Quantum instability has a geometric speed limit that stops itself

When you drive a quantum system hard enough, it becomes non-adiabatic — it can't keep up with the driving — and can go unstable. This work shows that the standard measure of that danger is literally just the speed of the quantum state along a geometric distance measure called the Fubini-Study metric. Better still, in nonlinear bosonic systems, high occupation slows that geometric speed down automatically, creating a self-limiting instability that never fully runs away.