Heisenberg uniqueness pairs and the Klein-Gordon equation

The notion of a Heisenberg Uniqueness Pair (HUP) is introduced. This amounts to asking which collections of exponentials are weak-star fundamental in $L^\infty$ on a planar curve. In the case when the curve is a hyperbola, we can give a complete answer if the frequencies are restricted to equally spaced points on a lattice-cross. As a consequence, we solve a problem on the density of algebras generated by two inner functions raised by Matheson and Stessin.