  We propose a feasible and scalable quantum-dot-based implementation of a
singlet-only spin qubit which is to leading order intrinsically insensitive to
random effective magnetic fields set up by fluctuating nuclear spins in the
host semiconductor. Our proposal thus removes an important obstacle for further
improvement of spin qubits hosted in high-quality III-V semiconductors such as
GaAs. We show how the resulting qubit could be initialized, manipulated, and
read out by electrical means only, in a way very similar to a triple-dot
exchange-only spin qubit. Due to the intrinsic elimination of the effective
nuclear fields from the qubit Hamiltonian, we find an improvement of the
dephasing time $T_2^*$ of several orders of magnitude as compared to similar
existing spin qubits.
