Quantum complexity and localization in random and time-periodic unitary circuits

Abstract: We study the growth and saturation of Krylov spread (K-) complexity under random quantum circuits. In Haar-random unitary evolution, we show that, for large system sizes, K-complexity grows linearly before saturating at a late-time value of $D/2$, where $D$ is the Hilbert space dimension, at timescales $\sim D$. Our numerical analysis encompasses two classes of random circuits: brick-wall random unitary circuits and Floquet random circuits. In brick-wall case, K-complexity exhibits dynamics consistent with Haar-random unitary evolution, while the inclusion of measurements significantly slows its growth down. For Floquet random circuits, we show that localized phases lead to reduced late-time saturation values of K-complexity, forbye we utilize these saturation values to probe the transition between thermal and many-body localized phases.