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Attached please find two circuits for the IBM quantum composer, that show the outcome of Grover's algorithm with 3 QuBits after 2 iterations for the situation, where the solution is at address 111 and for the situation, where the solution is at address 101.

I am astonished, that the probabilities differ so much.

Is that possible, that finding the solution at 111 is so much clearly than finding the solution at 101 ?enter image description here

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There is a missing CNOT gate from your oracle. It should be:

enter image description here

And the whole circuit should be:

enter image description here

This is because you want to add a phase when the state is $|101\rangle$. The first CNOT will change this state to $|111\rangle$ so you can apply the same oracle used in your other circuit. Then apply CNOT again to undo the effect of the first CNOT.

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  • $\begingroup$ Wow ! You are right ! That works ! Thank you so much ! $\endgroup$
    – Oliver Vornberger
    Mar 4 at 6:34
  • $\begingroup$ Dear Egretta.Thula: When I run this circuit on the IBM composer, then after one oracle and one reflection I get the following result: 0.177 0.177 0.177 0.177 0.177 0.884 0.177 0.177 but all state vectors are painted in red, so that means negative amplitude, right ? However, they should all be positive. What am I missing here ? $\endgroup$
    – Oliver Vornberger
    Mar 28 at 5:29

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