In HHL algorithm, after the quantum phase estimation, what is the angle of rotation i.e. what is the value of $\theta$ in R$_y$($\theta$) that is used in rotating ancilla qubit? How to find this angle of rotation without knowing the eigenvalues of matrix $A$? Is there any general method to find this angle of rotation for any $n$x$n$ Hermitian matrix $A$?

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  • $\begingroup$ note that you can use equations in posts via mathjax, see e.g. quantumcomputing.meta.stackexchange.com/q/49/55 $\endgroup$
    – glS
    Mar 17, 2021 at 11:12
  • $\begingroup$ That's why it's controlled off "register". That tells you (approximately) the eigenvalue and so you compute the rotation angle based upon that. $\endgroup$
    – DaftWullie
    Mar 17, 2021 at 11:55
  • $\begingroup$ @glS , thanks for the suggestion. I have updated the question. $\endgroup$
    – quankid
    Mar 17, 2021 at 11:56
  • 1
    $\begingroup$ @DaftWullie Can you explain in detail what you want to say. Better to explain with an example. $\endgroup$
    – quankid
    Mar 17, 2021 at 12:00


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