  We demonstrate a novel scheme for Raman-pulse and Bragg-pulse atom
interferometry based on the $5\mathrm{S} - 6\mathrm{P}$ blue transitions of
$^{87}$Rb that provides an increase by a factor $\sim 2$ of the interferometer
phase due to accelerations with respect to the commonly used infrared
transition at 780 nm. A narrow-linewidth laser system generating more than 1 W
of light in the 420-422 nm range was developed for this purpose. Used as a
cold-atom gravity gradiometer, our Raman interferometer attains a stability to
differential acceleration measurements of $1\times10^{-8}$ $g$ at 1 s and
$2\times 10^{-10}$ $g$ after 2000 s of integration time. When operated on
first-order Bragg transitions, the interferometer shows a stability of
$6\times10^{-8}$ g at 1 s, averaging to $1\times10^{-9}$ g after 2000 s of
integration time. The instrument sensitivity, currently limited by the noise
due to spontaneous emission, can be further improved by increasing the laser
power and the detuning from the atomic resonance. The present scheme is
attractive for high-precision experiments as, in particular, for the
determination of the Newtonian gravitational constant.
