Approaching the Quantum Speed Limit in Quantum Gates with Geometric Control

Abstract: We present a geometric optimization method for implementing quantum gates by optimally controlling the Hamiltonian parameters, aiming to approach the Mandelstam-Tamm Quantum Speed Limit (MT-QSL). Achieving this bound requires satisfying precise geometric conditions governing the evolution of quantum states. We extend this geometric framework to quantum unitary operators in arbitrary dimensions and analyze the conditions necessary for saturation of the bound. Additionally, we demonstrate that the quantum brachistochrone, when generalized to operators, does not generally reach the MT-QSL bound. Finally, we propose a systematic optimal control strategy based on geometric principles to approach the quantum speed limit for unitary driving.