Individual addressing of ions using magnetic field gradients in a   surface-electrode ion trap
  Dense array of ions in microfabricated traps represent one possible way to scale up ion trap quantum computing. The ability to address individual ions is an important component of such a scheme. We demonstrate individual addressing of trapped ions in a microfabricated surface-electrode trap using a magnetic field gradient generated on-chip. A frequency splitting of 310(2) kHz for two ions separated by 5 um is achieved. Selective single qubit operations are performed on one of two trapped ions with an average of 2.2+/-1.0% crosstalk. Coherence time as measured by the spin-echo technique is unaffected by the field gradient. 
Shannon X. Wang, Jaroslaw Labaziewicz, Yufei Ge, Ruth Shewmon, Isaac   L. Chuang