  We demonstrate spectroscopy and thermometry of individual motional modes in a
mesoscopic 2D ion array using entanglement-induced decoherence as a method of
transduction. Our system is a $\sim$400 $\mu$m-diameter planar crystal of
several hundred $^9$Be$^+$ ions exhibiting complex drumhead modes in the
confining potential of a Penning trap. Exploiting precise control over the
$^9$Be$^+$ valence electron spins, we apply a homogeneous spin-dependent
optical dipole force to excite arbitrary transverse modes with an effective
wavelength approaching the interparticle spacing ($\sim$20 \nolinebreak$\mu$m).
Center-of-mass displacements below 1 nm are detected via entanglement of spin
and motional degrees of freedom.
