# Questions tagged [hhl-algorithm]

For questions related to the quantum algorithm for inverting linear systems of equations, developed by Harrow, Hassidim and Lloyd in 2009.

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### Exponentiating Hermitian Matrix for use in QPE/HHL

I am currently trying to understand HHL by implementing a very inefficient Qiskit simulation script that performs HHL on an arbitrary hermitian matrix $A$ and a vector $b$. Because I am currently ...
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### How to draw Qiskit's HHL algorithm as a circuit?

Qiskit Aqua has a module that implements the HHL algorithm: https://qiskit.org/documentation/_modules/qiskit/aqua/algorithms/single_sample/hhl/hhl.html How does one draw the implemented circuit? * ...
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### Running HHL tutorial on real backend

I tried to follow the HHL tutorial. Since the 8x8 problem is not realistically simulable (I stopped after a few minutes), I tried to make it run on another backend, starting on the easiest example: <...
107 views

### Matrix Inversion is BQP-complete proof in HHL and the probability of measuring $T+1 \leq t \leq 2T$

I am continuing to try and fully understand the argument why Matrix inversion is BQP-complete according to the proof given in the HHL paper here, and I have hit another snag. In this question here, ...
158 views

### Understanding the filter functions in the HHL algorithm

I am continuing my studies of the HHL algorithm here. In applying the controlled rotation conditioned on the eigenvalues of the matrix, the authors use so called filter functions in order to filter ...
125 views

### Apply the conditional Hamiltonian evolution (HHL)

I have a problem with the conditional Hamiltonian. In the original article on HHL (p.3) they wrote that applying the conditional Hamiltonian correspond to:  \sum_{\tau=0}^{T-1}|\tau\rangle\langle\...
180 views

### Showing that Matrix Inversion is BQP-complete - HHL Algorithm

I am trying to understand an argument that Matrix Inversion is BQP-complete for certain conditions on the matrix. This is explained here on page 39 (this paper is a primer to the HHL algorithm and ...
167 views

### Using the HHL algorithm to compute $A |b \rangle$ instead of $A^{-1} |b \rangle$

In the paper "Compiling basic linear algebra subroutines for quantum computers" here, they discuss (page 2 bottom right) using the HHL algorithm to multiply a vector by a matrix. However, after ...