539 research outputs found
Negativity and quantum discord in Davies environments
We investigate the time evolution of negativity and quantum discord for a
pair of non-interacting qubits with one being weakly coupled to a decohering
Davies--type Markovian environment. At initial time of preparation, the qubits
are prepared in one of the maximally entangled pure Bell states. In the
limiting case of pure decoherence (i.e. pure dephasing), both, the quantum
discord and negativity decay to zero in the long time limit. In presence of a
manifest dissipative dynamics, the entanglement negativity undergoes a sudden
death at finite time while the quantum discord relaxes continuously to zero
with increasing time. We find that in dephasing environments the decay of the
negativity is more propitious with increasing time; in contrast, the evolving
decay of the quantum discord proceeds weaker for dissipative environments.
Particularly, the slowest decay of the quantum discord emerges when the energy
relaxation time matches the dephasing time.Comment: submitted for publicatio
Non-Markovianity of local dephasing channels and time-invariant discord
We study non-Markovianity and information flow for qubits experiencing local dephasing with an Ohmic class spectrum. We demonstrate the existence of a temperature-dependent critical value of the Ohmicity parameter s for the onset of non-Markovianity and give a physical interpretation of this phenomenon by linking it to the form of the reservoir spectrum. We demonstrate that this link holds also for more general spectra. We unveil a class of initial states for which discord is forever frozen at a positive value. We connect time invariant discord to non-Markovianity and propose a physical system in which it could be observed
Asymmetric broadcasting of quantum correlations
In this work, we exhaustively investigate local and
nonlocal broadcasting of entanglement as well as correlations beyond
entanglement (geometric discord) using asymmetric Pauli cloners with most
general two qubit state as the resource. We exemplify asymmetric broadcasting
of entanglement using Maximally Entangled Mixed States. We demonstrate the
variation of broadcasting range with the amount of entanglement present in the
resource state as well as with the asymmetry in the cloner. We show that it is
impossible to optimally broadcast geometric discord with the help of these
asymmetric Pauli cloning machines. We also study the problem of broadcasting of entanglement using non-maximally entangled state (NME) as
the resource. For this task, we introduce a method we call successive
broadcasting which involves application of optimal cloning
machines multiple times. We compare and contrast the performance of this method
with the application of direct optimal cloning machines. We
show that optimal cloner does a better job at broadcasting
than the successive application of cloners and the successive
method can be beneficial in the absence of cloners. We also
bring out the fundamental difference between the tasks of cloning and
broadcasting in the final part of the manuscript. We create examples to show
that there exist local unitaries which can be employed to give a better range
for broadcasting. Such unitary operations are not only economical, but also
surpass the best possible range obtained using existing cloning machines
enabling broadcasting of lesser entangled states. This result opens up a new
direction in exploration of methods to facilitate broadcasting which may
outperform the standard strategies implemented through cloning transformations.Comment: Edited sections, changed figures, to be published in Physical Review
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