Observation of phase synchronization and alignment during free induction decay of quantum spins with Heisenberg interactions

TL;DR

This paper reports the discovery that quantum spins in a closed system can spontaneously synchronize their directions during free induction decay, a phenomenon robust to various perturbations and distinct from classical or dissipative quantum synchronization.

Contribution

It demonstrates, through numerical simulations, that both integrable and non-integrable quantum spin systems can exhibit phase synchronization during free induction decay, a novel phenomenon in closed quantum systems.

Findings

Quantum spins synchronize their directions during free induction decay.

Synchronization occurs in both integrable and non-integrable systems.

Synchronization is robust against random fluctuations and inhomogeneous fields.

Abstract

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. Contrary to dissipative systems, this synchronization is independent of whether the interaction is ferro- or antiferromagnetic. 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. The synchronization is very robust against for instance random fluctuations of the Heisenberg couplings and inhomogeneous magnetic fields. 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. For classical spin systems the fixed magnitude of individual spins effectively acts like additional $N$ conservation laws.