DEFINITION. “A quantum computation consists of applying a given unitary operator to a K‐qubit system. The complexity of the task, called , can be simply defined as the minimum number of gates that it takes to implement the unitary operator.”
https://onlinelibrary.wiley.com/doi/full/10.1002/prop.201500092?casa_token=8OO3b1OY4e4AAAAA%3ARqriAgTwtxnvsxl09CqnNy79m7l4ye7ZrkZxcLtObB9kHsTwYgzBuIV2X-xXQ7ezoR6VQPfQLCS-vnI