Hilbert–Schmidt Estimates for Fermionic 2-Body Operators

We prove that the 2-body operator γ2Ψ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma _{2}^{\Psi }$$\end{document} of a fermionic N-particle state Ψ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Psi $$\end{document} obeys ‖γ2Ψ‖HS≤5N\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Vert \gamma _{2}^{\Psi }\Vert _{\textrm{HS}}\le \sqrt{5}N$$\end{document}, which complements the bound of Yang (Rev Mod Phys 34:694, 1962) that ‖γ2Ψ‖op≤N\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Vert \gamma _{2}^{\Psi }\Vert _{\textrm{op}}\le N$$\end{document}. This estimate furthermore resolves a conjecture of Carlen–Lieb–Reuvers (Commun Math Phys 344:655–671, 2016) concerning the entropy of the normalized 2-body operator. We also prove that the Hilbert–Schmidt norm of the truncated 2-body operator γ2Ψ,T\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma _{2}^{\Psi ,T}$$\end{document} obeys the inequality ‖γ2Ψ,T‖HS≤5Ntr(γ1Ψ(1-γ1Ψ))\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Vert \gamma _{2}^{\Psi ,T}\Vert _{\textrm{HS}}\le \sqrt{5N\,\textrm{tr}\,(\gamma _{1}^{\Psi }(1-\gamma _{1}^{\Psi }))}$$\end{document}.