21,419 research outputs found
Maximal Entanglement - A New Measure of Entanglement
Maximal correlation is a measure of correlation for bipartite distributions.
This measure has two intriguing features: (1) it is monotone under local
stochastic maps; (2) it gives the same number when computed on i.i.d. copies of
a pair of random variables. This measure of correlation has recently been
generalized for bipartite quantum states, for which the same properties have
been proved. In this paper, based on maximal correlation, we define a new
measure of entanglement which we call maximal entanglement. We show that this
measure of entanglement is faithful (is zero on separable states and positive
on entangled states), is monotone under local quantum operations, and gives the
same number when computed on tensor powers of a bipartite state.Comment: 8 pages, presented at IWCIT 201
Linking a distance measure of entanglement to its convex roof
An important problem in quantum information theory is the quantification of
entanglement in multipartite mixed quantum states. In this work, a connection
between the geometric measure of entanglement and a distance measure of
entanglement is established. We present a new expression for the geometric
measure of entanglement in terms of the maximal fidelity with a separable
state. A direct application of this result provides a closed expression for the
Bures measure of entanglement of two qubits. We also prove that the number of
elements in an optimal decomposition w.r.t. the geometric measure of
entanglement is bounded from above by the Caratheodory bound, and we find
necessary conditions for the structure of an optimal decomposition.Comment: 11 pages, 4 figure
Optimal Entanglement Enhancement for Mixed States
We consider the actions of protocols involving local quantum operations and
classical communication (LQCC) on a single system consisting of two separated
qubits. We give a complete description of the orbits of the space of states
under LQCC and characterise the representatives with maximal entanglement of
formation. We thus obtain a LQCC entanglement concentration protocol for a
single given state (pure or mixed) of two qubits which is optimal in the sense
that the protocol produces, with non-zero probability, a state of maximal
possible entanglement of formation. This defines a new entanglement measure,
the maximum extractable entanglement.Comment: Final version: to appear in Phys. Rev. Let
Operational one-to-one mapping between coherence and entanglement measures
We establish a general operational one-to-one mapping between coherence
measures and entanglement measures: Any entanglement measure of bipartite pure
states is the minimum of a suitable coherence measure over product bases. Any
coherence measure of pure states, with extension to mixed states by convex
roof, is the maximum entanglement generated by incoherent operations acting on
the system and an incoherent ancilla. Remarkably, the generalized CNOT gate is
the universal optimal incoherent operation. In this way, all convex-roof
coherence measures, including the coherence of formation, are endowed with
(additional) operational interpretations. By virtue of this connection, many
results on entanglement can be translated to the coherence setting, and vice
versa. As applications, we provide tight observable lower bounds for
generalized entanglement concurrence and coherence concurrence, which enable
experimentalists to quantify entanglement and coherence of the maximal
dimension in real experiments.Comment: 14 pages, 1 figure, new results added, published in PR
Entanglement, quantum randomness, and complexity beyond scrambling
Scrambling is a process by which the state of a quantum system is effectively
randomized due to the global entanglement that "hides" initially localized
quantum information. In this work, we lay the mathematical foundations of
studying randomness complexities beyond scrambling by entanglement properties.
We do so by analyzing the generalized (in particular R\'enyi) entanglement
entropies of designs, i.e. ensembles of unitary channels or pure states that
mimic the uniformly random distribution (given by the Haar measure) up to
certain moments. A main collective conclusion is that the R\'enyi entanglement
entropies averaged over designs of the same order are almost maximal. This
links the orders of entropy and design, and therefore suggests R\'enyi
entanglement entropies as diagnostics of the randomness complexity of
corresponding designs. Such complexities form a hierarchy between information
scrambling and Haar randomness. As a strong separation result, we prove the
existence of (state) 2-designs such that the R\'enyi entanglement entropies of
higher orders can be bounded away from the maximum. However, we also show that
the min entanglement entropy is maximized by designs of order only logarithmic
in the dimension of the system. In other words, logarithmic-designs already
achieve the complexity of Haar in terms of entanglement, which we also call
max-scrambling. This result leads to a generalization of the fast scrambling
conjecture, that max-scrambling can be achieved by physical dynamics in time
roughly linear in the number of degrees of freedom.Comment: 72 pages, 4 figures. Rewritten version with new title. v3: published
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Analysis of entanglement measures and LOCC maximized quantum fisher information of general two qubit systems
PubMed ID: 24957694Entanglement has been studied extensively for unveiling the mysteries of non-classical correlations between quantum systems. In the bipartite case, there are well known measures for quantifying entanglement such as concurrence, relative entropy of entanglement (REE) and negativity, which cannot be increased via local operations. It was found that for sets of non-maximally entangled states of two qubits, comparing these entanglement measures may lead to different entanglement orderings of the states. On the other hand, although it is not an entanglement measure and not monotonic under local operations, due to its ability of detecting multipartite entanglement, quantum Fisher information (QFI) has recently received an intense attraction generally with entanglement in the focus. In this work, we revisit the state ordering problem of general two qubit states. Generating a thousand random quantum states and performing an optimization based on local general rotations of each qubit, we calculate the maximal QFI for each state. We analyze the maximized QFI in comparison with concurrence, REE and negativity and obtain new state orderings. We show that there are pairs of states having equal maximized QFI but different values for concurrence, REE and negativity and vice versa.Publisher's Versio
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