We briefly describe the use of GRAPE pulses to implement quantum logic gates
in NMR quantum computers, and discuss a range of simple extensions to the core
technique. We then consider a range of difficulties which can arise in
practical implementations of GRAPE sequences, reflecting non-idealities in the
experimental systems used.Comment: 15 pages rspublic including 4 figures. This is the original
manuscript preprint form which differs slightly from the final accepted
version (Phil Trans Roy Soc A in press