  The problem of Phase Estimation (or Amplitude Estimation) admits a quadratic
quantum speedup. Wang, Higgott and Brierley [2019, Phys. Rev. Lett. 122 140504]
have shown that there is a continuous trade-off between quantum speedup and
circuit depth (by defining a family of algorithms known as $\alpha$-QPE). In
this work, we show that the scaling of $\alpha$-QPE can be naturally and
succinctly derived within the framework of Quantum Singular Value
Transformation (QSVT). From the QSVT perspective, a greater number of coherent
oracle calls translates into a better polynomial approximation to the sign
function, which is the key routine for solving Phase Estimation. The better the
approximation to the sign function, the fewer samples one needs to determine
the sign accurately. With this idea, we simplify the proof of $\alpha$-QPE,
while providing a new interpretation of the interpolation parameters, and show
that QSVT is a promising framework for reasoning about classical-quantum
interpolations.
