149 research outputs found
Quantum discord for general two--qubit states: Analytical progress
We present a reliable algorithm to evaluate quantum discord for general
two--qubit states, amending and extending an approach recently put forward for
the subclass of X--states. A closed expression for the discord of arbitrary
states of two qubits cannot be obtained, as the optimization problem for the
conditional entropy requires the solution to a pair of transcendental equations
in the state parameters. We apply our algorithm to run a numerical comparison
between quantum discord and an alternative, computable measure of non-classical
correlations, namely the geometric discord. We identify the extremally
non-classically correlated two--qubit states according to the (normalized)
geometric discord, at fixed value of the conventional quantum discord. The
latter cannot exceed the square root of the former for systems of two qubits.Comment: 8 pages, 2 figure
Local Quantum Uncertainty in Two-Qubit Separable States: A Case Study
Recent findings suggest, separable states, which are otherwise of no use in
entanglement dependent tasks, can also be used in information processing tasks
that depend upon the discord type general non classical correlations. In this
work, we explore the nature of uncertainty in separable states as measured by
local quantum uncertainty. Particularly in two-qubit system, we find separable
X-state which has maximum local quantum uncertainty. Interestingly, this
separable state coincides with the separable state, having maximum geometric
discord. We also search for the maximum amount of local quantum uncertainty in
separable Bell diagonal states. We indicate an interesting connection to the
tightness of entropic uncertainty with the state of maximum uncertainty.Comment: 11 pages, 2 figures, latex2e, comments welcome, to appear in qi
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