Abstract
<title>Abstract</title> <p>Driven isolated quantum systems lack a universal local energy scale that characterizes the instantaneous intensity of coherent nonequilibrium evolution. Existing descriptions rely on transition probabilities, spectral gaps, excitation densities or long-time effective ensembles, but none of these quantities is determined solely by the geometry of the evolving quantum trajectory. Here we show that requiring locality, gauge invariance, basis independence and the correct energy dimension is sufficient to fix such a scale uniquely among first-order local scalars built from the unitarily invariant Riemannian line element of projective Hilbert space, and that projective Hilbert-space geometry provides it: for any smooth pure-state unitary evolution, the instantaneous Fubini–Study speed v_FS generates a local geometric energy E_geo=ℏv_FS, which is gauge invariant, basis independent and determined only by the local trajectory of the state. The same scale is the local fidelity-loss rate, coincides with the quantum-speed-limit energy scale, and governs the onset of non-adiabatic excitation. Beyond this energetic characterization, the hyperbolic structure of the evolution generator gives this energy a distinct thermodynamic representation, k_B T_geo=ℏκ_g/2π, whose 2π normalization is derived — not postulated — from the exact thermal scattering structure of the generator, by the same analytic-continuation mechanism that fixes the Unruh and Hawking temperatures. The resulting temperature saturates the Maldacena–Shenker–Stanford bound on chaos algebraically, a formal correspondence of the linearized single-mode dynamics rather than a demonstration of many-body chaos. Our results establish intrinsic geometric energy as a state-evolution variable of driven quantum systems and provide a common foundation for non-adiabatic excitation, geometric self-limitation and dynamical bounds on information scrambling.</p>