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The interaction between two quantum bits enables entanglement, the two-particle correlations that are at the heart of quantum information science. In semiconductor quantum dots much work has focused on demonstrating single spin qubit control using optical techniques. However, optical control of entanglement of two spin qubits remains a major challenge for scaling from a single qubit to a full-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 tunneling, where the tunneling rate determines how rapidly entangling operations can be performed. The two-qubit gate speeds achieved here are over an order of magnitude faster than in other systems. These results demonstrate the viability and advantages of optically controlled quantum dot spins for multi-qubit systems.Semiconductor quantum dots (QDs) were among the first candidates proposed for solid-state qubits 1 . Self-assembled InAs QDs are a versatile physical platform, because they are epitaxially grown in a semiconductor wafer and can be fabricated into a monolithic architecture containing both electronic 2 and photonic 3 circuit elements. Individual QDs themselves can be organized into more complex "molecules" in one, two, and three dimensions. 4 With these engineering advantages, one can envision building an entire quantum network with the scalability and stability of a solid-state system.The elementary optical excitation of a QD, the exciton, has a resonance frequency in the optical regime, giving QDs a great speed advantage over nuclear spins (radio frequency) or electron spins (microwaves). With the giant optical dipole of a semiconductor quantum dot, quantum operations can be performed at a terahertz rate or faster 5 . Coherent manipulations of pure exciton qubits were the first quantum gate demonstrations in the solid state 6,

A single hole spin in a semiconductor quantum dot has emerged as a quantum bit that is potentially superior to an electron spin. A key feature of holes is that they have a greatly reduced hyperfine interaction with nuclear spins, which is one of the biggest difficulties in working with an electron spin. It is now essential to show that holes are viable for quantum information processing by demonstrating fast quantum gates and scalability. To this end we have developed InAs/GaAs quantum dots coupled through coherent tunneling and charged with controlled numbers of holes. We report fast, single qubit gates using a sequence of short laser pulses. We then take the important next step toward scalability of quantum information by optically controlling two interacting hole spins in separate dots.Comment: 5 figure