We study the non-Markovian effect on the dynamics of the quantum discord by exactly solving a model consisting of two independent qubits subject to two zero-temperature non-Markovian reservoirs, respectively. Considering the two qubits initially prepared in Bell-like or extended Werner-like states, we show that there is no occurrence of the sudden death, but only instantaneous disappearance of the quantum discord at some time points, in comparison to the entanglement sudden death in the same range of the parameters of interest. It implies that the quantum discord is more useful than the entanglement to describe quantum correlation involved in quantum systems.Comment: 5 pages, 5 figure

Memory (non-Markovian) effect is found to be able to accelerate quantum evolution [S. Deffner and E. Lutz, Phys. Rev. Lett. 111, 010402 (2013)]. In this work, for an atom in a structured reservoir, we show that the mechanism for the speedup is not only related to non-Markovianity but also to the population of excited states under a given driving time. In other words, it is the competition between non-Markovianity and population of excited states that ultimately determines the acceleration of quantum evolution in memory environment. A potential experimental realization for verifying the above phenomena is discussed by using a nitrogen-vacancy (N-V) center embedded in a planar photonic crystal cavity (PCC) under the current experimental conditions.Comment: 6 pages, 4 figures, published versio

The measurement outcomes of two incompatible observables on a particle can be precisely predicted when it is maximally entangled with a quantum memory, as quantified recently [Nature Phys. 6, 659 (2010)]. We explore the behavior of the uncertainty relation under the influence of local unital and nonunital noisy channels. While the unital noises only increase the amount of uncertainty, the amplitude-damping nonunital noises may amazingly reduce the amount of uncertainty in the longtime limit. This counterintuitive phenomenon could be justified by different competitive mechanisms between quantum correlations and the minimal missing information after local measurement.PACS numbers: 03.65. Ta, 05.40.Ca, 03.65.Yz One of the most remarkable features of quantum mechanics is the restriction of our ability to simultaneously predict the measurement outcomes of two incompatible observables with certainty, which is called Heisenberg's uncertainty principle [1]. Nowadays, the more modern approach to characterize the uncertainty principle is the use of entropic measures rather than with standard deviations [2]. If we denote the probability of the outcome x by p(x) when a given quantum state ρ is measured by an observable X, the Shannon entropy H (X) = − x p(x) log 2 p(x) characterizes the amount of uncertainty about X before we learn its measurement outcomes [3]. For two non-commuting observables Q and R, the entropic uncertainty relation can be expressed aswith |φ α and |ϕ β the eigenstates of Q and R, respectively. Since c is independent of the states of system to be measured, the widely studied entropic uncertainty relation provides us with a more general framework of quantifying uncertainty than the standard deviations (See a review in [4]).However, the entropic uncertainty relation may be violated if a particle is initially entangled with another one [5]. In the extreme case, an observer holding the particle A, maximally entangled with particle B (quantum memory), is able to precisely predict the outcomes of two incompatible observables Q and R acting on A. A stronger entropic uncertainty relation was conjectured by Renes and Boileau [6], and later proved by Berta et al. where S (A|B) = S(ρ AB ) − S(ρ B ) is the conditional von Neumann entropy with S(ρ) = −tr(ρ log 2 ρ) the von Neumann entropy [3]. S (X|B) with X ∈ (Q, R) is the conditional von Neumann entropy of the post-measurementquantum system A is measured by X, where {|ψ x } are the eigenstates of the observable X and ½ is the identity operator. Although the proof of this quantum-memoryassisted entropic uncertainty relation is rather complex, the meaning is clear: the entanglement of systems A and B may lead to a negative conditional entropy S(A|B) [8], which will in turn beat the lower bound log 2 1 c . Especially when A and B are maximally entangled, the simultaneous measurement of Q and R can be precisely predicted [7,9]. In recent, two parallel experiments [10,11] have confirmed the quantum-memory-assisted entropic uncertainty relation.Quantum objects are i...

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We study the ultimate limits to the decoherence rate associated with dephasing processes. Fluctuating chaotic quantum systems are shown to exhibit extreme decoherence, with a rate that scales exponentially with the particle number, thus exceeding the polynomial dependence of systems with fluctuating k-body interactions. Our findings suggest the use of quantum chaotic systems as a natural test-bed for spontaneous wave function collapse models. We further discuss the implications on the decoherence of AdS/CFT black holes resulting from the unitarity loss associated with energy dephasing.

Quantum-speed-limit (QSL) time captures the intrinsic minimal time interval for a quantum system evolving from an initial state to a target state. In single qubit open systems, it was found that the memory (non-Markovian) effect of environment plays an essential role in shortening QSL time or, say, increasing the capacity for potential speedup. In this paper, we investigate the QSL time for multiqubit open systems. We find that for a certain class of states the memory effect still acts as the indispensable requirement for cutting the QSL time down, while for another class of states this takes place even when the environment is of no memory. In particular, when the initial state is in product state |111...1>, there exists a sudden transition from no capacity for potential speedup to potential speedup in a memoryless environment. In addition, we also display evidence for the subtle connection between QSL time and entanglement that weak entanglement may shorten QSL time even more.Comment: 5pages, 3 figure

The present study investigates the relationship between a firm's R&D intensity and the risk of its common stock, by analysing a sample of firms which are more profitable, larger in market capitalization and more R&D intensive than the universe of US-listed firms. The results from the portfolio analysis, Monte Carlos simulations and correlation analysis of our sample show that: (i) R&D intensity is positively related to systematic risk in the stock market; (ii) the greater systematic risk is largely attributable to the greater intrinsic business risk and the greater operating risk of R&D-intensive firms; (iii) R&D-intensive firms carry marginally less financial leverage but they do not differ from other firms in terms of operating leverage; and (iv) our results are particularly strong in the manufacturing sector. For the non-manufacturing sector, the results are not robust for different study periods. Copyright (c) 2004 Accounting and Finance Association of Australia and New Zealand.

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