Go to Cianwood City,talk to the guy in the pharmacy(not pokemart) and he will give you it.
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you go down to the house next to the Pokemon center and inside is this guy, talk to him and he will give a secret potion.
You must surf on Routes 40 and 41 to get to Cianwood City. In the city, there will be a pharmacy where you can get a Secret Potion. Give this to Jasmine.
Cynthia will give it to you.
You get the secret potion by talking to the pharmacist in Cianwood City. His house is in the south, south east of the gym.
First you have to go to Cinawood city and then go to the pharmacy. When you talk to the guy he will give the secret potion, beat the gym (if you want to), go back to olivine and fight the trainers until you get up to the light room. Give the secret potion to jasmine and she will unlock the door.