Loss and DiVincenzo proposed in 1998 that quantum dots define excellent building blocks for a fault-tolerant quantum computer. By confining individual electrons in quantum dots, qubits can be defined on the electron spin states, and controlled using electrical signals. Two decades of intensive research on spin qubits in quantum dots has resulted in many proof-of-principle demonstrations. However, a realization of the original proposal remained elusive. In their proposal, Loss and DiVincenzo envisaged that if the magnetic orientation varies between quantum dots, hopping an electron between the two quantum dots would allow for fast and efficient qubit logic. Practically, it has remained too challenging to create such variations in the magnetic orientation. Here, we demonstrate that changes in the spin quantization axis can be realized, by making use of spin qubits in germanium. This is achieved by exploiting the strong spin-orbit interaction in germanium, which can cause orientations that differ by tens of degrees. In addition, we establish hopping of an electron between two quantum dots, where the spin remains coherent over effective length scales beyond a millimeter. This allows to realize qubit fidelities up to 99.97% and two-qubit gates of 99.3%. We then apply our findings to larger quantum circuits to explore pathways toward fault-tolerant quantum computation.

This event starts at 3:30pm with refreshments, followed at 3:45pm by a short presentation by Yinqi Chen (PhD student Maxim Vavilov group) titled "Voltage-Controlled Quantum Entanglement in Superconductor-Semiconductor Devices". The invited presentation starts at 4pm.