Initial-Slip Dynamics Enables the Quantum Mpemba Effect

We investigate the quantum Mpemba effect, whereby a system initially farther from equilibrium relaxes faster than one initially closer to equilibrium, emerging in a quantum harmonic oscillator linearly coupled to a bosonic environment. Starting from factorized thermal states at different temperatures, we derive exact analytical criteria for the occurrence of the quantum Mpemba effect valid also beyond the typically considered weak-coupling and Markovian approximations. Most importantly, we show that the effect is enabled by the initial-slip dynamics generated by the sudden switch-on of the system-bath interaction. Neglecting this contribution completely suppresses the anomalous relaxation, identifying transient system-bath correlations as the key ingredient underlying the quantum Mpemba effect.