On the origin of quantum many-body scars

The recent observation of long time oscillations in the dynamics of a highly excited state of  50 Rydberg atoms has spurred a lot of interest. This surprisingly defies the natural hypothesis that interacting systems should internally thermalize on fast time scales, and thus kill any coherent oscillation. This phenomenon is reminiscent of "scars" known to occur in quantum billiards, where single particle wavefunctions retain strong traces of periodic classical orbits, which ensures the long time persistence of periodic motion of suitably prepared quantum wavepackets. 

While interesting recent work has elucidated various aspects of the exceptional dynamics in quantum many-body chains, their potential connection with single particle scars has remained obscure. Here I will show that the quasiperiodic quantum dynamics can be understood as a remnant of an entire family of extraordinarily stable periodic orbits in the semiclassical limit of the many body spin chains.