  We present a novel continuous dynamical decoupling scheme for the
construction of a robust qubit in a three-level system. By means of a clock
transition adjustment, we first show how robustness to environmental noise is
achieved, while eliminating drive-noise, to first-order. We demonstrate this
scheme with the spin sub-levels of the NV-centre's electronic ground state. By
applying drive fields with moderate Rabi frequencies, the drive noise is
eliminated and an improvement of 2 orders of magnitude in the coherence time is
obtained compared to the pure dephasing time. We then show how the clock
transition adjustment can be tuned to eliminate also the second-order effect of
the environmental noise with moderate drive fields. A further improvement of
more than 1 order of magnitude in the coherence time is expected and confirmed
by simulations. Hence, our scheme prolongs the coherence time towards the
lifetime-limit using a relatively simple experimental setup.
