  Recently Levy has shown that quantum computation can be performed using an
ABAB.. chain of spin-1/2 systems with nearest-neighbor Heisenberg interactions.
Levy notes that all necessary elementary computational `gates' can be achieved
by using spin-resonance techniques involving modulating the spin-spin
interaction strength at high frequency. Here we note that, as an alternative to
that approach, it is possible to perform the elementary gates with simple,
non-oscillatory pulses.
