In an isolated system, the time evolution of a given observable in the Heisenberg picture can be efficiently represented in Krylov space. In this representation, an initial operator becomes increasingly complex as time goes by, a feature that can be quantified by the Krylov complexity. We introduce a fundamental and universal limit to the growth of the Krylov complexity by formulating a Robertson uncertainty relation, involving the Krylov complexity operator and the Liouvillian, as generator of time evolution. We further show the conditions for this bound to be saturated and illustrate its validity in the paradigmatic models of quantum chaos.

Ref.: Niklas Hörnedal, Nicoletta Carabba, Apollonas S. Matsoukas-Roubeas, Adolfo del Campo, Ultimate Physical Limits to the Growth of Operator Complexity,
https://doi.org/10.48550/arXiv.2202.05006