  Building robust instruments capable of making interferometric measurements
with precision beyond the standard quantum limit remains an important goal in
many metrology laboratories. We describe here the basic concepts underlying
spin squeezing experiments that allow one to surpass this limit. In priniciple
it is possible to reach the so-called Heisenberg limit, which constitutes an
improvement in precision by a factor $\sqrt{N}$, where $N$ is the number of
particles on which the measurement is carried out. In particular, we focus on
recent progress toward implementing spin squeezing with a cloud of beryllium
ions in a Penning ion trap, via the geometric phase gate used more commonly for
performing two-qubit entangling operations in quantum computing experiments.
