A compelling story would have a unique and risky experiment idea. It should also have characters that are deeply involved and whose lives are changed by the outcome.
The most important thing is a fresh and exciting experiment concept. Good character development and a sense of tension or mystery throughout the story can also make it compelling. And a surprising or thought-provoking ending would be a plus.
For a short story about the most ambitious experiment to be compelling, it needs a clear goal and high stakes. The characters' motivations and conflicts should be well-defined. Also, the description of the experiment process and its potential impact should be vivid and engaging.