CFT complexity and penalty factors

Journal of High Energy Physics, vol. 2026

Abstract

A bstract Quantum complexity of conformal field theory (CFT) states has recently gained significant attention, both as a diagnostic tool in condensed matter systems and in connection with holographic observables probing black hole interiors. Previous studies have primarily focused on cases where all generators of the conformal group contribute equally to the cost of building a circuit. In this work, we present a general framework for studying the complexity of circuits in generic Lie groups, where penalty factors assign relative weights to different generators. Our approach constructs a metric on the coset space of quantum states, induced from a (pseudo-)Riemannian norm on the space of unitary circuits. The geodesics of this metric are interpreted as optimal circuits. The method builds on the formalism of (pseudo-)Riemannian submersions and connects naturally to other prescriptions in the literature, including cost function minimization along stabilizer directions and constructions based on coadjoint orbits. As a concrete application, we compute state complexity for states in one- and two-dimensional CFTs. For specific choices of penalty factors, our prescription yields a positive-definite metric with a viable interpretation as complexity; in other cases, the resulting metric is indefinite. In the viable regime, we derive analytic results when a specific penalty factor is turned off, develop perturbative expansions for small values of the penalty factors, and provide numerical results in the general case. We comment on the relation of our measure of complexity to holography.

Authors 4

1. Stefano Baiguera corresponding

   Università Cattolica del Sacro Cuore · Istituto Nazionale di Fisica Nucleare, Sezione di Perugia

   Affiliation as printed

   INFN Sezione di Perugia

   Università Cattolica del Sacro Cuore

2. Nicolas Chagnet Aachen

   Leiden University

   Affiliation as printed

   ∆-ITP, Leiden University

3. Shira Chapman

   Ben-Gurion University of the Negev

   Affiliation as printed

   Ben-Gurion University of the Negev

4. Osher Shoval

   Ben-Gurion University of the Negev

   Affiliation as printed

   Ben-Gurion University of the Negev