To do this, you need to go back to experiment number 2. Before entering the door, you will be asked if you want to take Rosette with you or not. Do not take her with you.
How do I go back to Experiment 2 if I've already completed it? Does this mean that I need to start from the beginning?
Not entirely. If you have a save file from before that scene, you can use it.
If not, you can start a new game and skip ahead quite a bit in the story (to day 60 or 70, if I remember correctly).
Okay, thanks for the information
