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Emergent decoherence induced by quantum chaos in a many-body system: A Loschmidt echo observation through NMR

In the long quest to identify and compensate the sources of decoherence in many-body systems far from the ground state, the varied family of Loschmidt echoes (LEs) became an invaluable tool in several experimental techniques. A LE involves a time-reversal procedure to assess the effect of perturbations in a quantum excitation dynamics. However, when addressing macroscopic systems one is repeatedly confronted with limitations that seem insurmountable. This led to formulate the \textit{central hypothesis of irreversibility} stating that the time-scale of decoherence, $T_3$, is proportional to the time-scale of the many-body interactions we reversed, $T_2$. We test this by implementing two experimental schemes based on Floquet Hamiltonians where the effective strength of the dipolar spin-spin coupling, i.e. $1/T_2$, is reduced by a variable scale factor $k$. This extends the perturbations time scale, $T_Σ$, in relation to $T_2$. Strikingly, we observe the superposition of the normalized Loschmidt echoes for the bigger values of $k$. This manifests the dominance of the intrinsic dynamics over the perturbation factors, even when the Loschmidt echo is devised to reverse that intrinsic dynamics. Thus, in the limit where the reversible interactions dominate over perturbations, the LE decays within a time-scale, $T_3\approx T_2/R$ with $R=(0.15 \pm 0.01)$, confirming the emergence of a perturbation independent regime. These results support the central hypothesis of irreversibility.