Quantumness of quantum phase estimation algorithms

Quantum metrology promises enhanced precision in measurements, with quantum phase estimation algorithms (QPEAs) playing a central role. QPEAs achieve enhanced precision in phase estimation, even without explicit entanglement generation or the use of non-classical states. This paper elucidates the quantum resources underpinning this advantage. It is shown that while entanglement may not be generated by the controlled unitary gates within QPEAs, it is in general required to implement those gates. Furthermore, by comparing QPEAs to their quasi-classical counterparts, it is provided evidence that the quantum metrological advantage stems from the ability to control and measure individual particles at the microscopic level, highlighting the subtle interplay of quantum features in these algorithms. These findings have implications for the design and implementation of efficient quantum metrological protocols across various quantum computing platforms, and for operational resource theories of non-classicality.