A new upper bound for sets with no square differences

We show that if $A\subset \{1,\ldots,N\}$ has no solutions to $a-b=n^2$ with
$a,b\in A$ and $n\geq 1$ then \[|A|\ll \frac{N}{(\log N)^{c\log\log \log N}}\]
for some absolute constant $c>0$. This improves upon a result of
Pintz-Steiger-Szemer\'edi.