我的代码写法如下:
\documentclass[12pt]{article}
\renewcommand{\baselinestretch}{1.5}
\usepackage{hyperref}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{esint}
\usepackage{geometry}
\usepackage{titlesec}
\usepackage{graphicx}
\usepackage{subfigure}
\usepackage{fancyhdr}
\usepackage{xcolor}
\usepackage{xpatch}
\usepackage{booktabs}
\usepackage{float}
\usepackage{physics}
\usepackage{bigstrut}
\usepackage{multirow}
\usepackage{textcomp}
\usepackage{gensymb}
\usepackage{amsthm}
\usepackage{algorithm,algorithmicx,algpseudocode}
\usepackage{mathtools}
\usepackage[version=4]{mhchem}
\begin{document}
The state of the system is expressed as $\ket{\chi_1\ldots\chi_N}$
\end{document}