arXiv subjects

Mazhar Ali

Publications and source records attributed to Mazhar Ali.

35 records · Page 2Linked to original sources

arXiv2017

Time-invariant entanglement and sudden death of nonlocality for multipartite systems under collective dephasing

We investigate the dynamics of entanglement and nonlocality for multipartite quantum systems under collective dephasing. Using an exact and computable measure for genuine entanglement, we demonstrate the possibility of a non trivial phenomenon of time-invariant entanglement for multipartite quantum systems. We find that for four qubits, there exist quantum states, which are changing continously nevertheless their genuine entanglement remains constant. Based on our numerical results, we conjecture that there is no evidence of time-invariant entanglement for quantum states of three qubits. We point out that quantum states exhibiting time-invariant entanglement must live in both decoherence free subspace and in the subspaces orthogonal to it. The previous studies on this feature for two qubits can be recovered from our studies as a special case. We also study the nonlocality of quantum states under collective dephasing. We find that although genuine entanglement of quantum states may not change, however their nonlocality changes. We discuss the possibility of finite time end of genuine nonlocality.

quant-ph↗

arXiv2016

Restoring genuine tripartite entanglement under local amplitude damping

We investigate the possibility to restore genuine tripartite entanglement under local amplitude damping. We show that it is possible to protect genuine entanglement using CNOT and Hadamard gates. We analyze several ordering of such recovery operations. We find that for recovery operations applied after exposing qubits to decoherence, there is no enhancement in lifetime of genuine entanglement. Actual retrieval of entanglement is only possible when reversal scheme is applied before and after the decoherence process. We find that retrieval of entanglement for mixture of $|\widetilde{W}\rangle$ state with white noise is more evident than the respective mixture of $|W\rangle$ state. We also find the retrieval of entanglement for similar mixture of $|GHZ\rangle$ state as well.

quant-ph↗

arXiv2015

The Effect of Dopants on the Magnetoresistance of WTe2

Elucidating the nature of the large, non-saturating magnetoresistance in WTe2 is a significant step in functionalizing this phenomenon for applications. Here, Mo, Re, and Ta doped WTe2 are compared to determine whether isovalent and aliovalent substitutions have different effects on the large magnetoresistance. By 1% substitution, isoelectronic doping reduces the magnetoresistance by a factor of 1.2 with an apparent linear trend, whereas aliovalent doping reduces the effect by over an order of magnitude while following a higher-order decay. The apparent increased sensitivity of the magnetoresistive effect to aliovalent doping over simple isoelectronic disorder supports the conclusion that the large magnetoresistance in WTe2 arises from interactions between balanced hole and electron populations.

cond-mat.mtrl-sci↗

arXiv2015

Robustness of genuine tripartite entanglement under collective dephasing

We study robustness of genuine multipartite entanglement for a system of three qubits under collective dephasing. Using a computable entanglement monotone for multipartite systems, we found that almost every state is quite robust under this type of decoherence. We analyze random states and weighted graph states at infinity and found all of them to be genuinely entangled.

quant-ph↗

arXiv2014

Dynamics of genuine multipartite entanglement under local non-Markovian dephasing

We study dynamics of genuine entanglement for quantum states of three and four qubits under non-Markovian dephasing. Using a computable entanglement monotone for multipartite systems, we find that GHZ state is quite resilient state whereas the W state is the most fragile. We compare dynamics of chosen quantum states with dynamics of random pure states and weighted graph states.

quant-ph↗

arXiv2014

Generating genuine multipartite entanglement via XY-interaction and via projective measurements

We have studied the generation of multipartite entangled states for the superconducting phase qubits. The experiments performed in this direction have the capacity to generate several specific multipartite entangled states for three and four qubits. Our studies are also important as we have used a computable measure of genuine multipartite entanglement whereas all previous studies analyzed certain probability amplitudes. As a comparison, we have reviewed the generation of multipartite entangled states via von Neumann projective measurements.

quant-ph↗

arXiv2014

Sudden change in dynamics of genuine multipartite entanglement of cavity-reservoir qubits

We study the dynamics of genuine multipartite entanglement for a system of four qubits. Using a computable entanglement monotone for multipartite systems, we investigate the as yet unexplored aspects of a cavity-reservoir system of qubits. For one specific initial state, we observe a sudden transition in the dynamics of genuine entanglement for the four qubits. This sudden change occurs only during a time window where neither cavity-cavity qubits nor reservoir-reservoir qubits are entangled. We show that this sudden change in dynamics of this specific state is extremely sensitive to white noise.

quant-ph↗

arXiv2014

Robustness of multiparticle entanglement: specific entanglement classes and random states

We investigate the robustness of genuine multiparticle entanglement under decoherence. We consider different kinds of entangled three- and four-qubit states as well as random pure states. For amplitude damping noise, we find that the W-type states are most robust, while other states are not more robust than generic states. For phase damping noise the GHZ state is the most robust state, and for depolarizing noise several states are significantly more robust than random states.

quant-ph↗

arXiv2013

Quantum dissonance provide power to deterministic quantum computation with single qubit

Mixed state quantum computation can perform certain tasks which are believed to be efficiently intractable on a classical computer. For a specific model of mixed state quantum computation, namely, {\it deterministic quantum computation with a single qubit} (DQC1), recent investigations suggest that quantum correlations other than entanglement might be responsible for the power of DQC1 model. However, strictly speaking, the role of entanglement in this model of computation was not entirely clear. We provide conclusive evidence that there are instances where quantum entanglement is not present in any part of this model, nevertheless we have advantage over classical computation. This establishes the fact that quantum dissonance (quantum correlations) present in fully separable states provide power to DQC1 model.

quant-ph↗

arXiv2010

Quantum discord for a two-parameter class of states in $2 \otimes d$ quantum systems

Quantum discord witnesses the nonclassicality of quantum states even when there is no entanglement in these quantum states. This type of quantum correlation also has some interesting and significant applications in quantum information processing. Quantum discord has been evaluated explicitly only for certain class of two-qubit states. We extend the previous studies to $2 \otimes d$ quantum systems and derive an analytical expression for quantum discord for a two-parameter class of states for $d \geq 3$. We compare quantum discord, classical correlation, and entanglement for qubit-qutrit systems to demonstrate that different measures of quantum correlation are not identical and conceptually different.

quant-ph↗

arXiv2010

Quantum discord for two-qubit X-states

Quantum discord, a kind of quantum correlation, is defined as the difference between quantum mutual information and classical correlation in a bipartite system. In general, this correlation is different from entanglement, and quantum discord may be nonzero even for certain separable states. Even in the simple case of bipartite quantum systems, this different kind of quantum correlation has interesting and significant applications in quantum information processing. So far, quantum discord has been calculated explicitly only for a rather limited set of two-qubit quantum states and expressions for more general quantum states are not known. In this paper, we derive explicit expressions for quantum discord for a larger class of two-qubit states, namely, a seven-parameter family of so called X-states that have been of interest in a variety of contexts in the field. We also study the relation between quantum discord, classical correlation, and entanglement for a number of two-qubit states to demonstrate that they are independent measures of correlation with no simple relative ordering between them.

quant-ph↗

arXiv2009

Distillability sudden death in qutrit-qutrit systems under amplitude damping

Recently it has been discovered that certain two-qutrit entangled states interacting with global and/or multi-local decoherence undergo distillability sudden death (DSD). We investigate this phenomenon for qutrit-qutrit systems interacting with statistically independent zero-temperature reservoirs. We show that certain initially prepared free-entangled states become bound-entangled in a finite time due to the action of Markovian dissipative environment. Moreover, in contrast with local dephasing, simple local unitary transformations can completely avoid distillability sudden death under amplitude damping.

quant-ph↗

arXiv2009

Distillability sudden death in qutrit-qutrit systems under global decoherence

Recently Song {\it et al}., Phys. Rev. A {\bf 80}, 012331 (2009), have discovered that certain two-qutrit entangled states interacting with multi-local decoherence undergo distillability sudden death whereas their locally equivalent states do not exhibit such behavior. We have found that their basic equation of motion missed some terms and therefore the subsequent calculations and conclusions are not correct/valid except that distillability sudden death may occur. We provide the missing terms and point out the corrections which must be done in the literature to remove confusion. Moreover, we have studied this feature for certain entangled states under global, collective and multi-local decoherence. We have found that entanglement sudden death and distillability sudden death may happen under global and multi-local noises. Moreover, local unitary transformations can not avoid distillability sudden death.

quant-ph↗

arXiv2008

Manipulating sudden death of entanglement of two-qubit X-states in thermal reservoirs

Manipulation of sudden death of entanglement (ESD) of two qubits interacting with statistically uncorrelated thermal reservoirs is investigated. It is shown that for initially prepared X-states of the two qubits a simple (necessary and sufficient) criterion for ESD can be derived with the help of the Peres-Horodecki criterion. This criterion implies that, in contrast to the zero-temperature case, at finite temperature of at least one of the reservoirs all initially prepared two-qubit X-states exhibit ESD. General conditions are derived under which ESD can be hastened, delayed, or averted.

quant-ph↗

arXiv2008

Hastening, delaying, or averting sudden death of quantum entanglement

The finite time end of entanglement between two decohering qubits can be modified by local, unitary actions performed during the decohering process. Depending on the time when such action is taken, the end can be speeded up or slowed down, or even averted all together. This phenomenon offers practical applications for the stabilization of entangled quantum states. Details concerning hastening or delaying the finite time end of entanglement are presented for two qubits which decay spontaneously into statistically independent reservoirs.

quant-ph↗

arXiv2007

Disentanglement in qubit-qutrit systems

We examine the phenomenon of {\it entanglement sudden death} (ESD) for $(2 \times 3)$-dimensional systems. As for $2 \times 2$ systems, the negativity vanishes in finite time for some entangled pure as well as mixed states. While locally equivalent pure states do so asymptotically. Interference between the decay of the two upper levels to the lowest one in the qutrit adds further richness to ESD in this systems.

quant-ph↗

arXiv2007

The Jamiołkowski isomorphism and a conceptionally simple proof for the correspondence between vectors having Schmidt number $k$ and $k$-positive maps

Positive maps which are not completely positive are used in quantum information theory as witnesses for convex sets of states, in particular as entanglement witnesses and more generally as witnesses for states having Schmidt number not greater than k. It is known that such witnesses are related to k-positive maps. In this article we propose a new proof for the correspondence between vectors having Schmidt number k and k-positive maps using Jamiolkowski's criterion for positivity of linear maps; to this aim, we also investigate the precise notion of the term "Jamiolkowski isomorphism". As consequences of our proof we get the Jamiolkowski criterion for complete positivity, and we find a special case of a result by Choi, namely that k-positivity implies complete positivity, if k is the dimension of the smaller one of the Hilbert spaces on which the operators act.

quant-ph↗