Sharp Spectral-Cluster Restriction Bounds for Orthonormal Systems

For a smooth $k$-dimensional submanifold $\Sigma$ of a $d$-dimensional compact Riemannian manifold $M$, we extend the $L^p(\Sigma)$ restriction bounds of Burq-G\'erard-Tzvetkov -- originally proved for individual Laplace--Beltrami eigenfunction -- to arbitrary systems of $L^2(M)$-orthonormal functions. Our bounds are essentially optimal for every triple $(k,d,p)$ with $p\ge2$, except possibly when $d\ge3,\quad k=d-1,\quad 2\le p\le4.$ This work is inspired by a work of Frank and Sabin, who established analogous $L^p(M)$ bounds for $L^2(M)$-orthonormal systems.