Geometric Power Capacity of Coherent Ergotropy in Quantum Batteries

Abstract

We explore coherent ergotropy extraction in quantum batteries from a resource-geometric point of view. For an initial state $ρ$, we quantify the coherent extraction process by the coherent ergotropy $\mathcal{E}_c(ρ)$ and the coherent extraction distance $D_c^{\rm ext}(ρ)$ between the active state $σ_ρ$ and the passive state $P_ρ$. This defines the geometric power capacity $Π_c(ρ)=\mathcal{E}_c(ρ)/D_c^{\rm ext}(ρ)$, which measures the coherent ergotropy released unit minimal unitary distance. We prove that, for any driving Hamiltonian satisfying $\|V_t\|\leqν$, the actual coherent discharging power is bounded by $P_c^{\rm ext}(ρ;V_t)\leq νΠ_c(ρ)$, showing that $Π_c(ρ)$ is a capacity under unit driving norm rather than the power of a particular protocol. General bounds on $Π_c(ρ)$ are derived by combining relative entropy bounds on coherent ergotropy with geometric bounds on the coherent extraction distance. We also formulate coherence measure induced bounds and protocol-corrected capacities involving the effective speed of a given Hamiltonian. Qubit and qutrit examples demonstrate that $Π_c(ρ)$ captures a resource-geometric feature of coherent discharging beyond coherent ergotropy or coherence measures alone.