What are the most valuable skills to learn to be considered for a quantum optics PhD position? I have a master degree in thin-films interferometry, and I want to receive a Ph.D position in the area of Quantum optics. How can I assure receiving this position? What are the required skills or certificates to be accepted?
You can get a PhD position without specific knowledge of the research area.  For quantum optics, commonly desired skills include:


English language skills
Advanced mathematical skills, including statistics
Broad education in physics
Experience conducting research


You do not need a "certificate."  You will need a masters degree (which can substitute for a bachelors degree in countries that require a bachelors degree).