  An error-corrected quantum processor will require millions of qubits,
accentuating the advantage of nanoscale devices with small footprints, such as
silicon quantum dots. However, as for every device with nanoscale dimensions,
disorder at the atomic level is detrimental to qubit uniformity. Here we
investigate two spin qubits confined in a silicon double-quantum-dot artificial
molecule. Each quantum dot has a robust shell structure and, when operated at
an occupancy of 5 or 13 electrons, has single spin-$\frac{1}{2}$ valence
electron in its $p$- or $d$-orbital, respectively. These higher electron
occupancies screen atomic-level disorder. The larger multielectron
wavefunctions also enable significant overlap between neighbouring qubit
electrons, while making space for an interstitial exchange-gate electrode. We
implement a universal gate set using the magnetic field gradient of a
micromagnet for electrically-driven single qubit gates, and a
gate-voltage-controlled inter-dot barrier to perform two-qubit gates by pulsed
exchange coupling. We use this gate set to demonstrate a Bell state preparation
between multielectron qubits with fidelity 90.3%, confirmed by two-qubit state
tomography using spin parity measurements.
