Quantum discord in spin systems with dipole–dipole interaction

The behavior of total quantum correlations (discord) in dimers consisting of dipolar-coupled spins 1/2 are studied. We found that the discord Q = 0 at absolute zero temperature. As the temperature T increases, the quantum correlations in the system increase at first from zero to its maximum and then decrease to zero according to the asymptotic law T - 2 . It is also shown that in absence of external magnetic field B , the classical correlations C at T → 0 are, vice versa, maximal. Our calculations predict that in crystalline gypsum CaSO 4 · 2H 2 O the value of natural ( B = 0 ) quantum discord between nuclear spins of hydrogen atoms is maximal at the temperature of 0.644 μ K, and for 1,2-dichloroethane H 2 ClC– CH 2 Cl the discord achieves the largest value at T = 0.517 μ K. In both cases, the discord equals Q ≈ 0.083 bit/dimer what is 8.3 % of its upper limit in two-qubit systems. We estimate also that for gypsum at room temperature Q ∼ 10 - 18 bit/dimer, and for 1,2-dichloroethane at T = 90 K the discord is Q ∼ 10 - 17 bit per a dimer.