  The optical beam splitter is a widely-used device in photonics-based quantum
information processing. Specifically, linear optical networks demand large
numbers of beam splitters for unitary matrix realization. This requirement
comes from the beam splitter property that a photon cannot go back out of the
input ports, which we call "directionally-biased". Because of this property,
higher dimensional information processing tasks suffer from rapid device
resource growth when beam splitters are used in a feed-forward manner.
Directionally-unbiased linear-optical devices have been introduced recently to
eliminate the directional bias, greatly reducing the numbers of required beam
splitters when implementing complicated tasks. Analysis of some originally
directional optical devices and basic principles of their conversion into
directionally-unbiased systems form the base of this paper. Photonic quantum
walk implementations are investigated as a main application of the use of
directionally-unbiased systems. Several quantum walk procedures executed on
graph networks constructed using directionally-unbiased nodes are discussed. A
significant savings in hardware and other required resources when compared with
traditional directionally-biased beam-splitter-based optical networks is
demonstrated.
