Efficient explicit circuit for quantum state preparation of piece-wise continuous functions

Abstract: Efficiently uploading data into quantum states is essential for many quantum algorithms to achieve advantage across various applications. In this paper, we address this challenge by proposing a method to upload a polynomial function $f(x)$ on the interval $x \in (a, b)$ into a pure quantum state consisting of qubits, where a discretized $f(x)$ is the amplitude of this state. The preparation cost has $\mathcal{O}(n\log n)$ scaling in the number of qubits $n$ and linear scaling with the degree of the polynomial $Q$. This efficiency allows the preparation of states whose amplitudes correspond to high-degree polynomials, enabling the approximation of almost any continuous function. We introduce an explicit algorithm for uploading such functions using four real polynomials that meet specific parity and boundedness conditions. We also generalize this approach to piece-wise polynomial functions, with the algorithm scaling linearly with the number of piecewise parts. Our method achieves efficient quantum circuit implementation and we present detailed gate counting and resource analysis.