Decomposing complex unitary evolution into a series of constituent components is a cornerstone of practical quantum information processing. For the decompostion of an n×n unitary into a series of 2×2 subunitaries there are well established algorithms which allow for a linear optics realization of large unitaries by beamsplitters and phase shifters. Motivated by recent advances in the production of multi mode devices, we present an algorithm that allows a decomposition of large unitaries into a series of m×m multimode devices and prove that it is asymptotically optimal.

[This is Joint Work with Christian Arends, Lasse Wolf, Jasmin Meinecke, Sonja Barkhofen and Tim Bartley]