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I understand that CNOT and Toffoli (CCNOT) gates are not universal for quantum computation since they (alone) cannot create superposition. Moreover, it is totally possible to perform a CNOT operation where the control qubit is in superposition, $\alpha\vert 0 \rangle + \beta\vert 1 \rangle$, and target qubit to be $\vert 0 \rangle$.

My question is can we have a control qubit to be in superposition while using CCNOT, something like the following?

Toffoli Gate containing superposition

If yes, what will be the values for $p, q, $ and $r$? If not, why not?

The related material I found are universality of the toffoli gate and CNOT in superposition.

Thank you in advance!

M. Al Jumaily
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1 Answers1

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Control qubit can be in superposition, why not? This is how entangled qubits are created. But the circuit you have drawn does not entangle qubits, it actually does not change qubit states at all, and $|p\rangle = \alpha| 0 \rangle + \beta| 1 \rangle$, $|q\rangle=|0\rangle$, $|r\rangle=|0\rangle$.

It would be more interesting if the input value of the 2nd qubit was $|1\rangle$; then the 2nd output value $|q\rangle=|1\rangle$, but there is no separated $|p\rangle$ and $|r\rangle$ output values, instead you have 2-qubit entangled state

$$|pr\rangle=\alpha|00\rangle + \beta|1 1 \rangle$$

kludg
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