  In their seminal paper, Lubotzky, Phillips and Sarnak (LPS) defined the
notion of regular Ramanujan graphs and gave an explicit construction of
infinite families of $(p+1)$-regular Ramanujan Cayley graphs, for infinitely
many $p$. In this paper we extend the work of LPS and its successors to
bigraphs (biregular bipartite graphs), in several aspects: we investigate the
combinatorial properties of various generalizations of the notion of Ramanujan
graphs, define a notion of Cayley bigraphs, and give explicit constructions of
infinite families of $(p^3+1,p+1)$-regular Ramanujan Cayley bigraphs, for
infinitely many $p$.
  Both the LPS graphs and our ones are arithmetic, arising as quotients of
Bruhat-Tits trees by congruence subgroups of arithmetic lattices in a $p$-adic
group, $PGL_2(\mathbb{Q}_p)$ for LPS and $PU_3(\mathbb{Q}_p)$ for us. In both
cases the Ramanujan property relates to the Generalized Ramanujan Conjecture
(GRC), on the respective groups. But while for $PGL_2$ the GRC holds
unconditionally, this is not so in the case of $PU_3$. We find explicit cases
where the GRC does and does not hold, and use this to construct arithmetic
non-Ramanujan Cayley bigraphs as well, and prove that nevertheless they satisfy
the Sarnak-Xue density hypothesis.
  On the combinatorial side, we present a pseudorandomness characterization of
Ramanujan bigraphs, and a more general notion of biexpanders. We also show that
the graphs we construct exhibit the cutoff phenomenon with bounded window size
for the mixing time of non-backtracking random walks, either as a consequence
of the Ramanujan property, or the density hypothesis. Finally, we present some
other applications of our work: super golden gates for $PU_3$, Ramanujan and
non-Ramanujan complexes of type $\tilde{A}_2$, optimal strong approximation for
$p$-arithmetic subgroups of $PSU_3$ and vanishing of Betti numbers of Picard
modular surfaces.
