Classical codes violate the conjectured square-root bound for quantum random access codes
This work investigates whether quantum random access codes satisfy the conjectured square-root bound $p \leq (1 + \sqrt{m/n})/2$. By embedding classical random access codes with private randomness into quantum schemes using diagonal encoding states and commuting POVM measurements, the authors construct the first classical counterexamples that violate this bound, revealing that classical coding rates are key to the separation from the quantum limit. Leveraging spectral properties of decoding measurements, they establish a new restricted bound and prove that for any fixed $p \in (1/2, 1)$, counterexamples exist when the input length is sufficiently large. Moreover, they achieve optimal logarithmic qubit scaling when the recovery bias is $\sqrt{\log_2 n / n}$.