My question should be fairly simple, though I did not find an answer to it here or anywhere else.

I have been working on an algorithm which, similarly to the HHL algorithm, provides a state $|x\rangle$ for which I want to extract some global quantity at the end. I now want to build a simple few-qubits implementation of such algorithm and for that I would like to find the $\texttt{Absolute Average}$ given by $\frac{1}{N}\big|\sum_{i=1}^Nx_i\big|$, which is clearly possible according to this tutorial to Qiskit's HHL implementation.

However, I am struggling to understand which observable $M$ I should apply in order to get this quantity from $\langle x|M|x\rangle$.


1 Answer 1


Let $|+\rangle$ denote the uniform superposition state with zero relative phases. Then

$$ \langle +|x\rangle = \frac{1}{\sqrt{N}}\begin{bmatrix}1&1&\dots&1\end{bmatrix}\begin{bmatrix}x_1\\x_2\\\dots\\x_N\end{bmatrix} = \frac{1}{\sqrt{N}}\sum_{i=1}^Nx_i. $$

For $M$ defined as $M:=|+\rangle\langle+|$, we have

$$ \langle x|M|x\rangle = \langle x|+\rangle\langle+|x\rangle = \frac{1}{N}\left|\sum_{i=1}^Nx_i\right|^2 $$

so we can obtain the absolute average from the measurement of $\langle x|M|x\rangle$ using

$$ \frac{1}{N}\left|\sum_{i=1}^Nx_i\right| = \sqrt{\frac{\langle x|M|x\rangle}{N}}. $$


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