Quantum coding in non-inertial frames

Freezing of quantum and classical correlations via trace and Hellinger distances

Environment and initial state engineered dynamics of quantum and classical correlations

We have investigated the quantum phase transition in the ground state of collective Lipkin-Meshkov-Glick model (LMG model) subjected to decoherence due to its interaction, represented by a quantum channel, with an environment. We discuss the behavior of quantum and classical pair wise correlations in the system, with the quantumness of correlations measured by quantum discord (QD), entanglement of formation (EOF), measurement-induced disturbance (MID) and the Clauser-Horne-Shimony-Holt-Bell function (CHSH-Bell function). The time evolution established by system-environment interactions is assumed to be Markovian in nature and the quantum channels studied include the amplitude damping (AD), phase damping (PD), bit-flip (BF), phase-flip (PF), and bit-phase-flip (BPF) channels. One can identify appropriate quantities associated with the dynamics of quantum correlations signifying quantum phase transition in the model. Surprisingly, the CHSH-Bell function is found to detect all the phase transitions, even when quantum and classical correlations are zero for the relevant ground state.

It is well known that many operations in quantum information processing depend largely on a special kind of quantum correlation, that is, entanglement. However, there are also quantum tasks that display the quantum advantage without entanglement. Distinguishing classical and quantum correlations in quantum systems is therefore of both fundamental and practical importance. In consideration of the unavoidable interaction between correlated systems and the environment, understanding the dynamics of correlations would stimulate great interest. In this study, we investigate the dynamics of different kinds of bipartite correlations in an all-optical experimental setup. The sudden change in behaviour in the decay rates of correlations and their immunity against certain decoherences are shown. Moreover, quantum correlation is observed to be larger than classical correlation, which disproves the early conjecture that classical correlation is always greater than quantum correlation. Our observations may be important for quantum information processing.

Dynamics of coded information over Bloch channels is investigated. We show that the coded information is sent with high accuracy over the Bloch channel by increasing the absolute equilibrium value of the information carrier or decreasing the ratio of relaxation time. The robustness of coded information in maximum and partial entangled states is discussed. It is shown that, the maximum entangled states are more robust than the partial entangled state over these types of channels. The dynamics of the local and the non-local information is investigated for different values of the channel's parameters and the initial state setting. It is found that by increasing the absolute equilibrium values for both qubits, the local information of both qubits decreases faster and consequently the information gained by the eavesdropper increases.

Initial decoherence and disentanglement of open two-qubit systems

Local noise can produce quantum correlations on an initially classically\ncorrelated state, provided that it is not represented by a unital or\nsemi-classical channel \\cite{DagmarBruss}. We find the power of any given local\nchannel for producing quantum correlations on an initially classically\ncorrelated state. We introduce a computable measure for quantifying the quantum\ncorrelations in quantum-classical states, which is based on the\nnon-commutativity of ensemble states in one party of the composite system.\nUsing this measure we show that the amount of quantum correlations produced, is\nproportional to the classical correlations in the initial state. The power of\nan arbitrary channel for producing quantum correlations is found by averaging\nover all possible initial states. Finally we compare our measure with the\ngeometrical measure of quantumness for a subclass of quantum-classical sates,\nfor which we have been able to find a closed analytical expression.\n

The property of quantum correlation has been studied in recent years, especially for the quantum and classical correlations affected by environment. The dynamics of quantum and classical correlations in two-qubit system under identical local noise channels have been investigated recently. Here we will consider the dynamics of quantum and classical correlations when the local noise channels of two sides are not identical. We investigate the dynamics of quantum and classical correlations with three types of local noise channels in both Markovian and non-Markovian conditions, and show the decay rules of quantum and classical correlations with different types and parameter times of local noise channels.

Classical and quantum correlations for two-mode coherent-state superposition

In this paper, some concepts of quantum correlations, such as one-way quantum deficit, purity, and entanglement of formation are investigated between the modes of Dirac fields in noninertial frames. We consider two bipartite divisions of a tripartite system constructed by initial Werner states, which is shared between two relatively accelerated observers. The degradation of these quantum correlations is shown when the acceleration parameter increases. However, a nonzero amount of correlations survives even at infinite acceleration limit. Furthermore, we study the inevitable influence of mixedness factor of initial state on the quantum correlations behaviour. Finally, we review our calculation beyond single-mode approximation and point out some differences and similarities between the two regimes.

We derive the capacity of time-varying channels with memory that have causal channel side information (CSI) at the sender and receiver. We obtain capacity of block-memoryless and asymptotically block-memoryless channels with block-memoryless or weakly decorrelating side information. Our coding theorems rely on causal generation of the codewords relative to the causal transmitter CSI. The CSI need not be perfect, and we consider the case where the transmitter and receiver have the same causal CSI as well as the case where the transmitter CSI is a deterministic function of the receiver CSI. For block-memoryless and asymptotically block-memoryless channels, our coding strategy averages mutual information density over multiple transmission blocks to achieve the maximum average mutual information. We apply the coding theorem associated with the block-memoryless channel to determine the capacity and optimal input distribution of intersymbol interference (ISI) time-varying channels with causal perfect CSI about the time-varying channel. The capacity of this channel cannot be found through traditional decomposition methods

Revisiting the behavior of quantum correlations under nondissipative decoherence by means of the correlation matrix

Quantum correlations from classically correlated states

We use the quantum correlation of both logarithmic negativity and mutual information between a qubit and a relativistic boson to analyze the dynamics of Universe expansion. These dynamical quantum correlations can encode the information about underlying spacetime structure, which suggests a promising application in observational cosmology. We find that the dynamics of both logarithmic negativity and mutual information between the qubit and the boson are very similar. They decrease monotonically with the growth of the expansion volume and the expansion rate. Smaller momentum and medium-sized mass of boson are more favourable for extracting the information about history of Universe expansion. The quantum correlation between the qubit and the antiboson however has very different behavior: the logarithmic negativity is always zero and the mutual information can be generated through the expansion of Universe. Smaller momentum and medium-sized mass of antiboson are beneficial for the production of mutual information. Finally, the trigger phenomenon and conservation for mutual information are witnessed.

Many dynamical models which have been analyzed in the context of system theory, can also be viewed as communication channels with memory. In this interpretation, the system's input is a transmitted message and the observation, or output, is the received message. Information-theoretic measures like entropy, mutual information and capacity can therefore be employed, and key concepts in system thery, such as observability, controllability and stability, can be expressed in information-theoretic terms. In this paper we study certain linear Markovian models from this viewpoint Observability of a Markovian linear discrete-in system is shown to be related to entropies of the initial state and the output observation. Stability is found to pertain to the capacity of the channel which represents the system. The derived relations expose the role of information flow in dynamical system behavior and suggest applications for other liner and nonlinear models.