Indeed the behavior you've described is the default in revtex4-1.
This isn't "in effect", as you said, just because you're loading the caption package, which is incompatible with that class.
So, don't load either caption nor subcaption and you will have your captions in a \small font and justified.
MWE
\documentclass[reprint,aps,prl,twocolumn ,groupedaddress,nobibnotes]{revtex4-1}
\usepackage[demo]{graphicx}
\begin{document}
\begin{figure}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{This is a long sentence to test whether the caption is justified
or not. Unfortunately it is not.}
\end{figure}
\end{document}
Output
EDIT (in response to the OP's comment)
minipages are not incompatible with revtex4-1.
Here's an example of how to have two images side by side, in one column and in two columns mode.
\documentclass[reprint,aps,prl,twocolumn ,groupedaddress,nobibnotes]{revtex4-1}
\usepackage[demo]{graphicx}
\usepackage{lipsum}
\begin{document}
\begin{figure}
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{This is a long sentence to test whether the caption is justified
or not. Unfortunately it is not.}
\end{minipage}\hfill%
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{Another figure.}
\end{minipage}%
\end{figure}
\begin{figure*}
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{This is a long sentence to test whether the caption is justified
or not. Unfortunately it is not.}
\end{minipage}\hfill%
\begin{minipage}[t]{0.48\linewidth}
\includegraphics[width=.9\linewidth]{circuits.png}
\caption{Another figure.}
\end{minipage}%
\end{figure*}
\end{document}
Output (two images in the same column)
Output (two images spanning two columns)
Do make sure that your tex file doesn't include any whitespace characters in the file name.
If the name of the tex file is 1st draft updated.tex, LaTeX creates an associated aux file named 1st draft updated.aux. Among the pieces of information written to the aux file are the name(s) of the bib file(s), the bibliography style that's supposed to be employed, and the arguments of \cite-like instructions. BibTeX peruses the aux file, not the tex file, in order to find out what it's supposed to do. As you've (re)discovered, BibTeX cannot handle whitespace characters in file names.
Summing up: The issue you've encountered is not related to either the revtex4-2 document class or the apsrev4-2 bibliography style. To "fix" the problem, simply change the name of the tex file from 1st draft updated.tex to 1stdraftupdated.tex (or some other name that doesn't contain space characters) and perform a full recompile cycle: LaTeX, BibTeX, and LaTeX twice more.
The name of the file that the contains the following code is test.tex -- note the absence of whitespace.
%%% test.tex
\RequirePackage{filecontents}
\begin{filecontents}{ttt.bib}
@article{hillery1999,
title = {Quantum secret sharing},
author = {Hillery, Mark and Bu{\v z}ek, Vladim{\'i}r and Berthiaume, Andr{\'e}},
journal = {Phys.\ Rev.~A},
volume = {59},
issue = {3},
pages = {1829--1834},
numpages = {0},
year = {1999},
month = {Mar},
publisher= {American Physical Society},
doi = {10.1103/PhysRevA.59.1829},
url = {https://link.aps.org/doi/10.1103/PhysRevA.59.1829}
}
\end{filecontents}
\documentclass{revtex4-2}
\bibliographystyle{apsrev4-2}
\frenchspacing % optional
\begin{document}
\cite{hillery1999}
\bibliography{ttt}
\end{document}
Best Answer
The
captionpackage, hencesubcaptionas well, is not compatible withrevtex4-2. You getThe error you get is due to
caption=false, anyhow.You can use
subfiginstead.There's no need for
tabularx, just use a bit less than 1/3 of the line width (which is better called\columnwidth) and filler glue between images.I'd recommend against using
commath. See https://tex.stackexchange.com/a/123408/4427, https://tex.stackexchange.com/a/135985/4427 and https://tex.stackexchange.com/search?q=commath+user%3A4427