A common trick for designing faster quantum adiabatic algorithms is to apply
the adiabaticity condition locally at every instant. However it is often
difficult to determine the instantaneous gap between the lowest two
eigenvalues, which is an essential ingredient in the adiabaticity condition. In
this paper we present a simple linear algebraic technique for obtaining a lower
bound on the instantaneous gap even in such a situation. As an illustration, we
investigate the adiabatic unordered search of van Dam et al. (How powerful is
adiabatic quantum computation? Proc. IEEE FOCS, pp. 279-287, 2001) and Roland
and Cerf (Physical Review A 65, 042308, 2002) when the non-zero entries of the
diagonal final Hamiltonian are perturbed by a polynomial (in logN, where
N is the length of the unordered list) amount. We use our technique to derive
a bound on the running time of a local adiabatic schedule in terms of the
minimum gap between the lowest two eigenvalues.Comment: 11 page
