Qubit-optimal quantum phase estimation of block-encoded Hamiltonians

Abstract: Block-encodings have become one of the most common oracle assumptions in the circuit model. I present an algorithm that uses von Neumann's measurement procedure to measure a phase, using time evolution on a block-encoded Hamiltonian as a subroutine. This produces an extremely simple algorithm for quantum phase estimation, which can be performed with a pointer system of $\mathcal{O}(1)$ qubits. I then use recent results for block-encoding implementations, showing that one can efficiently prepare QPE beginning from a linear combination of Pauli strings. Using this, I give the Clifford + T complexity bound for QPE with respect to model-relevant parameters of the Hamiltonian and the desired precision. In the process, I provide a very general error analysis for Clifford + T implementations of QSP, quantum eigenvalue transformation, or quantum singular value transformation circuits.