Central extensions of the Heisenberg algebra

We study the non-trivial central extensions $CEHeis$ of the Heisenberg algebra $Heis$ recently constructed in {AccBouCE}. We prove that a real form of $CEHeis$ is one the fifteen classified real four--dimensional solvable Lie algebras. We also show that $CEHeis$ can be realized (i) as a sub--Lie--algebra of the Schroedinger algebra and (ii) in terms of two independent copies of the canonical commutation relations (CCR). This gives a natural family of unitary representations of $CEHeis$ and allows an explicit determination of the associated group by exponentiation. In contrast with $Heis$, the group law for $CEHeis$ is given by nonlinear (quadratic) functions of the coordinates.