Improvement of the fractional uncertainty of a neutral-atom calcium optical frequency standard to 2 x 10-14

We have investigated the two major effects that limit the accuracy of an optical frequency standard based on laser-cooled neutral calcium atoms, i.e. the residual Doppler shift and atomic collisions. A new correction method was applied to reduce the contribution of the residual Doppler effect to the total fractional uncertainty to 1 x 10(-14). Measurements of the shift of the clock transition frequency due to cold collisions allowed us to reduce their contribution to 4 x 10(-15). With these improvements we have reduced the total fractional frequency uncertainty of the standard by nearly an order of magnitude to 2 x 10(-14).