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What is the inverse of the cyclic shift operator?

Physics Asked by enco909 on November 4, 2020

I have given the cyclic shift operator $X(x) in mathbf{R}^{d times d}$, the generalized pauli operator, for a qudit system. This operator is defined by the action it takes on an arbitary ket vector $left| j rangle right. in mathbf{R}^d$ with: $$X(x) left| j rangle right. = left| x oplus j rangle right..$$ Thereby $oplus$ describes the cyclic addition operator defined by: $$x oplus j = (x + j) text{mod}(d).$$ Now I’m supposed to show that the inverse of this operator equals $X(-x)$. My approach is to look at how this operator acts on an arbitary ket vector. This should be something like: $$X(x) left| j rangle right. = left| x oplus j rangle right. langle j left| j rangle right. = left| x oplus j rangle right..$$
So the inverse of this operator should act as:
$$ X^{-1}(x) left| j rangle right. = (left| x oplus j rangle right. langle j left| right.)^{-1} left| j rangle right. = left| j rangle right. langle x oplus j left| right. jrangle.$$
But at this point I have no clue how to show that this expression equals $X(-x)$.

Edit:

With the correction by By Symmetry I’ve tried to invert the operator directly. So my new approach is:

$$X(x)=sum_j left| right. x oplus j rangle langle j left. right|.$$

So the inverse operator is given by:

begin{align}
X^{-1}(x)
&= left( sum_j left| right. x oplus j rangle langle j left. right| right)^{-1}
&= sum_j left| right. j rangle langle x oplus j left. right|
&= sum_j X^{-1}(-x) X(-x) left| right. j rangle langle x oplus j left. right|
&= sum_j X^{-1}(-x) left| right. -x oplus j rangle langle x oplus j left. right|
&= sum_j left| right. -x oplus j rangle X(-x) langle x oplus j left. right|
&= sum_j left| right. -x oplus j rangle langle x-x oplus j left. right|
&= sum_j left| right. -x oplus j rangle langle j left. right| = X(-x).
end{align}

I hope I got this right.

One Answer

Inverse operators are unique, so it is sufficent to show that $X(-x)$ is an inverse to show that it is the inverse. With that in mind begin{align} X(-x)X(x)|jrangle &= X(-x)|xoplus jrangle &= |-x oplus (xoplus j)rangle &= |(-x oplus x)oplus jrangle &= |0 oplus jrangle &= |jrangle end{align} so $X(-x)X(x)$ acts trivially on any basis vector $|jrangle$. By linearity this extends to an arbitrary state $|psirangle$, so $X(-x)X(x) = 1$. Therefore $X(-x) = X^{-1}(x)$

Correct answer by By Symmetry on November 4, 2020

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