1,859 research outputs found
Population-only decay map for n-qubit n-partite inseparability detection
We introduce a new positive linear map for a single qubit. This map is a
decay only in populations of a single-qubit density operator. It is shown that
an n-fold product of this map may be used for a detection of n-partite
inseparability of an n-qubit density operator (i.e., detection of impossibility
of representing a density operator in the form of a convex combination of
products of density operators of individual qubits). This product map is also
investigated in relation to a variant of the entanglement detection method
mentioned by Laskowski and Zukowski.Comment: 5 pages, 1 figure, RevTex4, v2 minor grammatical changes, typos
correcte
Dynamics of Atom-Field Entanglement from Exact Solutions: Towards Strong Coupling and Non-Markovian Regimes
We examine the dynamics of bipartite entanglement between a two-level atom
and the electromagnetic field. We treat the Jaynes-Cummings model with a single
field mode and examine in detail the exact time evolution of entanglement,
including cases where the atomic state is initially mixed and the atomic
transition is detuned from resonance. We then explore the effects of other
nearby modes by calculating the exact time evolution of entanglement in more
complex systems with two, three, and five field modes. For these cases we can
obtain exact solutions which include the strong coupling regimes. Finally, we
consider the entanglement of a two-level atom with the infinite collection of
modes present in the intracavity field of a Fabre-Perot cavity. In contrast to
the usual treatment of atom-field interactions with a continuum of modes using
the Born-Markov approximation, our treatment in all cases describes the full
non-Markovian dynamics of the atomic subsystem. Only when an analytic
expression for the infinite mode case is desired do we need to make a weak
coupling assumption which at long times approximates Markovian dynamics.Comment: 12 pages, 5 figures; minor changes in grammar, wording, and
formatting. One unnecessary figure removed. Figure number revised (no longer
counts subfigures separately
Entanglement requirements for implementing bipartite unitary operations
We prove, using a new method based on map-state duality, lower bounds on
entanglement resources needed to deterministically implement a bipartite
unitary using separable (SEP) operations, which include LOCC (local operations
and classical communication) as a particular case. It is known that the Schmidt
rank of an entangled pure state resource cannot be less than the Schmidt rank
of the unitary. We prove that if these ranks are equal the resource must be
uniformly (maximally) entangled: equal nonzero Schmidt coefficients. Higher
rank resources can have less entanglement: we have found numerical examples of
Schmidt rank 2 unitaries which can be deterministically implemented, by either
SEP or LOCC, using an entangled resource of two qutrits with less than one ebit
of entanglement.Comment: 7 pages Revte
Efficient generation of random multipartite entangled states using time optimal unitary operations
We review the generation of random pure states using a protocol of repeated
two qubit gates. We study the dependence of the convergence to states with Haar
multipartite entanglement distribution. We investigate the optimal generation
of such states in terms of the physical (real) time needed to apply the
protocol, instead of the gate complexity point of view used in other works.
This physical time can be obtained, for a given Hamiltonian, within the
theoretical framework offered by the quantum brachistochrone formalism. Using
an anisotropic Heisenberg Hamiltonian as an example, we find that different
optimal quantum gates arise according to the optimality point of view used in
each case. We also study how the convergence to random entangled states depends
on different entanglement measures.Comment: 5 pages, 2 figures. New title, improved explanation of the algorithm.
To appear in Phys. Rev.
Information theoretic treatment of tripartite systems and quantum channels
A Holevo measure is used to discuss how much information about a given POVM
on system is present in another system , and how this influences the
presence or absence of information about a different POVM on in a third
system . The main goal is to extend information theorems for mutually
unbiased bases or general bases to arbitrary POVMs, and especially to
generalize "all-or-nothing" theorems about information located in tripartite
systems to the case of \emph{partial information}, in the form of quantitative
inequalities. Some of the inequalities can be viewed as entropic uncertainty
relations that apply in the presence of quantum side information, as in recent
work by Berta et al. [Nature Physics 6, 659 (2010)]. All of the results also
apply to quantum channels: e.g., if \EC accurately transmits certain POVMs,
the complementary channel \FC will necessarily be noisy for certain other
POVMs. While the inequalities are valid for mixed states of tripartite systems,
restricting to pure states leads to the basis-invariance of the difference
between the information about contained in and .Comment: 21 pages. An earlier version of this paper attempted to prove our
main uncertainty relation, Theorem 5, using the achievability of the Holevo
quantity in a coding task, an approach that ultimately failed because it did
not account for locking of classical correlations, e.g. see [DiVincenzo et
al. PRL. 92, 067902 (2004)]. In the latest version, we use a very different
approach to prove Theorem
Entanglement Measures for Intermediate Separability of Quantum States
We present a family of entanglement measures R_m which act as indicators for
separability of n-qubit quantum states into m subsystems for arbitrary 2 \leq m
\leq n. The measure R_m vanishes if the state is separable into m subsystems,
and for m = n it gives the Meyer-Wallach measure while for m = 2 it reduces, in
effect, to the one introduced recently by Love et al. The measures R_m are
evaluated explicitly for the GHZ state and the W state (and its modifications,
the W_k states) to show that these globally entangled states exhibit rather
distinct behaviors under the measures, indicating the utility of the measures
R_m for characterizing globally entangled states as well.Comment: 8 pages, 8 figure
Collective Uncertainty Entanglement Test
For a given pure state of a composite quantum system we analyze the product
of its projections onto a set of locally orthogonal separable pure states. We
derive a bound for this product analogous to the entropic uncertainty
relations. For bipartite systems the bound is saturated for maximally entangled
states and it allows us to construct a family of entanglement measures, we
shall call collectibility. As these quantities are experimentally accessible,
the approach advocated contributes to the task of experimental quantification
of quantum entanglement, while for a three-qubit system it is capable to
identify the genuine three-party entanglement.Comment: 4 pages, 3 figure
Microscopic Origin of Quantum Chaos in Rotational Damping
The rotational spectrum of Yb is calculated diagonalizing different
effective interactions within the basis of unperturbed rotational bands
provided by the cranked shell model. A transition between order and chaos
taking place in the energy region between 1 and 2 MeV above the yrast line is
observed, associated with the onset of rotational damping. It can be related to
the higher multipole components of the force acting among the unperturbed
rotational bands.Comment: 7 pages, plain TEX, YITP/K-99
Lower and upper bounds on the fidelity susceptibility
We derive upper and lower bounds on the fidelity susceptibility in terms of
macroscopic thermodynamical quantities, like susceptibilities and thermal
average values. The quality of the bounds is checked by the exact expressions
for a single spin in an external magnetic field. Their usefulness is
illustrated by two examples of many-particle models which are exactly solved in
the thermodynamic limit: the Dicke superradiance model and the single impurity
Kondo model. It is shown that as far as divergent behavior is considered, the
fidelity susceptibility and the thermodynamic susceptibility are equivalent for
a large class of models exhibiting critical behavior.Comment: 19 page
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