Decoherence Bounds on Quantum Computation with Trapped Ions
  Using simple physical arguments we investigate the capabilities of a quantum computer based on cold trapped ions. From the limitations imposed on such a device by spontaneous decay, laser phase coherence, ion heating and other sources of error, we derive a bound between the number of laser interactions and the number of ions that may be used. The largest number which may be factored using a variety of species of ion is determined. 
Richard J. Hughes, Daniel F. V. James, Emanuel H. Knill, Raymond   Laflamme and Albert G. Petschek