# In the rotation part of HHL algorithm, how do you decompose $R$ in terms of universal gates?

In this paper, there is a diagram explaining how HHL works, which I attached below: My question is, for the rotation part, how do you write $$R$$ as a combination of universal set of gates without knowing the eigenvalues a priori? Or what is a generic way of writing $$R$$ without doing any classical pre-computation regarding the eigenvalues?

In most of the implementations that I found online, it is assumed that the eigenvalues are known, and then one can find the angles of rotation by calculating for each eigenvalue: $$\theta_i = arccos(\frac{C}{\lambda_i})$$. After that, one can append for the ancilla register multiple $$R_y$$ gates with the aforementioned angles of rotation as arguments.

Thanks in advance for the help!

We are given a system of $$N$$ linear equations with $$N$$ unknowns which can be expressed as $$Ax=b$$, where $$x$$ is a vector of unknowns, $$A$$ is the matrix of coeﬃcients and $$b$$ is the vector of solutions. If $$A$$ is an invertible matrix, then we can write that the solution is given by $$x=A^{−1}b$$.
From this we can see that it is in fact $$x$$, not $$A$$, that is the unknown to be found. So, since you know what $$A$$ is, you do in fact know the eigenvalues of $$A$$ beforehand. In fact, since $$A$$ must be known to implement $$U$$, you must even know them in steps a) and c) also! Hence what you describe as "pre-computation" is actually a necessary step, encoding the problem statement into the gates in the algorithm :)
• Knowing the exact eigenvalues of $A$ is not a requirement. The only requirement you have is to know an upper-bound and a lower-bound on the eigenvalues (see quantumcomputing.stackexchange.com/q/2604/1386). You can also perform Hamiltonian Simulation without knowing the eigenvalues, else VQE would never have been a thing. May 14, 2020 at 9:48