  In the article we present explicit expressions for quantum fluctuations of
spacetime in the case of $(4+n)$-dimensional spacetimes, and consider their
holographic properties and some implications for clocks, black holes and
computation. We also consider quantum fluctuations and their holographic
properties in ADD model and estimate the typical size and mass of the clock to
be used in precise measurements of spacetime fluctuations. Numerical
estimations of phase incoherence of light from extra-galactic sources in ADD
model are also presented.
