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Say I have

$$\dfrac{1}{\sqrt{2}}\bigl(|1\rangle|221\rangle|0\rangle + |3\rangle|73\rangle|2\rangle\bigr).$$

How can I change that into

$$\dfrac{1}{\sqrt{2}}\bigl(|1\rangle|221\rangle|1\rangle + |3\rangle|73\rangle|2\rangle\bigr)?$$

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  • $\begingroup$ What do you mean by a certain term? If it is what it I think you are saying then you are not following linearity. But I just want to clarify. $\endgroup$ – AHusain Mar 14 at 22:28
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Assuming that your last spin is of dimension 3, why not just apply the unitary $$ \left(\begin{array}{ccc} 0 & 1 & 0 \\ 1 & 0 & 0 \\ 0 & 0 & 1 \end{array}\right)? $$

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  • $\begingroup$ How is this achieving what the op has in his example $\endgroup$ – bilanush Mar 15 at 13:26
  • $\begingroup$ It is a unitary which changes 0 to 1, and leaves 2 unchanged. That is all the OP needed to solve his example. $\endgroup$ – DaftWullie Mar 15 at 15:59
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The first register the way you write it is about two qubits if not more. In the state, it is either 01 (1) or 11 (3). Then I would just use a X/NOT controlled by the two qubits of the register and only applied when the control is 01. This is done by Toffoli where you use X gate in between the first qubit so the control is applied when it is 0.

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