  We measured the ground state electric dipole polarizability of sodium,
potassium, and rubidium using a Mach-Zehnder atom interferometer with an
electric field gradient. We find alpha_Na=24.11(2)_stat(18)_sys x 10^-24 cm^3,
alpha_K=43.06(14)(33), and alpha_Rb=47.24(12)(42). Since these measurements
were all performed in the same apparatus and subject to the same systematic
errors we can present polarizability ratios with 0.3% precision. We find
alpha_Rb/alpha_Na=1.959(5), alpha_K/alpha_Na=1.786(6), and
alpha_Rb/alpha_K=1.097(5). We combine our ratio measurements with the higher
precision measurement of sodium polarizability by Ekstrom et al. [Phys. Rev. A
51, 3883 (1995)] to find alpha_K=43.06(21) and alpha_Rb=47.24(21).
