11.7 Find all values of the parameter \(a\), for each of which the system of equations 
\[
\left\{
\begin{array}{l}
x^{2}+y^{2}=2 a, \\ 
x+\log _{2}\left(y^{2}+1\right)=a
\end{array}
\right.
\]
has a unique solution.