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Is it true that the whole circuit, viewed as a map from the input registers to the end is a unitary operator ?

eternalstudent
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    If all your gates are unitary, then yes. And most gates are unitary. The primary non-unitary one that some systems have is "reset this qubit to 0". – Frank Yellin Dec 19 '22 at 03:36
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    depends what you mean exactly. Non-unitary operations are possible and often considered. But also, any such quantum operation can be represented as a unitary operation in a possibly enlarged space – glS Dec 21 '22 at 01:02

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There are non-unitary operations other than measurement, such as introducing a qubit or discarding a qubit or reseting a qubit or applying noise to a qubit.

Craig Gidney
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