Ultrafast optical control of entanglement between two quantum-dot spins

The interaction between two quantum bits enables the creation of entanglement, the two-particle correlations that are at the heart of quantum information science. In semiconductor quantum dots, much work has focused on demonstrating control over single spin qubits using optical techniques. However, optical control of two spin qubits remains a major challenge for scaling to a fully fledged quantum-information platform. Here, we combine advances in vertically stacked quantum dots with ultrafast laser techniques to achieve optical control of the entangled state of two electron spins. Each electron is in a separate InAs quantum dot, and the spins interact through tunnelling, where the tunnelling rate determines how rapidly entangling operations can be carried out. We achieve two-qubit gates with an interaction rate of 30 GHz, more than an order of magnitude faster than demonstrated in any other system so far. These results demonstrate the viability and advantages of optically controlled quantum-dot spins for multi-qubit systems.