We consider the coherence-vector representation of a bipartite state and obtain a necessary and sufficient condition for a zero-discord state. Based on this, a measure of quantum, classical, and total amount of correlations in bipartite states is proposed in this representation. Analytical expressions for this measure are available for any bipartite states. Our measure of nonclassical correlation coincides with the geometric measure of quantum discord for some particular states. I. INTRODUCTIONCharacterizing and quantifying the correlations in bipartite states is a basic and significant problem in quantum physics. The study of quantum correlations has traditionally focused on entanglement [1], which plays an important role in quantum computation and quantum communication [2]. However, quantum entanglement is not the only kind of correlation and is not necessary for deterministic quantum computation with one pure qubit (DQC1) [3,4]. These led to the studies on other kinds of nonclassical correlations over the past decade. One of the measures for nonclassical correlations, the quantum discord, initially introduced by Ollivier and Zurek [5] and by Henderson and Vedral [6], has attracted much attention recently [7][8][9][10][11][12][13][14].Given a bipartite state ρ AB on a composite Hilbert space H AB = H A ⊗ H B , the total amount of correlations is quantified by the quantum mutual information I (ρ AB ) = S(ρ A ) + S(ρ B ) − S(ρ AB ), with S(ρ) = −Trρ log 2 ρ the von Neumann entropy and ρ A(B) = Tr B(A) ρ AB the reduced density matrices for subsystem A(B). The quantum discord is defined as Q(ρ AB ) = I (ρ AB ) − J A (ρ AB ), the discrepancy between the quantum mutual information I (ρ AB ) and its classical version J A (ρ AB ) = S(ρ B ) − min { A

In our paper, the parameter b in Eqs. (25), (27)-(29) should be replaced by 2b. The conclusions of our paper are not affected. Consequently, the b parameter in Fig. 1 should read 0.25 instead of 0.5.In detail, Eqs. (25), (27)-(29) should read

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We theoretically investigate a Rashba spin-orbit coupled Fermi gas near Feshbach resonances, by using mean-field theory and a two-channel model that takes into account explicitly Feshbach molecules in the close channel. In the absence of spin-orbit coupling, when the channel coupling g between the closed and open channels is strong, it is widely accepted that the two-channel model is equivalent to a single-channel model that excludes Feshbach molecules. This is the so-called broad resonance limit, which is well-satisfied by ultracold atomic Fermi gases of 6 Li atoms and 40 K atoms in current experiments. Here, with Rashba spin-orbit coupling we find that the condition for equivalence becomes much more stringent. As a result, the single-channel model may already be insufficient to describe properly an atomic Fermi gas of 40 K atoms at a moderate spin-orbit coupling. We determine a characteristic channel coupling strength gc as a function of the spin-orbit coupling strength, above which the single-channel and two-channel models are approximately equivalent. We also find that for narrow resonance with small channel coupling, the pairing gap and molecular fraction is strongly suppressed by SO coupling. Our results can be readily tested in 40 K atoms by using optical molecular spectroscopy.