Functional Quantum Computing: An Optical Approach
  A new model of quantum computing has recently been proposed which, in analogy with a classical lambda-calculus, exploits quantum processes which operate on other quantum processes. One such quantum meta-operator takes N unitary transformations as input, coherently permutes their ordering, and outputs a new composite operator which can be applied to a quantum state. Here we propose an optical device which implements this type of coherent operator permutation. This device requires only one physical implementation of each operator to be permuted. 
Timothy Rambo, Joseph Altepeter, Giacomo Mauro D'Ariano, Prem Kumar
quant-ph