1,416 research outputs found
Multi-copy and stochastic transformation of multipartite pure states
Characterizing the transformation and classification of multipartite
entangled states is a basic problem in quantum information. We study the
problem under two most common environments, local operations and classical
communications (LOCC), stochastic LOCC and two more general environments,
multi-copy LOCC (MCLOCC) and multi-copy SLOCC (MCSLOCC). We show that two
transformable multipartite states under LOCC or SLOCC are also transformable
under MCLOCC and MCSLOCC. What's more, these two environments are equivalent in
the sense that two transformable states under MCLOCC are also transformable
under MCSLOCC, and vice versa. Based on these environments we classify the
multipartite pure states into a few inequivalent sets and orbits, between which
we build the partial order to decide their transformation. In particular, we
investigate the structure of SLOCC-equivalent states in terms of tensor rank,
which is known as the generalized Schmidt rank. Given the tensor rank, we show
that GHZ states can be used to generate all states with a smaller or equivalent
tensor rank under SLOCC, and all reduced separable states with a cardinality
smaller or equivalent than the tensor rank under LOCC. Using these concepts, we
extended the concept of "maximally entangled state" in the multi-partite
system.Comment: 8 pages, 1 figure, revised version according to colleagues' comment
On-demand generation of entanglement of atomic qubits via optical interferometry
The problem of on-demand generation of entanglement between single-atom
qubits via a common photonic channel is examined within the framework of
optical interferometry. As expected, for a Mach-Zehnder interferometer with
coherent laser beam as input, a high-finesse optical cavity is required to
overcome sensitivity to spontaneous emission. We show, however, that with a
twin-Fock input, useful entanglement can in principle be created without
cavity-enhancement. Both approaches require single-photon resolving detectors,
and best results would be obtained by combining both cavity-feedback and
twin-Fock inputs. Such an approach may allow a fidelity of using a
two-photon input and currently available mirror and detector technology. In
addition, we study interferometers based on NOON states and show that they
perform similarly to the twin-Fock states, yet without the need for
high-precision photo-detectors. The present interferometrical approach can
serve as a universal, scalable circuit element for quantum information
processing, from which fast quantum gates, deterministic teleportation,
entanglement swapping , can be realized with the aid of single-qubit
operations.Comment: To be published in PR
Greenberger-Horne-Zeilinger state protocols for fully connected qubit networks
We generalize the recently proposed Greenberger-Horne-Zeilinger (GHZ)
tripartite protocol [A. Galiautdinov, J. M. Martinis, Phys. Rev. A 78,
010305(R) (2008)] to fully connected networks of weakly coupled qubits
interacting by way of anisotropic Heisenberg exchange g(XX+YY)+g1*ZZ. Our model
adopted here differs from the more familiar Ising-Heisenberg chain in that here
every qubit interacts with every other qubit in the circuit. The assumption of
identical couplings on all qubit pairs allows an elegant proof of the protocol
for arbitrary N. In order to further make contact with experiment, we study
fidelity degradation due to coupling imperfections by numerically simulating
the N=3 and N=4 cases. Our simulations indicate that the best fidelity at
unequal couplings is achieved when (a) the system is initially prepared in the
uniform superposition state (similarly to how it is done in the ideal case),
and (b) the entangling time and the final rotations on each of the qubits are
appropriately adjusted.Comment: 11 pages, 1 figur
Full characterization of a three-photon GHZ state using quantum state tomography
We have performed the first experimental tomographic reconstruction of a
three-photon polarization state. Quantum state tomography is a powerful tool
for fully describing the density matrix of a quantum system. We measured 64
three-photon polarization correlations and used a "maximum-likelihood"
reconstruction method to reconstruct the GHZ state. The entanglement class has
been characterized using an entanglement witness operator and the maximum
predicted values for the Mermin inequality was extracted.Comment: 3 pages, 3 figure
Generalized Quantum Theory: Overview and Latest Developments
The main formal structures of Generalized Quantum Theory are summarized.
Recent progress has sharpened some of the concepts, in particular the notion of
an observable, the action of an observable on states (putting more emphasis on
the role of proposition observables), and the concept of generalized
entanglement. Furthermore, the active role of the observer in the structure of
observables and the partitioning of systems is emphasized.Comment: 14 pages, update in reference
Hardy's proof of nonlocality in the presence of noise
We extend the validity of Hardy's nonlocality without inequalities proof to
cover the case of special one-parameter classes of non-pure statistical
operators. These mixed states are obtained by mixing the Hardy states with a
completely chaotic noise or with a colored noise and they represent a realistic
description of imperfect preparation processes of (pure) Hardy states in
nonlocality experiments. Within such a framework we are able to exhibit a
precise range of values of the parameter measuring the noise affecting the
non-optimal preparation of an arbitrary Hardy state, for which it is still
possible to put into evidence genuine nonlocal effects. Equivalently, our work
exhibits particular classes of bipartite mixed states whose constituents do not
admit any local and deterministic hidden variable model reproducing the quantum
mechanical predictions.Comment: 9 pages, 2 figures, RevTex, revised versio
Channel Capacities versus Entanglement Measures in Multiparty Quantum States
For quantum states of two subsystems, entanglement measures are related to
capacities of communication tasks -- highly entangled states give higher
capacity of transmitting classical as well as quantum information. However, we
show that this is no more the case in general: quantum capacities of
multi-access channels, motivated by communication in quantum networks, do not
have any relation with genuine multiparty entanglement measures. Along with
revealing the structural richness of multi-access channel capacities, this
gives us a tool to classify multiparty quantum states from the perspective of
its usefulness in quantum networks, which cannot be visualized by known
multiparty entanglement measures.Comment: 6 pages, 2 figures, RevTeX4; v2: minor changes, some implications
strengthene
Greenberger-Horne-Zeilinger argument of nonlocality without inequalities for mixed states
We generalize the Greenberger-Horne-Zeilinger nonlocality without
inequalities argument to cover the case of arbitrary mixed statistical
operators associated to three-qubits quantum systems. More precisely, we
determine the radius of a ball (in the trace distance topology) surrounding the
pure GHZ state and containing arbitrary mixed statistical operators which
cannot be described by any local and realistic hidden variable model and which
are, as a consequence, noncompletely separable. As a practical application, we
focus on certain one-parameter classes of mixed states which are commonly
considered in the experimental realization of the original GHZ argument and
which result from imperfect preparations of the pure GHZ state. In these cases
we determine for which values of the parameter controlling the noise a
nonlocality argument can still be exhibited, despite the mixedness of the
considered states. Moreover, the effect of the imperfect nature of measurement
processes is discussed.Comment: 8 pages, RevTex; added references, corrected typo
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