Evidence for and against Zauner's MUB Conjecture in ${\mathbb{C}}^6$

Abstract

The problem of finding provably maximal sets of mutually unbiased bases in ${\mathbb{C}}^d$, for composite dimensions $d$ which are not prime powers, remains completely open. In the first interesting case, $d=6$, Zauner predicted that there can exist no more than three MUBs. We explore possible algebraic solutions in $d=6$ by looking at their `shadows' in vector spaces over finite fields. The main result is that if a counter-example to Zauner's conjecture were to exist, then it would leave no such shadow upon reduction modulo several different primes, forcing its algebraic complexity level to be much higher than that of current well-known examples. In the case of prime powers $q\equiv 5\mathrm{mod}12$, however, we are able to show some curious evidence which -- at least formally -- points in the opposite direction. In ${\mathbb{C}}^6$, not even a single vector has ever been found which is mutually unbiased to a set of three MUBs. Yet in these finite fields we find sets of three `generalised MUBs' together with an orthonormal set of four vectors of a putative fourth MUB, all of which lifts naturally to a number field.

arXiv comment15 pages

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