  The coherent control of interacting spins in semiconductor quantum dots is of
strong interest for quantum information processing as well as for studying
quantum magnetism from the bottom up. Here, we present a $2\times4$ germanium
quantum dot array with full and controllable interactions between
nearest-neighbor spins. As a demonstration of the level of control, we define
four singlet-triplet qubits in this system and show two-axis single-qubit
control of each qubit and SWAP-style two-qubit gates between all neighbouring
qubit pairs, yielding average single-qubit gate fidelities of
99.49(8)-99.84(1)% and Bell state fidelities of 73(1)-90(1)%. Combining these
operations, we experimentally implement a circuit designed to generate and
distribute entanglement across the array. A remote Bell state with a fidelity
of 75(2)% and concurrence of 22(4)% is achieved. These results highlight the
potential of singlet-triplet qubits as a competing platform for quantum
computing and indicate that scaling up the control of quantum dot spins in
extended bilinear arrays can be feasible.
