Sevgili Aurelien,
It is generally said that, 99% of Turkish population is Sunni Muslim. The other
cults, religions are in total 1%. But, recently, Prime Minister Erdogan said that the 10% of population is non-Muslim, during the visit of Pope. Of course, many people objected this statement.
I don't know if there is an official survey about religion statistics. Probably, this result comes from the data on the religion field of Turkish identity.
This statistics in Turkey are not realistic for many reasons. First of all, there is no reliable institution that is responsible for this kind of survey. Secondly, non-Sunni Turkish population is afraid to tell about their real cult or religion or atheism. Even the alewis are afraid. Think of the atheists! *Unless Turkish society accepts the diversity in religion as a natural human choice, there won't be any reliable statistics, unfortunately.
I am sorry, but I'm unable to make an assumption about the atheists' ratio to total population.
Take care.