What are the most valuable skills to learn to be considered for a quantum optics PhD position?
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).