New test of local Lorentz invariance using a 21Ne-Rb-K comagnetometer.

We develop a new comagnetometer using (21)Ne atoms with nuclear spin I=3/2 and Rb atoms polarized by spin exchange with K atoms to search for tensor interactions that violate local Lorentz invariance. We frequently reverse the orientation of the experiment and search for signals at the first and second harmonics of the sidereal frequency. We constrain 4 of the 5 spatial Lorentz-violating coefficients c(jk)(n) that parametrize anisotropy of the maximum attainable velocity of a neutron at a level of 10(-29), improving previous limits by 2 to 4 orders of magnitude and placing the most stringent constraint on deviations from local Lorentz invariance.