Structural Phase Transition of Trapped Ions in the Quantum Regime

We experimentally characterize the 1D linear to 2D zigzag structural transition for arrays of ions confined in a linear radio-frequency Paul trap and cooled to near their ground state of motion. The transition is controllably quenched by a ramp of the confining potential. Raman sideband spectroscopy is used to probe both the energy level structure and the motional population distribution of the zigzag vibrational mode, which softens near the transition critical point. A robust critical point is achieved through stabilization of the trap potential, and significant ground state occupation is demonstrated crossing the transition. Near the critical point we resolve biases arising from ion-trap asymmetries that change the nature of the transition, and show how they can be suppressed.