QUROPE Quantum Information Processing and Communication in Europe

D. Kim, Z. Shi, C. B. Simmons, D. R. Ward, J. R. Prance, T. S. Koh, J. K. Gamble, D. E. Savage, M. G. Lagally, M. Friesen, S. N. Coppersmith, M. A. Eriksson
Nature 511, 70-74 (2014)

Encoding quantum information in semiconductor quantum dots offers long coherence times. However, their manipulation is often slower than desired. Previous work has increased the speed of spin qubit rotations by making use of integrated micromagnets, dynamic pumping of nuclear spins or the addition of a third quantum dot. However, these alternatives increase the complexity of the setup, which, in turn, make scalability and manufacturability more challenging.

In their work, Kim and co-workers demonstrate a quantum-dot qubit that is a hybrid of spin and charge. It is simple, requiring neither nuclear-state preparation nor micromagnets. The hybrid qubit contains three electrons in two dots and has the advantage of allowing all-electrical qubit manipulation without the need for microwave driving or local magnetic field gradients. The name “hybrid” derives from the fact that the qubit has both spin and charge character. The charge aspect allows also for very fast manipulation, though at the same time it limits the qubit coherence compared to its relatives. The researchers demonstrate that hybrid qubits allow for fast rotations along two axes of the Bloch sphere with fidelities of 85% in the X direction and 95% in the Z direction.