  Quantum bits or qubits naturally decohere by becoming entangled with
uncontrollable environments. Dynamical decoupling is thereby required to
disentangle qubits from an environment by periodically reversing the qubit
bases, but this causes rotation error to accumulate. Whereas a conventional
qubit is rotated within the SU(2) two-level system, a geometric qubit defined
in the degenerate subspace of a V-shaped SU(3) three-level system is
geometrically rotated via the third ancillary level to acquire a geometric
phase. We here demonstrate that, simply by introducing detuning, the dynamical
decoupling of the geometric qubit on a spin triplet electron in a
nitrogen-vacancy center in diamond can be made to spontaneously suppress error
accumulation. The geometric dynamical decoupling extends the coherence time of
the geometric qubit up to 1.9 ms, limited by the relaxation time, with 128
decoupling gates at room temperature. Our technique opens a route to holonomic
quantum memory for use in various quantum applications requiring sequential
operations
