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Jul 18, 2020 at 0:53 history edited keisuke.akira CC BY-SA 4.0
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Jul 18, 2020 at 0:52 comment added keisuke.akira One way to obtain the result in $d$ dimensions is to use the $d^2$ Heisenberg-Weyl operators (or the finite dimensional representation of the Heisenberg-Weyl algebra). If $X(i)Z(j)$ is the $(i,j)$th operator then, we have, $\frac{1}{d^{2}} \sum_{i, j=0}^{d-1} X(i) Z(j) \rho Z^{\dagger}(j) X^{\dagger}(i)=\frac{\mathbb{I}}{d}$. See, for example, Page 176 of this book.
Jul 17, 2020 at 8:16 comment added Amplituhedron Are you assuming the state $\rho$ is written in terms of the Pauli matrices? For a higher dimension, I found it very difficult to compute the commutation relation with unitaries and Hermitian matrices given in a quantum tomography for $\rho$. (I tried with the U(n) basis.) Nevertheless in some dimensions like 2^n dimensions, we can construct such a basis with tensor products of Pauli matrices.
Jul 15, 2020 at 3:06 history answered keisuke.akira CC BY-SA 4.0