662 research outputs found
Measuring entanglement in condensed matter systems
We show how entanglement may be quantified in spin and cold atom many-body
systems using standard experimental techniques only. The scheme requires no
assumptions on the state in the laboratory and a lower bound to the
entanglement can be read off directly from the scattering cross section of
Neutrons deflected from solid state samples or the time-of-flight distribution
of cold atoms in optical lattices, respectively. This removes a major obstacle
which so far has prevented the direct and quantitative experimental study of
genuine quantum correlations in many-body systems: The need for a full
characterization of the state to quantify the entanglement contained in it.
Instead, the scheme presented here relies solely on global measurements that
are routinely performed and is versatile enough to accommodate systems and
measurements different from the ones we exemplify in this work.Comment: 6 pages, 2 figure
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
Entanglement in general two-mode continuous-variable states: local approach and mapping to a two-qubit system
We present a new approach to the analysis of entanglement in smooth bipartite
continuous-variable states. One or both parties perform projective filterings
via preliminary measurements to determine whether the system is located in some
region of space; we study the entanglement remaining after filtering. For small
regions, a two-mode system can be approximated by a pair of qubits and its
entanglement fully characterized, even for mixed states. Our approach may be
extended to any smooth bipartite pure state or two-mode mixed state, leading to
natural definitions of concurrence and negativity densities. For Gaussian
states both these quantities are constant throughout configuration space.Comment: 4 pages, RevTeX 4, one figure. Further modifications in response to
journal referees, correction to expression for negativit
Bounds on relative entropy of entanglement for multi-party systems
We present upper and lower bounds to the relative entropy of entanglement of
multi-party systems in terms of the bi-partite entanglements of formation and
distillation and entropies of various subsystems. We point out implications of
our results to the local reversible convertibility of multi-party pure states
and discuss their physical basis in terms of deleting of information.Comment: 4 pages, no figure
Observable estimation of entanglement of formation and quantum discord for bipartite mixed quantum states
We present observable lower and upper bounds for the entanglement of
formation (EOF) and quantum discord (QD), which facilitates estimates of EOF
and QD for arbitrary experimental unknown states in finite-dimensional
bipartite systems. These bounds can be easily obtained by a few experimental
measurements on a twofold copy of the mixed states.
Based on our results, we use the experimental measurement data of the real
experiment given by Schmid \textit{et al.} [Phys. Rev. Lett. \textbf{101},
260505 (2008)] to obtain the lower and upper bounds of EOF and QD for the
experimental unknown state.Comment: 8 pages, 5 figure
Entanglement and permutational symmetry
We study the separability of permutationally symmetric quantum states. We
show that for bipartite symmetric systems most of the relevant entanglement
criteria coincide. However, we provide a method to generate examples of bound
entangled states in symmetric systems, for the bipartite and the multipartite
case. These states shed some new light on the nature of bound entanglement.Comment: 5 pages, no figures, revtex4; v3: published versio
Tripartite entanglement and quantum relative entropy
We establish relations between tripartite pure state entanglement and
additivity properties of the bipartite relative entropy of entanglement. Our
results pertain to the asymptotic limit of local manipulations on a large
number of copies of the state. We show that additivity of the relative entropy
would imply that there are at least two inequivalent types of asymptotic
tripartite entanglement. The methods used include the application of some
useful lemmas that enable us to analytically calculate the relative entropy for
some classes of bipartite states.Comment: 7 pages, revtex, no figures. v2: discussion about recent results, 2
refs. added. Published versio
Manipulating the quantum information of the radial modes of trapped ions: Linear phononics, entanglement generation, quantum state transmission and non-locality tests
We present a detailed study on the possibility of manipulating quantum
information encoded in the "radial" modes of arrays of trapped ions (i.e., in
the ions' oscillations orthogonal to the trap's main axis). In such systems,
because of the tightness of transverse confinement, the radial modes pertaining
to different ions can be addressed individually. In the first part of the paper
we show that, if local control of the radial trapping frequencies is available,
any linear optical and squeezing operation on the locally defined modes - on
single as well as on many modes - can be reproduced by manipulating the
frequencies. Then, we proceed to describe schemes apt to generate unprecedented
degrees of bipartite and multipartite continuous variable entanglement under
realistic noisy working conditions, and even restricting only to a global
control of the trapping frequencies. Furthermore, we consider the transmission
of the quantum information encoded in the radial modes along the array of ions,
and show it to be possible to a remarkable degree of accuracy, for both
finite-dimensional and continuous variable quantum states. Finally, as an
application, we show that the states which can be generated in this setting
allow for the violation of multipartite non-locality tests, by feasible
displaced parity measurements. Such a demonstration would be a first test of
quantum non-locality for "massive" degrees of freedom (i.e., for degrees of
freedom describing the motion of massive particles).Comment: 21 pages; this paper, presenting a far more extensive and detailed
analysis, completely supersedes arXiv:0708.085
Entangled light from white noise.
Peer reviewe
Quantifying mixed-state quantum entanglement by optimal entanglement witness
We develop an approach of quantifying entanglement in mixed quantum states by
the optimal entanglement witness operator. We identify the convex set of mixed
states for which a single witness provides the exact value of an entanglement
measure, and show that the convexity, properties, and symmetries of
entanglement or of a target state considerably fix the form of the optimal
witness. This greatly reduces difficulty in computing and experimentally
determining entanglement measures. As an example, we show how to experimentally
quantify bound entanglement in four-qubit noisy Smolin states and three-qubit
Greenberger-Horne-Zeilinger (GHZ) entanglement under white noise. For general
measures and states, we provide a numerical method to efficiently optimize
witness.Comment: Supplemental material is include
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