235 research outputs found
Configuration of separability and tests for multipartite entanglement in Bell-type experiments
We derive tight quadratic inequalities for all kinds of hybrid
separable-inseparable -particle density operators on an arbitrary
dimensional space. This methodology enables us to truly derive a tight
quadratic inequality as tests for full -partite entanglement in various
Bell-type correlation experiments on the systems that may not be identified as
a collection of qubits, e.g., those involving photons measured by incomplete
detectors. It is also proved that when the two measured observables are assumed
to precisely anti-commute, a stronger quadratic inequality can be used as a
witness of full -partite entanglement.Comment: To appear in Phys. Rev. Lett. (submitted on Jul. 4, 2002
Photonic multipartite entanglement conversion using nonlocal operations
We propose a simple setup for the conversion of multipartite entangled states
in a quantum network with restricted access. The scheme uses nonlocal
operations to enable the preparation of states that are inequivalent under
local operations and classical communication, but most importantly does not
require full access to the states. It is based on a flexible linear optical
conversion gate that uses photons, which are ideally suited for distributed
quantum computation and quantum communication in extended networks. In order to
show the basic working principles of the gate, we focus on converting a
four-qubit entangled cluster state to other locally inequivalent four-qubit
states, such as the GHZ and symmetric Dicke state. We also show how the gate
can be incorporated into extended graph state networks, and can be used to
generate variable entanglement and quantum correlations without entanglement
but nonvanishing quantum discord.Comment: 10 pages, 6 figures, correction of reference list, add Journal ref.
and DO
Fidelity criterion for quantum-domain transmission and storage of coherent states beyond unit-gain constraint
We generalize the experimental success criterion for quantum
teleportation/memory in continuous-variable quantum systems to be suitable for
non-unit-gain condition by considering attenuation/amplification of the
coherent-state amplitude. The new criterion can be used for a non-ideal quantum
memory and long distance quantum communication as well as quantum devices with
amplification process. It is also shown that the framework to measure the
average fidelity is capable of detecting all Gaussian channels in quantum
domain.Comment: 4pages, No figures, Accepted for publication in PR
Entanglement of orbital angular momentum states between an ensemble of cold atoms and a photon
Recently, atomic ensemble and single photons were successfully entangled by
using collective enhancement [D. N. Matsukevich, \textit{et al.}, Phys. Rev.
Lett. \textbf{95}, 040405(2005).], where atomic internal states and photonic
polarization states were correlated in nonlocal manner. Here we experimentally
clarified that in an ensemble of atoms and a photon system, there also exists
an entanglement concerned with spatial degrees of freedom. Generation of
higher-dimensional entanglement between remote atomic ensemble and an
application to condensed matter physics are also discussed.Comment: 5 pages, 3 figure
Faithful qubit distribution assisted by one additional qubit against collective noise
We propose a distribution scheme of polarization states of a single photon
over collective-noise channel. By adding one extra photon with a fixed
polarization, we can protect the state against collective noise via a
parity-check measurement and post-selection. While the scheme succeeds only
probabilistically, it is simpler and more flexible than the schemes utilizing
decoherence-free subspace. An application to BB84 protocol through collective
noise channel, which is robust to the Trojan horse attack, is also given.Comment: 4 pages, 3 figures; published version in Phys. Rev. Let
Unconditionally Secure Key Distribution Based on Two Nonorthogonal States
We prove the unconditional security of the Bennett 1992 protocol, by using a
reduction to an entanglement distillation protocol initiated by a local
filtering process. The bit errors and the phase errors are correlated after the
filtering, and we can bound the amount of phase errors from the observed bit
errors by an estimation method involving nonorthogonal measurements. The angle
between the two states shows a trade-off between accuracy of the estimation and
robustness to noises.Comment: 5 pages, 1 figur
Entanglement cost of generalised measurements
Bipartite entanglement is one of the fundamental quantifiable resources of
quantum information theory. We propose a new application of this resource to
the theory of quantum measurements. According to Naimark's theorem any rank 1
generalised measurement (POVM) M may be represented as a von Neumann
measurement in an extended (tensor product) space of the system plus ancilla.
By considering a suitable average of the entanglements of these measurement
directions and minimising over all Naimark extensions, we define a notion of
entanglement cost E_min(M) of M.
We give a constructive means of characterising all Naimark extensions of a
given POVM. We identify various classes of POVMs with zero and non-zero cost
and explicitly characterise all POVMs in 2 dimensions having zero cost. We
prove a constant upper bound on the entanglement cost of any POVM in any
dimension. Hence the asymptotic entanglement cost (i.e. the large n limit of
the cost of n applications of M, divided by n) is zero for all POVMs.
The trine measurement is defined by three rank 1 elements, with directions
symmetrically placed around a great circle on the Bloch sphere. We give an
analytic expression for its entanglement cost. Defining a normalised cost of
any d-dimensional POVM by E_min(M)/log(d), we show (using a combination of
analytic and numerical techniques) that the trine measurement is more costly
than any other POVM with d>2, or with d=2 and ancilla dimension 2. This
strongly suggests that the trine measurement is the most costly of all POVMs.Comment: 20 pages, plain late
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