Key to open door on seperate level

[drunkmonk]

Hey everyone, i'm having trouble trying to figure out how to script the player picking up a key in one map and using that key to unlock a level door in a different map entirely. So far what I have is the player picks up a key in the east wing guest quarters, and they have to take that key to the lab and use it on a door to the upper levels. I have never done something like this before and I have tried it out myself with no luck, any help would be greatly appriectiated.
Thanks.