Quantum discord quantifies nonclassical correlations in a quantum system including those not captured by entanglement. Thus, only states with zero discord exhibit strictly classical correlations. We prove that these states are negligible in the whole Hilbert space: typically a state picked out at random has positive discord and, given a state with zero discord, a generic arbitrarily small perturbation drives it to a positive-discord state. These results hold for any Hilbert-space dimension and have direct implications for quantum computation and for the foundations of the theory of open systems. In addition, we provide a simple necessary criterion for zero quantum discord. Finally, we show that, for almost all positive-discord states, an arbitrary Markovian evolution cannot lead to a sudden, permanent vanishing of discord.
|Number of pages||6|
|Journal||Physical Review A (Atomic, Molecular, and Optical Physics)|
|Publication status||Published - 17 May 2010|
Ferraro, A., Aolita, L., Cavalcanti, D., Cucchietti, F. M., & Acín, A. (2010). Almost all quantum states have nonclassical correlations. Physical Review A (Atomic, Molecular, and Optical Physics), 81(5), . https://doi.org/10.1103/PhysRevA.81.052318