381 research outputs found
Continuous quantum error correction by cooling
We describe an implementation of quantum error correction that operates
continuously in time and requires no active interventions such as measurements
or gates. The mechanism for carrying away the entropy introduced by errors is a
cooling procedure. We evaluate the effectiveness of the scheme by simulation,
and remark on its connections to some recently proposed error prevention
procedures.Comment: 8 pages, 5 figures. Published version. Minor change in conten
Polarized ensembles of random pure states
A new family of polarized ensembles of random pure states is presented. These
ensembles are obtained by linear superposition of two random pure states with
suitable distributions, and are quite manageable. We will use the obtained
results for two purposes: on the one hand we will be able to derive an
efficient strategy for sampling states from isopurity manifolds. On the other,
we will characterize the deviation of a pure quantum state from separability
under the influence of noise.Comment: 14 pages, 1 figur
Zeno Dynamics in Quantum Statistical Mechanics
We study the quantum Zeno effect in quantum statistical mechanics within the
operator algebraic framework. We formulate a condition for the appearance of
the effect in W*-dynamical systems, in terms of the short-time behaviour of the
dynamics. Examples of quantum spin systems show that this condition can be
effectively applied to quantum statistical mechanical models. Further, we
derive an explicit form of the Zeno generator, and use it to construct Gibbs
equilibrium states for the Zeno dynamics. As a concrete example, we consider
the X-Y model, for which we show that a frequent measurement at a microscopic
level, e.g. a single lattice site, can produce a macroscopic effect in changing
the global equilibrium.Comment: 15 pages, AMSLaTeX; typos corrected, references updated and added,
acknowledgements added, style polished; revised version contains corrections
from published corrigend
Local Hamiltonians for Maximally Multipartite Entangled States
We study the conditions for obtaining maximally multipartite entangled states
(MMES) as non-degenerate eigenstates of Hamiltonians that involve only
short-range interactions. We investigate small-size systems (with a number of
qubits ranging from 3 to 5) and show some example Hamiltonians with MMES as
eigenstates.Comment: 6 pages, 3 figures, published versio
Classical limit of the quantum Zeno effect
The evolution of a quantum system subjected to infinitely many measurements
in a finite time interval is confined in a proper subspace of the Hilbert
space. This phenomenon is called "quantum Zeno effect": a particle under
intensive observation does not evolve. This effect is at variance with the
classical evolution, which obviously is not affected by any observations. By a
semiclassical analysis we will show that the quantum Zeno effect vanishes at
all orders, when the Planck constant tends to zero, and thus it is a purely
quantum phenomenon without classical analog, at the same level of tunneling.Comment: 10 pages, 2 figure
Classical Statistical Mechanics Approach to Multipartite Entanglement
We characterize the multipartite entanglement of a system of n qubits in
terms of the distribution function of the bipartite purity over balanced
bipartitions. We search for maximally multipartite entangled states, whose
average purity is minimal, and recast this optimization problem into a problem
of statistical mechanics, by introducing a cost function, a fictitious
temperature and a partition function. By investigating the high-temperature
expansion, we obtain the first three moments of the distribution. We find that
the problem exhibits frustration.Comment: 38 pages, 10 figures, published versio
Quantum Zeno tomography
We show that the resolution "per absorbed particle" of standard absorption
tomography can be outperformed by a simple interferometric setup, provided that
the different levels of "gray" in the sample are not uniformly distributed. The
technique hinges upon the quantum Zeno effect and has been tested in numerical
simulations. The scheme we propose could be implemented in experiments with
UV-light, neutrons or X-rays.Comment: 8 pages, 5 figure
Zeno dynamics and constraints
We investigate some examples of quantum Zeno dynamics, when a system
undergoes very frequent (projective) measurements that ascertain whether it is
within a given spatial region. In agreement with previously obtained results,
the evolution is found to be unitary and the generator of the Zeno dynamics is
the Hamiltonian with hard-wall (Dirichlet) boundary conditions. By using a new
approach to this problem, this result is found to be valid in an arbitrary
-dimensional compact domain. We then propose some preliminary ideas
concerning the algebra of observables in the projected region and finally look
at the case of a projection onto a lower dimensional space: in such a situation
the Zeno ansatz turns out to be a procedure to impose constraints.Comment: 21 page
On the local unitary equivalence of states of multi-partite systems
Two pure states of a multi-partite system are alway are related by a unitary
transformation acting on the Hilbert space of the whole system. This
transformation involves multi-partite transformations. On the other hand some
quantum information protocols such as the quantum teleportation and quantum
dense coding are based on equivalence of some classes of states of bi-partite
systems under the action of local (one-particle) unitary operations. In this
paper we address the question: ``Under what conditions are the two states
states, and , of a multi-partite system locally unitary
equivalent?'' We present a set of conditions which have to be satisfied in
order that the two states are locally unitary equivalent. In addition, we study
whether it is possible to prepare a state of a multi-qudit system. which is
divided into two parts A and B, by unitary operations acting only on the
systems A and B, separately.Comment: 6 revtex pages, 1 figur
Multipartite entanglement characterization of a quantum phase transition
A probability density characterization of multipartite entanglement is tested
on the one-dimensional quantum Ising model in a transverse field. The average
and second moment of the probability distribution are numerically shown to be
good indicators of the quantum phase transition. We comment on multipartite
entanglement generation at a quantum phase transition.Comment: 10 pages, 6 figures, final versio
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