Thermal Quantum Correlations in Coupled Andreev Spin Qubits: Interplay of Superconducting Phase and Spin–Orbit Interaction

This study investigates the thermal quantum correlations of a hybrid system consisting of two coupled superconducting spin qubits. By embedding quantum dots within a Josephson junction, the researchers create a platform where spin and charge degrees of freedom are highly tunable. The paper focuses on how external parameters—specifically the superconducting phase difference, tunneling amplitude, and spin-orbit interaction—influence the system's nonclassical properties at finite temperatures.

The authors utilize two primary quantifiers of quantum correlations: Local Quantum Fisher Information (LQFI) and Local Quantum Uncertainty (LQU). These measures are evaluated using an effective Hamiltonian that describes the spin-qubit interactions. The study derives analytical expressions for the thermal density matrix, allowing for a systematic analysis of how the system's energy spectrum and thermal fluctuations affect its quantum coherence and correlation robustness.

The researchers demonstrate that quantum correlations decrease monotonically as temperature increases. However, the system's robustness is significantly improved by increasing the tunneling strength and spin-orbit interaction. A central finding is that the superconducting phase difference acts as a control knob, inducing periodic behavior in the correlations through interference effects. Furthermore, the study shows that the stability of these quantum resources is directly linked to the energy gap between the ground and first excited states; a larger gap effectively suppresses thermal noise. The authors also note that LQFI consistently provides a higher value than LQU, indicating its greater sensitivity to quantum fluctuations.

As scalable quantum computing remains a major challenge, hybrid architectures like Andreev spin qubits offer a promising path forward by combining the coherence of spin qubits with the control and scalability of superconducting circuits. Understanding the microscopic mechanisms that protect quantum correlations in these systems is essential for designing more robust quantum gates and sensors.