Holographic Spread Complexity at Fixed Charge: Routhians, Branes and Strings

Abstract

Holographic spread (Krylov) complexity relates the growth of a boundary state's complexity to the proper radial momentum of a probe falling into the bulk. Unitary evolution makes spread complexity an even function of time. We show that this requirement fails whenever a probe carries a conserved Noether charge and is described by its unreduced Lagrangian. The cure is simple and universal: passing to the Routhian of the fixed-charge sector restores the correct short-time behaviour of the complexity. We establish this prescription from first principles and test it across an extensive family of probes: charged particles, non-BPS D-branes with detuned tension and charge, branes excited along internal isometries, worldvolume gauge fields, a fluctuating D0-brane in AdS$_4\times \mathbb{CP}^3$ and fundamental strings combining winding with rotation in AdS$_5\times \mathrm{S}^5$ complemented by further examples in AdS$_3\times \mathrm{S}^3\times T^4$, ABJM, and the charged Anabalón-Ross background. We then translate these results into Krylov-chain data, extracting Lanczos coefficients and Krylov-number correlators, and propose that complexity for charged, extended probes organises naturally into collective, fluctuation, charge and mixed contributions. This decomposition opens a concrete path toward a genuinely field-theoretic, multi-seed construction of holographic complexity.