Cosmological large scale structure $N$-body simulations are
computation-light, memory-heavy problems in supercomputing. The considerable
amount of memory is usually dominated by an inefficient way of storing more
than sufficient phase space information of particles. We present a new
parallel, information-optimized, particle-mesh based $N$-body code CUBE, in
which information- and memory-efficiency is increased by nearly an order of
magnitude. This is accomplished by storing particle's relative phase space
coordinates instead of global values, and in the format of fixed point as light
as 1-byte. The remaining information is given by complimentary density and
velocity fields (negligible in memory space) and proper ordering of particles
(no extra memory). Our numerical experiments show that this
information-optimized $N$-body algorithm provides accurate results within the
error of the particle-mesh algorithm. This significant lowering of
memory-to-computation ratio breaks the bottleneck of scaling up and speeding up
large cosmological $N$-body simulations on multi-core and heterogenous
computing systems.