We further advance the study of the notion of computational complexity for 2d CFTs based on a gate set built out of conformal symmetry transformations. Previously, it was shown that by choosing a suitable cost function, the resulting complexity functional is equivalent to geometric (group) actions on coadjoint orbits of the Virasoro group, up to a term that originates from the central extension. We show that this term can be recovered by modifying the cost function, making the equivalence exact. Moreover, we generalize our approach to Kac-Moody symmetry groups, finding again an exact equivalence between complexity functionals and geometric actions. We then determine the optimal circuits for these complexity measures and calculate the corresponding costs for several examples of optimal transformations. In the Virasoro case, we find that for all choices of reference state except for the vacuum state, the complexity only measures the cost associated to phase changes, while assigning zero cost to the non-phase changing part of the transformation. For Kac-Moody groups in contrast, there do exist non-trivial optimal transformations beyond phase changes that contribute to the complexity, yielding a finite gauge invariant result. Furthermore, we also show that the alternative complexity proposal of path integral optimization is equivalent to the Virasoro proposal studied here. Finally, we sketch a new proposal for a complexity definition for the Virasoro group that measures the cost associated to non-trivial transformations beyond phase changes. This proposal is based on a cost function given by a metric on the Lie group of conformal transformations. The minimization of the corresponding complexity functional is achieved using the Euler-Arnold method yielding the Korteweg-de Vries equation as equation of motion.
Kac--Moody groups and automorphic forms in low dimensional supergravity theories
Kac-Moody群$G$ over $\mathbb{R}$已经被证明是作为超引力理论的对称群出现的，这些对称群在维数上被简化到小于3，它们的积分形式$G(\mathbb{Z})$对量子化的对称性进行了编码。本文简要介绍了超引力中Kac-Moody群的几何对称性及其已知的证据。我们描述的Kac-穆迪集团$\R$和$\Z$使用某些选择的基本模块，被认为是具有物理相关性的建设。在超弦理论的低能量极限中，已知有限维代数群上的爱森斯坦级数编码量子修正。本文简要地描述了Eisenstein级数的构造如何推广到Kac-Moody群。我们预言了$E_9$、$E_{10}$和$E_{11}$上的Eisenstein级数的常数项编码微扰弦论修正。
