coined walk by a quantum optical network built from passive, linear optical elements such as beam splitters and phase shifters? It turned out that such a realization is possible and, in fact, it leads to a natural generalization of the coined walk, the scattering quantum random walk [@Kosik2005Scattering-mode]. The SQRW on the hypercube allows for a quantum optical implementation of the SKW search algorithm [@shenvi:052307]. Having a proposal for a physical realization at hand we are in the position to analyze in some detail the effects hindering its successful operation.

Noise and decoherence strongly influence quantum walks. For a recent review on this topic see [@Kendon2006Decoherence-in-]. The first investigations in this direction indicated that a small amount of decoherence can actually enhance the mixing property [@Kendon2003Decoherence-can]. For a continuous QRW on a hypercube there is a threshold for decoherence, beyond which the walk behaves classically [@Alagic2005Decoherence-in-]. Košik [*et al*]{} analyzed SQRW with randomized phase noise on a $d$ dimensional lattice [@Kosik2006Quantum-walks-w]. The quantum walk on the line has been studied by several authors in the linear optical context, with the emphasis on the effect of various initial states, as well as on the impact of decoherence [@jeong:pra68.012310; @pathak:pra75.032351]. The quantum 