A combination of two laser beams lets atoms pass through in one direction but not the other, as if a demon were opening or closing a microscopic gate. The barrier beam repels atoms only when they are in an excited state. Atoms approaching from the left are in their lowest-energy state so they go through, but those coming from the right get kicked into an excited state by the pumping beam, and then bounce back after reaching the barrier.
Credit: Steck
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