Better looking quantum states

Question

I'm having a spacing glitch with the last of these four quantum states, and I'm wondering if there's a way to get a better output:

\documentclass[12pt,letterpaper,twoside]{book}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{tensor}
\usepackage{mathtools}
\newcommand*{\kp}[1]{\mskip+#1mu}
\newcommand*{\kn}[1]{\mskip-#1mu}
\begin{document}
\begin{subequations}
\begin{align}
    |, 0~0 , \rangle &= \frac{1}{\sqrt{2}} \kp1 (\kp2 |\kn1+ - \kp2 \rangle - |\kn1- + \kp2 \rangle), \
    |, 1~1 , \rangle &= |\kn1+ + \kp2 \rangle, \
    |,1~0 , \rangle &= \frac{1}{\sqrt{2}} \kp1 (\kp2 |\kn1+ - \kp2 \rangle + |\kn1- + \kp2 \rangle), \
    |, 1!-!1 , \rangle &= |\kn1 - - \kp2 \rangle.
\end{align}
\end{subequations}
\end{document}

Preview of what this code is doing:

The last expression is ugly to my eyes, because of the minus sign. Is there a way to get something that matches the size of the other lines, like a smaller minus sign?

@campa, you mean $|, 1, -1 \rangle$? It's still too large compared to others... — Cham, Mar 01 '24 at 15:20
amssymb has something which looks like a short minus https://tex.stackexchange.com/a/469724/82917 — campa, Mar 01 '24 at 15:22

score 3 · Accepted Answer · answered Mar 01 '24 at 19:43

Perhaps not exactly what you want, but too long for a comment.

I suggest you use \DeclarePairedDelimiterX to keep the spacing on your ket uniform. You can write \ket00, but I recommend \ket{0}{0}.
You can add a phantom space to keep your kets aligned. Rather than squeezing a strange small + or - sign into your last line, I suggest you increase the spacing on the others.

\documentclass{article}
\usepackage{mathtools}
\DeclarePairedDelimiterX{\ket}[2]{\lvert}{\rangle}{,{#1};{#2},}
\newcommand{\ps}{\phantom{+}}
\begin{document}
\begin{align}
    \ket{0}{\ps0} &=\frac{1}{\sqrt{2}} (\ket+- -\ket-+),\
    \ket{1}{\ps1} &=\ket++,\
    \ket{1}{\ps0} &=\frac{1}{\sqrt{2}} (\ket+- +\ket-+),\
    \ket{1}{-1} &=\ket--.
\end{align}
\end{document}

score 1 · Answer 2 · answered Mar 01 '24 at 22:43

I suggest to use a syntax such as \qs{a,b} so we can process the list. In this case, putting between any two items

\mathclose{}\;\;\mathopen{}

keeps the spacing right and -1 will be interpreted with unary minus.

\documentclass{article}
\usepackage{amsmath}
\ExplSyntaxOn
\NewDocumentCommand{\qs}{m}
 {
  \lvert
  \clist_use:nn {#1} {\mathclose{};;\mathopen{}} 
  \rangle
}
\begin{document}
\begin{align}
\qs{0,0}  &= \frac{1}{\sqrt{2}}(\qs{+,-} - \qs{-,+}) \
\qs{1,1}  &= \qs{+,+} \
\qs{1,0}  &= \qs{+,-} - \qs{-,+} \
\qs{1,-1} &= \qs{-,-}
\end{align}
\end{document}

If you prefer thin spaces at either end

\documentclass{article}
\usepackage{amsmath}
\ExplSyntaxOn
\NewDocumentCommand{\qs}{m}
 {
  \lvert,
  \clist_use:nn {#1} {\mathclose{};;\mathopen{}} 
  ,\rangle
}
\begin{document}
\begin{align}
\qs{0,0}  &= \frac{1}{\sqrt{2}}(\qs{+,-} - \qs{-,+}) \
\qs{1,1}  &= \qs{+,+} \
\qs{1,0}  &= \qs{+,-} - \qs{-,+} \
\qs{1,-1} &= \qs{-,-}
\end{align}
\end{document}

Better looking quantum states

2 Answers2