Two-Mode Squeezed States and Their Superposition in the Motion of Two   Trapped Ions
  We propose a method to create two-mode squeezed states and their superposition in the center-of-mass mode and breathing mode of two-trapped ions. Each ion is illuminated simultaneously by two standing waves. One of the fields is tuned to excite resonantly and simultaneously both upper sidebands of the two normal modes, while the other field tuned to the corresponding lower sidebands. 
Hao-Sheng Zeng, Le-Man Kuang and Ke-Lin Gao