70 research outputs found
Direct versus measurement assisted bipartite entanglement in multi-qubit systems and their dynamical generation in spin systems
We consider multi-qubit systems and relate quantitatively the problems of
generating cluster states with high value of concurrence of assistance, and
that of generating states with maximal bipartite entanglement. We prove an
upper bound for the concurrence of assistance. We consider dynamics of spin-1/2
systems that model qubits, with different couplings and possible presence of
magnetic field to investigate the appearance of the discussed entanglement
properties. We find that states with maximal bipartite entanglement can be
generated by an XY Hamiltonian, and their generation can be controlled by the
initial state of one of the spins. The same Hamiltonian is capable of creating
states with high concurrence of assistance with suitably chosen initial state.
We show that the production of graph states using the Ising Hamiltonian is
controllable via a single-qubit rotation of one spin-1/2 subsystem in the
initial multi-qubit state. We shown that the property of Ising dynamics to
convert a product state basis into a special maximally entangled basis is
temporally enhanced by the application of a suitable magnetic field. Similar
basis transformations are found to be feasible in the case of isotropic XY
couplings with magnetic field.Comment: (14 pages, 7 figures, RevTeX4
Choi representation of completely positive maps: a technical introduction
This is a very brief operational introduction to the Choi representation of
completely positive maps, i.e. quantum channels. It focuses on certain useful
calculational techniques which are presented in full detail
Quantum scissors: teleportation of single-mode optical states by means of a nonlocal single photon
We employ the quantum state of a single photon entangled with the vacuum
(|1,0>-|0,1>), generated by a photon incident upon a symmetric beam splitter,
to teleport single-mode quantum states of light by means of the Bennett
protocol. Teleportation of coherent states results in truncation of their Fock
expansion to the first two terms. We analyze the teleported ensembles by means
of homodyne tomography and obtain fidelities of up to 99 per cent for low
source state amplitudes. This work is an experimental realization of the
quantum scissors device proposed by Pegg, Phillips and Barnett (Phys. Rev.
Lett. 81, 1604 (1998)
Teleportation: from probability distributions to quantum states
The role of the off-diagonal density matrix elements of the entangled pair is
investigated in quantum teleportation of a qbit. The dependence between them
and the off-diagonal elements of the teleported density matrix is shown to be
linear. In this way the ideal quantum teleportation is related to an entirely
classical communication protocol: the one-time pad cypher. The latter can be
regarded as the classical counterpart of Bennett's quantum teleportation
scheme. The quantum-to-classical transition is demonstrated on the statistics
of a gedankenexperiment.Comment: 11 pages, 1 figure, accepted for publication in J. Phys. A (Math.
Gen.
Quantum homogenization and state randomization in semi-quantal spin systems
We investigate dynamics of semi-quantal spin systems in which quantum bits
are attached to classically and possibly stochastically moving classical
particles. The interaction between the quantum bits takes place when the
respective classical particles get close to each other in space. We find that
with Heisenberg XX couplings quantum homogenization takes place after a time
long enough, regardless of the details of the underlying classical dynamics.
This is accompanied by the development of a stationary bipartite entanglement.
If the information on the details of the motion of a stochastic classical
system is disregarded, the stationary state of the whole quantum subsystem is
found to be a complete mixture in the studied cases, though the transients
depend on the properties of the classical motion.Comment: 10 pages, 10 figures (included
Wigner-function description of quantum teleportation in arbitrary dimensions and continuous limit
We present a unified approach to quantum teleportation in arbitrary
dimensions based on the Wigner-function formalism. This approach provides us
with a clear picture of all manipulations performed in the teleportation
protocol. In addition within the framework of the Wigner-function formalism all
the imperfections of the manipulations can be easily taken into account.Comment: 8 pages, LaTeX, 1 figure (included). Accepted for publication in
Phys. Rev. A A minor correction added on May 2
Pulse-mode quantum projection synthesis: Effects of mode mismatch on optical state truncation and preparation
Quantum projection synthesis can be used for phase-probability-distribution
measurement, optical-state truncation and preparation. The method relies on
interfering optical lights, which is a major challenge in experiments performed
by pulsed light sources. In the pulsed regime, the time frequency overlap of
the interfering lights plays a crucial role on the efficiency of the method
when they have different mode structures. In this paper, the pulsed mode
projection synthesis is developed, the mode structure of interfering lights are
characterized and the effect of this overlap (or mode match) on the fidelity of
optical-state truncation and preparation is investigated. By introducing the
positive-operator-valued measure (POVM) for the detection events in the scheme,
the effect of mode mismatch between the photon-counting detectors and the
incident lights are also presented.Comment: 11 pages, 4 figures, submitted to Phys. Rev.
Discrete Wigner functions and the phase space representation of quantum teleportation
We present a phase space description of the process of quantum teleportation
for a system with an dimensional space of states. For this purpose we
define a discrete Wigner function which is a minor variation of previously
existing ones. This function is useful to represent composite quantum system in
phase space and to analyze situations where entanglement between subsystems is
relevant (dimensionality of the space of states of each subsystem is
arbitrary). We also describe how a direct tomographic measurement of this
Wigner function can be performed.Comment: 8 pages, 1 figure, to appear in Phys Rev
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