Entanglement-induced exponential advantage in amplitude estimation via state matrixization

Estimating quantum amplitude, or the overlap between two quantum states, is a fundamental task in quantum computing and underpins numerous quantum algorithms. In this work, we introduce a novel algorithmic framework for quantum amplitude estimation by transforming pure states into their matrix forms (Matrixization) and encoding them into non-diagonal blocks of density operators and diagonal blocks of unitary operators. Utilizing the construction details of state preparation circuits, we systematically reconstruct amplitude estimation algorithms within the novel matrixization framework through a technique known as channel block encoding. Compared with the standard approach, amplitude estimation through matrixization can have a different complexity that depends on the entanglement properties of the two quantum states. Specifically, our new algorithm can have exponentially smaller gate complexity when one of the two quantum states is prepared by a linear-depth quantum circuit that is below maximal entanglement under a certain bi-partition and the other state is maximally entangled. We later generalize this result to broader regimes and discuss implications. Our results demonstrate that the near-optimal performance of the standard amplitude estimation algorithm can be surpassed in specific cases.