  We report detailed measurements of the low temperature magnetic phase diagram
of Er$_2$Ti$_2$O$_7$. Heat capacity and time-of-flight neutron scattering
studies of single crystals, subject to magnetic fields applied along the
crystallographic [110] direction, reveal unconventional low energy states.
Er$^{3+}$ magnetic ions reside on a pyrochlore lattice in Er$_2$Ti$_2$O$_7$,
where local XY anisotropy and antiferromagnetic interactions give rise to a
unique frustrated system. In zero field, the ground state exhibits coexisting
short and long range order, accompanied by soft collective spin excitations
previously believed to be absent. The application of finite magnetic fields
tunes the ground state continuously through a landscape of non-collinear
phases, divided by a zero temperature phase transition at $\mu_0 H_c \sim$ 1.5
T. The characteristic energy scale for spin fluctuations is seen to vanish at
the critical point, as expected for a second order quantum phase transition
driven by quantum fluctuations.
