Researchers at QuTech, TNO and Delft University of Technology have demonstrated a four-qubit quantum processor based on hole spins in germanium quantum dots. Furthermore, they defined the quantum dots in a two-by-two array and obtained controllable coupling along both directions. Qubit logic is implemented all-electrically and the exchange interaction can be pulsed to freely program one-qubit, two-qubit, three-qubit, and four-qubit operations, resulting in a compact and high-connectivity circuit.
They executed a quantum logic circuit that generates a four-qubit Greenberger-Horne-Zeilinger state and they obtained coherent evolution by incorporating dynamical decoupling.
These results are an important step towards quantum error correction and quantum simulation with quantum dots.