From The Big Bang Theory – Season 12 Episode 7: ‘The Grant Allocation Derivation’ (12×07)
Leonard Hofstadter: The europium laser is so cool. It has a four-level f-f transition which provides for high power output, and its quantum efficiency is off the charts.
Sheldon Cooper: What are you going to use it for?
Leonard Hofstadter: I have no idea.
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