Dual bounds for the positive definite functions approach to mutually unbiased bases

Afonso S. Bandeira, Nikolaus Doppelbauer, Dmitriy Kunisky

A long-standing open problem asks if there can exist 7 mutually unbiased bases (MUBs) in ${\mathbb{C}}^6$, or, more generally, $d+1$ MUBs in ${\mathbb{C}}^d$ for any $d$ that is not…