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I have an extension to the following question: How to get the relative phase of a qubit? How do I get the relative phase of a pair of entangled qubits such as $$\frac{1}{\sqrt{2}}(|00\rangle+e^{i\theta}|11\rangle)$$

in which I want to find theta. I tried to use a similar methodology as the answer in the linked question above, but with no success.

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  • $\begingroup$ In the thread that was mentioned in the question, there was a mistake in this answer, that is corrected now. $\endgroup$ Jul 27 '20 at 11:27
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  1. Apply a CNOT gate with one of the qubits as control and the other as target. You'll get

$$\frac{1}{\sqrt{2}}(|0\rangle+e^{i\theta}|1\rangle) \otimes |0\rangle$$

  1. Use the methodology from How to get the relative phase of a qubit? for the first qubit :-)
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  • $\begingroup$ Hmm I tried that in the simulator but for some reason it didn't work. Then most likely its just a silly-mistake type issue with the way I wrote it. Thanks $\endgroup$ Jul 22 '20 at 5:36
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    $\begingroup$ Shouldn't it be the state after CNOT $\frac{1}{\sqrt{2}}(|0\rangle + e^{i\theta}|1\rangle)\otimes |0\rangle$? $\endgroup$ Jul 22 '20 at 6:49
  • $\begingroup$ @MartinVesely Fixed, thank you - a typical off-by-one error :-) $\endgroup$ Jul 22 '20 at 9:21

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