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[h3]Why must Quantum gates be reversible?[/h3]
All quantum gates are reversible, because quantum mechanics is also reversible (i.e it is unitary, like the gates themselves).
This arises from the Schrdinger equation and so this reveribility is just a fact about the universe.
Classical computers also have to be reversible, it's just that we instead 'throw out' the unnecessary information as we go.
This is not possible for quantum computations as discarding the information would count as measurement and so would collapse the system.