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Understanding CNOT Gategate for Indirect Measurementindirect measurement

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Understanding CNOT Gate for Indirect Measurement

I am trying to write a simple circuit to understand the process of finding the set of parameters such that the cost function of that circuit in question is minimized. For that I understand one has to have a readout bit in order to calculate the cost function.

I have read on papers that through a control gate one can achieve this. My question is the following: do I have to have control gates as in between each step of the circuit (between the gates), or can I just apply the control gates at the end of circuit? Also: why is not advantageous to only use a readout bit instead of multiple readout bits?