161 research outputs found
On the quantumness of correlations in nuclear magnetic resonance
Nuclear Magnetic Resonance (NMR) was successfully employed to test several
protocols and ideas in Quantum Information Science. In most of these
implementations the existence of entanglement was ruled out. This fact
introduced concerns and questions about the quantum nature of such bench tests.
In this article we address some issues related to the non-classical aspects of
NMR systems. We discuss some experiments where the quantum aspects of this
system are supported by quantum correlations of separable states. Such
quantumness, beyond the entanglement-separability paradigm, is revealed via a
departure between the quantum and the classical versions of information theory.
In this scenario, the concept of quantum discord seems to play an important
role. We also present an experimental implementation of an analogous of the
single-photon Mach-Zehnder interferometer employing two nuclear spins to encode
the interferometric paths. This experiment illustrate how non-classical
correlations of separable states may be used to simulate quantum dynamics. The
results obtained are completely equivalent to the optical scenario, where
entanglement (between two field modes) may be present
Experimentally Witnessing the Quantumness of Correlations
The quantification of quantum correlations (other than entanglement) usually
entails laboured numerical optimization procedures also demanding quantum state
tomographic methods. Thus it is interesting to have a laboratory friendly
witness for the nature of correlations. In this Letter we report a direct
experimental implementation of such a witness in a room temperature nuclear
magnetic resonance system. In our experiment the nature of correlations is
revealed by performing only few local magnetization measurements. We also
compare the witness results with those for the symmetric quantum discord and we
obtained a fairly good agreement
Environment-induced sudden transition in quantum discord dynamics
Non-classical correlations play a crucial role in the development of quantum
information science. The recent discovery that non-classical correlations can
be present even in separable (unentangled) states has broadened this scenario.
This generalized quantum correlation has been increasing relevance in several
fields, among them quantum communication, quantum computation, quantum phase
transitions, and biological systems. We demonstrate here the occurrence of the
sudden-change phenomenon and immunity against some sources of noise for the
quantum discord and its classical counterpart, in a room temperature nuclear
magnetic resonance setup. The experiment is performed in a decohering
environment causing loss of phase relations among the energy eigenstates and
exchange of energy between system and environment, resulting in relaxation to a
Gibbs ensemble
Decoherence Dynamics of Measurement-Induced Nonlocality and comparison with Geometric Discord for two qubit systems
We check the decoherence dynamics of Measurement-induced Nonlocality(in
short, MIN) and compare it with geometric discord for two qubit systems. There
are quantum states, on which the action of dephasing channel cannot destroy MIN
in finite or infinite time. We check the additive dynamics of MIN on a qubit
state under two independent noise. Geometric discord also follows such additive
dynamics like quantum discord. We have further compared non-Markovian evolution
of MIN and geometric discord under dephasing and amplitude damping noise for
pure state and it shows distinct differences between their dynamics.Comment: 11 pages, 10 figures, Revte
Classical and quantum correlations under decoherence
Recently some authors have pointed out that there exist nonclassical
correlations which are more general, and possibly more fundamental, than
entanglement. For these general quantum correlations and their classical
counterparts, under the action of decoherence, we identify three general types
of dynamics that include a peculiar sudden change in their decay rates. We show
that, under suitable conditions, the classical correlation is unaffected by
decoherence. Such dynamic behavior suggests an operational measure of both
classical and quantum correlations that can be computed without any
extremization procedure.Comment: Published versio
Transverse Ising Model: Markovian evolution of classical and quantum correlations under decoherence
The transverse Ising Model (TIM) in one dimension is the simplest model which
exhibits a quantum phase transition (QPT). Quantities related to quantum
information theoretic measures like entanglement, quantum discord (QD) and
fidelity are known to provide signatures of QPTs. The issue is less well
explored when the quantum system is subjected to decoherence due to its
interaction, represented by a quantum channel, with an environment. In this
paper we study the dynamics of the mutual information , the
classical correlations and the quantum correlations
, as measured by the QD, in a two-qubit state the density matrix
of which is the reduced density matrix obtained from the ground state of the
TIM in 1d. The time evolution brought about by system-environment interactions
is assumed to be Markovian in nature and the quantum channels considered are
amplitude damping, bit-flip, phase-flip and bit-phase-flip. Each quantum
channel is shown to be distinguished by a specific type of dynamics. In the
case of the phase-flip channel, there is a finite time interval in which the
quantum correlations are larger in magnitude than the classical correlations.
For this channel as well as the bit-phase-flip channel, appropriate quantities
associated with the dynamics of the correlations can be derived which signal
the occurrence of a QPT.Comment: 8 pages, 7 figures, revtex4-1, version accepted for publication in
Eur. Phys. J.
Quantum and classical thermal correlations in the XY spin-1/2 chain
We investigate pairwise quantum correlation as measured by the quantum
discord as well as its classical counterpart in the thermodynamic limit of
anisotropic XY spin-1/2 chains in a transverse magnetic field for both zero and
finite temperatures. Analytical expressions for both classical and quantum
correlations are obtained for spin pairs at any distance. In the case of zero
temperature, it is shown that the quantum discord for spin pairs farther than
second-neighbors is able to characterize a quantum phase transition, even
though pairwise entanglement is absent for such distances. For finite
temperatures, we show that quantum correlations can be increased with
temperature in the presence of a magnetic field. Moreover, in the XX limit, the
thermal quantum discord is found to be dominant over classical correlation
while the opposite scenario takes place for the transverse field Ising model
limit
Algebraic characterization of X-states in quantum information
A class of two-qubit states called X-states are increasingly being used to
discuss entanglement and other quantum correlations in the field of quantum
information. Maximally entangled Bell states and "Werner" states are subsets of
them. Apart from being so named because their density matrix looks like the
letter X, there is not as yet any characterization of them. The su(2) X su(2) X
u(1) subalgebra of the full su(4) algebra of two qubits is pointed out as the
underlying invariance of this class of states. X-states are a seven-parameter
family associated with this subalgebra of seven operators. This recognition
provides a route to preparing such states and also a convenient algebraic
procedure for analytically calculating their properties. At the same time, it
points to other groups of seven-parameter states that, while not at first sight
appearing similar, are also invariant under the same subalgebra. And it opens
the way to analyzing invariant states of other subalgebras in bipartite
systems.Comment: 4 pages, 1 figur
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