Quantum discord in spin systems with dipole-dipole interaction

TL;DR

This paper investigates quantum discord in dipolar-coupled spin-1/2 systems, revealing temperature-dependent behavior with maximal quantum correlations at microkelvin temperatures and negligible discord at room temperature.

Contribution

It provides the first detailed analysis of quantum discord in dipolar spin systems, including predictions for natural materials like gypsum and dichloroethane at ultra-low temperatures.

Findings

Quantum discord is zero at absolute zero temperature.

Quantum discord peaks at microkelvin temperatures (~0.5-0.6 μK).

At room temperature, quantum discord is effectively negligible.

Abstract

The behavior of total purely quantum correlation (discord) in dimers consisting of dipolar-coupled spins 1/2 is studied. We found that the discord Q=0 at the absolute zero temperature. With increasing the temperature $T$, at first the quantum correlations in the system increase, smoothly reach the maximum, and then turn again into zero according to the asymptotic law $T^{-2}$. It is also shown that in absence of external magnetic field $B$, the classical correlation $C$ at $T\to0$ is, vice versa, maximal. Our calculations predict that in crystalline gypsum ${\rm CaSO_4\cdot2H_2O}$ the value of natural (B=0) quantum discord between nuclear spins of hydrogen atoms is maximal at the temperature of 0.644 $\mu$K, and for 1,2-dichloroethane ${\rm H_2ClC-CH_2Cl}$ the discord achieves the largest value at $T=0.517 \mu$K. In both cases, the discord equals $Q\approx0.083$ bit/dimer what is 8.3% of its upper limit in two-qubit systems. We estimate also that for gypsum at room temperature $Q\sim10^{-18}$ bit/dimer, and for 1,2-dichloroethane at T=90 K the discord is $Q\sim10^{-17}$ bit per a dimer.