An application of positive definite functions to the problem of MUBs
  We present a new approach to the problem of mutually unbiased bases (MUBs), based on positive definite functions on the unitary group. The method provides a new proof of the fact that there are at most $d+1$ MUBs in ${\mathbb C}^d$. It may also lead to a proof of non-existence of complete systems of MUBs in dimension 6 via a conjectured algebraic identity. 
Mihail N. Kolountzakis, M\'at\'e Matolcsi, and Mih\'aly Weiner
quant-ph