Spectroscopy and Thermometry of Drumhead Modes in a Mesoscopic   Trapped-Ion Crystal using Entanglement
  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. 
Brian C. Sawyer, Joseph W. Britton, Adam C. Keith, C.-C. Joseph Wang,   James K. Freericks, Hermann Uys, Michael J. Biercuk,...
quant-ph cond-mat.mes-hall physics.atom-ph