Mutually unbiased bases in dimension six containing a product-vector basis

Excluding the existence of four MUBs in (mathbb {C}^6) is an open problem in quantum information. We investigate the number of product-vectors in the set of four mutually unbiased bases (MUBs) in dimension six, by assuming that the set exists and contains a product-vector basis. We show that in most cases the number of product-vectors in each of the remaining three MUBs is at most two. We further construct the exceptional case in which the three MUBs respectively contain at most three, two and two product-vectors. We also investigate the number of vectors mutually unbiased to an orthonormal basis.