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Journal of the Physical Society of Japan

Quantum Correlations of Two Superconducting Charge Qubits in a Magnetic Field

Sujit Sarkar

Published Sep 26, 2014• 4 citations
Publisher

Abstract

We present different quantum correlations, for two superconducting charge qubits in the presence of an applied magnetic flux. We theoretically predict collapse and revival of entanglement in the presence of a magnetic flux without any permanent decay. We theoretically predict the permanent decay of entanglement only with temperature. We also theoretically predict a perfectly entangled state for very low temperatures for small values of the on-site Coulomb charging energies. The behavior of the quantum discord is different, a finite quantum discord also in the seperable state, there is no evidence of oscillations and permanent decay of quantum discord with magnetic flux and temperature. We theoretically predict that the quantum discord predicts the quantum mechanical correlation of the system over the quantum entanglement correlation of the system.

DOI
10.7566/jpsj.83.104003

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One-dimensional spin-12\frac{1}{2}21​ systems are well-known candidates to study the quantum correlations between particles. In condensed matter physics, studies often are restricted to first-neighbor particles. In this work, we consider the one-dimensional XXZXXZXXZ model in a transverse magnetic field (TF) which is not integrable except at specific points. Analytical expressions for quantum correlations (entanglement and quantum discord) between spin pairs at any distance are obtained for both zero and finite temperature by using the analytical approach proposed by Caux et al. [Phys. Rev. B 68, 134431 (2003)]. We compare the efficiency of the quantum discord (QD) with respect to the entanglement in the detection of critical points as the neighboring spin pairs go farther than the next-nearest neighbors. In the absence of the TF and at zero temperature, we show that the QD for spin pairs farther than the second neighbors is able to capture the critical points while the pairwise entanglement is absent. In contrast with the pairwise entanglement, two-site QD is effectively long range in the critical regimes where it decays algebraically with the distance between pairs. We also show that the thermal QD between neighbor spins possesses strong distinctive behavior at the critical point that can be seen at finite temperature and, therefore, spotlights the critical point while the entanglement fails in this task.
Quantum correlations (including entanglement and quantum discord) of two qubits symmetrically coupled to the surrounding XY spin chain in the presence of a transverse magnetic field are studied. We investigate the dynamic evolution of the quantum correlations of the two qubits when the surrounding spin chain is in the thermal equilibrium state. Numerical results show that the thermal effects can accelerate the decay process of the quantum correlations if the initial state of the composite system is not in the decoherence free space. When the temperature is high enough, the quantum phase transition driven by the external magnetic field completely disappear.