Following an idea of Lin, we prove that if
A and
B are two positive operators such that 0 <
mI ≤
A ≤
m′
I ≤
M′
I ≤
B ≤
MI, then
$${\Phi ^2}\left( {\frac{{A + B}}{2}} \right) \leqslant \frac{{{K^2}\left( h \right)}}{{{{\left( {1 + \frac{{{{\left( {\log \frac{{M'}}{{m'}}} \right)}^2}}}{8}} \right)}^2}}}{\Phi ^2}\left( {A\# B} \right),$$
and
$${\Phi ^2}\left( {\frac{{A + B}}{2}} \right) \leqslant \frac{{{K^2}\left( h \right)}}{{{{\left( {1 + \frac{{{{\left( {\log \frac{{M'}}{{m'}}} \right)}^2}}}{8}} \right)}^2}}}{\left( {\Phi \left( A \right)\# \Phi \left( B \right)} \right)^2},$$
where
\(K\left( h \right) = \frac{{{{\left( {h + 1} \right)}^2}}}{{4h}}\) and
\(h = \frac{M}{m}\) and Φ is a positive unital linear map.