Equilibration of observables in closed quantum systems that are described by a unitary time evolution is a meanwhile well-established phenomenon apart from a few equally well-established exceptions. Here we report the surprising theoretical observation that integrable as well as non-integrable spin rings with nearest-neighbor or long-range isotropic Heisenberg interaction not only equilibrate but moreover also synchronize the directions of the expectation values of the individual spins. We highlight that this differs from spontaneous synchronization in quantum dissipative systems. Here, we observe mutual synchronization of local spin directions in closed systems under unitary time evolution. In our numerical simulations, we investigate the free induction decay (FID) of an ensemble of up to $N = 25$ quantum spins with $s = 1/2$ each by solving the time-dependent Schr\"odinger equation numerically exactly. Our findings are related to, but not fully explained by conservation laws of the system. Even if we cannot provide a full understanding of the phenomenon, it is very robust against for instance random fluctuations of the Heisenberg couplings and inhomogeneous magnetic fields. The observed synchronization is independent of whether the interaction is ferro- or antiferromagnetic. Synchronization is not observed with strong enough symmetry-breaking interactions such as the dipolar interaction. We also compare our results to closed-system classical spin dynamics which does not exhibit phase synchronization due to the lack of entanglement and since the fixed magnitude of individual classical spins effectively acts like additional $N$ conservation laws.
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