191 research outputs found
Long-range quantum discord in critical spin systems
We show that quantum correlations as quantified by quantum discord can
characterize quantum phase transitions by exhibiting nontrivial long-range
decay as a function of distance in spin systems. This is rather different from
the behavior of pairwise entanglement, which is typically short-ranged even in
critical systems. In particular, we find a clear change in the decay rate of
quantum discord as the system crosses a quantum critical point. We illustrate
this phenomenon for first-order, second-order, and infinite-order quantum phase
transitions, indicating that pairwise quantum discord is an appealing quantum
correlation function for condensed matter systems
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
Nonclassical correlation in NMR quadrupolar systems
The existence of quantum correlation (as revealed by quantum discord), other
than entanglement and its role in quantum-information processing (QIP), is a
current subject for discussion. In particular, it has been suggested that this
nonclassical correlation may provide computational speedup for some quantum
algorithms. In this regard, bulk nuclear magnetic resonance (NMR) has been
successfully used as a test bench for many QIP implementations, although it has
also been continuously criticized for not presenting entanglement in most of
the systems used so far. In this paper, we report a theoretical and
experimental study on the dynamics of quantum and classical correlations in an
NMR quadrupolar system. We present a method for computing the correlations from
experimental NMR deviation-density matrices and show that, given the action of
the nuclear-spin environment, the relaxation produces a monotonic time decay in
the correlations. Although the experimental realizations were performed in a
specific quadrupolar system, the main results presented here can be applied to
whichever system uses a deviation-density matrix formalism.Comment: Published versio
Markovian evolution of classical and quantum correlations in transverse-field XY model
The transverse-field XY model in one dimension is a well-known spin model for
which the ground state properties and excitation spectrum are known exactly.
The model has an interesting phase diagram describing quantum phase transitions
(QPTs) belonging to two different universality classes. These are the
transverse-field Ising model and the XX model universality classes with both
the models being special cases of the transverse-field XY model. In recent
years, quantities related to quantum information theoretic measures like
entanglement, quantum discord (QD) and fidelity have been shown to provide
signatures of QPTs. Another interesting issue is that of decoherence to which a
quantum system is subjected due to its interaction, represented by a quantum
channel, with an environment. In this paper, we determine the dynamics of
different types of correlations present in a quantum system, namely, the mutual
information, the classical correlations and the quantum correlations, as
measured by the quantum discord, in a two-qubit state. The density matrix of
this state is given by the nearest-neighbour reduced density matrix obtained
from the ground state of the transverse-field XY model in 1d. We assume
Markovian dynamics for the time-evolution due to system-environment
interactions. The quantum channels considered include the bit-flip,
bit-phase-flip and phase-flip channels. Two different types of dynamics are
identified for the channels in one of which the quantum correlations are
greater in magnitude than the classical correlations in a finite time interval.
The origins of the different types of dynamics are further explained. For the
different channels, appropriate quantities associated with the dynamics of the
correlations are identified which provide signatures of QPTs. We also report
results for further-neighbour two-qubit states and finite temperatures.Comment: 10 pages, 11 figures, revtex4-1. arXiv admin note: text overlap with
arXiv:1205.130
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
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.
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
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
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