompanied with an increase of von Neumann
entropy of the system. We observe and discuss revivals of entanglement due to
unitary interaction of both subsystems. For some mixed states having different
marginal entropies of both subsystems (one of them larger than the global
entropy and the other one one smaller) we find an asymmetry in speed of
entanglement decay. The entanglement of these states decreases faster, if the
depolarizing channel acts on the "classical" subsystem, characterized by
smaller marginal entropy.Comment: 10 pages, Revtex, 10 figures, refined versio
Sudden death of effective entanglement
Sudden death of entanglement is a well-known effect resulting from the finite
volume of separable states. We study the case when the observer has a limited
measurement capability and analyse the effective entanglement, i.e.
entanglement minimized over the output data. We show that in the well defined
system of two quantum dots monitored by single electron transistors, one may
observe a sudden death of effective entanglement when real, physical
entanglement is still alive. For certain measurement setups, this occurs even
for initial states for which sudden death of physical entanglement is not
possible at all. The principles of the analysis may be applied to other
analogous scenarios, such as etimation of the parameters arising from quantum
process tomography.Comment: final version, 5 pages, 3 figure
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