The Quantum Phase Estimation algorithm is designed to estimate the eigenvalues of a unitary operator. It is a very important subroutine to many quantum algorithms, most famously Shor's algorithm for factoring and the HHL algorithm (named after the physicists Harrow, Hassidim and Lloyd) for solving linear systems of equations on a quantum computer. Moreover, eigenvalue problems can be found across many disciplines and application areas, including (for example) principal component analysis (PCA) as used in machine learning or the solution of differential equations as relevant across mathematics, physics, engineering and chemistry.