Quantum state testing with restricted measurements

Abstract

We study quantum state testing where the goal is to test whether $ρ=ρ_0\in\mathbb{C}^{d\times d}$ or $\|ρ-ρ_0\|_1>\varepsilon$, given $n$ copies of $ρ$ and a known state description $ρ_0$. In practice, not all measurements can be easily applied, even using unentangled measurements where each copy is measured separately. We develop an information-theoretic framework that yields unified copy complexity lower bounds for restricted families of non-adaptive measurements through a novel measurement information channel. Using this framework, we obtain the optimal bounds for a natural family of $k$-outcome measurements with fixed and randomized schemes. We demonstrate a separation between these two schemes, showing the power of randomized measurement schemes over fixed ones. Previously, little was known for fixed schemes, and tight bounds were only known for randomized schemes with $k\ge d$ and Pauli observables, a special class of 2-outcome measurements. Our work bridges this gap in the literature.