Entropic uncertainty relations and locking: Tight bounds for mutually unbiased bases
Manuel A. Ballester, Stephanie Wehner · Physical Review A · 2007
We prove tight entropic uncertainty relations for a large number of mutually unbiased measurements. In particular, we show that a bound derived from the result by Maassen and Uffink [Phys. Rev. Lett. 60, 1103 (1988)] for two such measurements can in fact be tight for up to $\sqrt{d}$ measurements in mutually unbiased bases. We then show that using more mutually unbiased bases does not always lead to a better locking effect. We prove that the optimal bound for the accessible information using up to $\sqrt{d}$ specific mutually unbiased bases is $\mathrm{log}\phantom{\rule{0.2em}{0ex}}d∕2$, which is the same as can be achieved by using only two bases. Our result indicates that merely using mutually unbiased bases is not sufficient to achieve a strong locking effect and we need to look for additional properties.