  We propose a scalable ion trap architecture for universal quantum
computation, which is composed of an array of ion traps with one ion confined
in each trap. The neighboring traps are designed capable of merging into one
single trap. The universal two-qubit $\sqrt{SWAP}$ gate is realized by direct
collision of two neighboring ions in the merged trap, which induces an
effective spin-spin interaction between two ions. We find that the
collision-induced spin-spin interaction decreases with the third power of two
ions' trapping distance. Even with a $200\ \mu m$ trapping distance between
atomic ions in Paul traps, it is still possible to realize a two-qubit gate
operation with speed in $0.1\ kHz$ regime. The speed can be further increased
up into $0.1\ MHz$ regime using electrons with $10\ mm$ trapping distance in
Penning traps.
