The group at QuTech, a collaboration between TU Delft and TNO, is led by of Menno Veldhorst and has built a four qubit array using germanium quantum dots.
“By putting four such qubits in a two-by-two grid, demonstrating universal control over all qubits, and operating a quantum circuit that entangles all qubits, we have made an important step forward in realizing a scalable approach for quantum computation,” said Veldhorst.
Electrons trapped in quantum dots have been studied for more than two decades as a platform for quantum information but scaling beyond two-qubit logic has remained elusive. To break this barrier, the groups of Menno Veldhorst and Giordano Scappucci decided to take an entirely different approach and started to work with holes (i.e. missing electrons) in germanium. Using this approach, the same electrodes needed to define the qubits could also be used to control and entangle them.
“No large additional structures have to be added next to each qubit such that our qubits are almost identical to the transistors in a computer chip,” said Nico Hendrickx, graduate student in the group of Menno Veldhorst and first author of a key article published in Nature.
“Furthermore, we have obtained excellent control and can couple qubits at will, allowing us to program one, two, three, and four-qubit gates, promising highly compact quantum circuits.”
After successfully creating the first germanium quantum dot qubit in 2019, the number of qubits on their chips has doubled every year. “Four qubits by no means makes a universal quantum computer, of course,” said Veldhorst. “But by putting the qubits in a two-by-two grid we now know how to control and couple qubits along different directions. Now that we know how to manufacture germanium and operate an array of qubits, the germanium quantum information route can truly begin.”