Construction of $\varepsilon_{d}$-ASIC-POVMs via $2$-to-$1$ PN functions and the Li bound
Symmetric informationally complete positive operator-valued measures (SIC-POVMs) in finite dimension $d$ are a particularly attractive case of informationally complete POVMs (IC-POVMs), which consist of $d^{2}$ subnormalized projectors with equal pairwise fidelity. However, it is difficult to construct SIC-POVMs, and it is not even clear whether there exists an infinite family of SIC-POVMs. To realize some possible applications in quantum information processing, Klappenecker et al. [37] introduced an approximate version of SIC-POVMs called approximately symmetric informationally complete POVMs (ASIC-POVMs). In this paper, we construct a class of $\varepsilon_{d}$-ASIC-POVMs in dimension $d=q$ and a class of $\varepsilon_{d}$-ASIC-POVMs in dimension $d=q+1$, respectively, where $q$ is a prime power. We prove that all $2$-to-$1$ perfect nonlinear (PN) functions can be used for constructing $\varepsilon_{q}$-ASIC-POVMs. We show that the set of vectors corresponding to the $\varepsilon_{q}$-ASIC-POVM forms a biangular frame. The construction of $\varepsilon_{q+1}$-ASIC-POVMs is based on a multiplicative character sum estimate called the Li bound. We show that the set of vectors corresponding to the $\varepsilon_{q+1}$-ASIC-POVM forms an asymptotically optimal codebook. We characterize "how close" the $\varepsilon_{q}$-ASIC-POVMs (resp. $\varepsilon_{q+1}$-ASIC-POVMs) are from being SIC-POVMs of dimension $q$ (resp. dimension $q+1$). Finally, we explain the significance of constructing $\varepsilon_{d}$-ASIC-POVMs.