It's a bit tricky to answer "what did Einstein use for his experiment" without more context. Einstein wasn't a hands-on experimentalist, he was primarily a theoretical physicist. He developed theories based on existing experiments and thought experiments.

Here are some possibilities depending on what you mean by "experiment":

* The photoelectric effect: This is a classic experiment that Einstein used to explain his theory of the photoelectric effect, which led to his Nobel Prize in 1921. The experiment itself involved shining light onto a metal surface and measuring the emitted electrons.

* Michelson-Morley experiment: This famous experiment was conducted by Albert Michelson and Edward Morley in the late 19th century. It aimed to detect the luminiferous aether, which was a hypothetical medium thought to carry light waves. The experiment's failure to find the aether was a key factor leading Einstein to develop his theory of special relativity.

* Thought experiments: Einstein was famous for using thought experiments, which are imagined scenarios used to explore theoretical concepts. For example, his famous "thought experiment" involving a person riding on a train was instrumental in formulating his theory of special relativity.

To give a more specific answer, please provide more details about the experiment you're interested in.