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How to calculate cnot gate(or toffoli) in red box

Why is there additional line?

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And is it same as the two toffoli gate?

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    $\begingroup$ where did you find this circuit? $\endgroup$
    – DaftWullie
    Oct 5, 2022 at 6:30

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If you scroll down in the paper you will find the definition of those elements in figure 3.

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  • $\begingroup$ In that paper, it says "The uncomputation circuit (bottom) uses a measure-and-fixup approach [15] that requires only Clifford gates, and so has a T-count of zero and measurement-depth of 1.",but what is measure-and-fixup approach? I also read reference [15] (Cody Jones. Low-overhead constructions for the fault-tolerant toffoli gate.), but i can't understand it. I simulated on qiskitm, but the result of measurement after Hadamard gate is half 0, half 1. what the measure after hadamard gate with controled z if condition means? $\endgroup$ Nov 11, 2022 at 9:10
  • $\begingroup$ @SeokhyeonLee A measure and fixup approach is an approach where you measure and then apply a fixup based on the result. It's self-descriptive. In the paper's case it's just a generic description of what's happening in the bottom part of figure 3. $\endgroup$ Nov 11, 2022 at 15:51
  • $\begingroup$ Does this mean that an additional ancilla for error correction should be applied to apply an error correction method? when I simulated it the bit, phase flip error cannot influence the result in some cases. So, even if after measure it, the result is the same, so it can't detect the error, so the circuit don't know that this result is invalid. so confused $\endgroup$ Nov 16, 2022 at 2:49

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