The quantum Mpemba effect in free-fermionic mixed states

The quantum Mpemba effect in free-fermionic mixed states

27th March 2025 | Filiberto Ares, Vittorio Vitale, Sara Murciano
The paper investigates the quantum Mpemba effect (QMPE) in free-fermionic mixed states, focusing on the XY spin chain model. The authors explore how the *mixedness* of initial states and non-unitary dynamics affect symmetry restoration. They use entanglement asymmetry, a quantum information-based observable, to probe the dynamics of symmetry breaking. The study reveals that the QMPE can occur even in the presence of dissipation or at finite temperature, though it is eventually suppressed as the state becomes more mixed. The key findings include: 1. **Unitary Dynamics**: In the absence of dissipation, the symmetry is dynamically restored in a subsystem for initial states that break symmetry more, leading to the QMPE. 2. **Dissipative Evolution**: The presence of gain and loss terms does not alter the criteria for the QMPE but diminishes the entanglement asymmetry, potentially affecting the timing of symmetry restoration. 3. **Local Dephasing**: Local dephasing also reduces the entanglement asymmetry, shifting the intersection times of entanglement asymmetry curves for different initial states. 4. **Finite Temperature**: The dynamics at finite temperature show a similar trend, with the QMPE still occurring but with modified conditions. The results highlight the complex interplay between symmetry breaking, non-unitary dynamics, and the nature of initial states in the context of the QMPE.The paper investigates the quantum Mpemba effect (QMPE) in free-fermionic mixed states, focusing on the XY spin chain model. The authors explore how the *mixedness* of initial states and non-unitary dynamics affect symmetry restoration. They use entanglement asymmetry, a quantum information-based observable, to probe the dynamics of symmetry breaking. The study reveals that the QMPE can occur even in the presence of dissipation or at finite temperature, though it is eventually suppressed as the state becomes more mixed. The key findings include: 1. **Unitary Dynamics**: In the absence of dissipation, the symmetry is dynamically restored in a subsystem for initial states that break symmetry more, leading to the QMPE. 2. **Dissipative Evolution**: The presence of gain and loss terms does not alter the criteria for the QMPE but diminishes the entanglement asymmetry, potentially affecting the timing of symmetry restoration. 3. **Local Dephasing**: Local dephasing also reduces the entanglement asymmetry, shifting the intersection times of entanglement asymmetry curves for different initial states. 4. **Finite Temperature**: The dynamics at finite temperature show a similar trend, with the QMPE still occurring but with modified conditions. The results highlight the complex interplay between symmetry breaking, non-unitary dynamics, and the nature of initial states in the context of the QMPE.
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