Is it true that the whole circuit, viewed as a map from the input registers to the end is a unitary operator ?
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1$\begingroup$ 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". $\endgroup$– Frank YellinDec 19, 2022 at 3:36
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1$\begingroup$ 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 $\endgroup$– glS ♦Dec 21, 2022 at 1:02
1 Answer
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.