High-fidelity entangling gate for double-quantum-dot spin qubits
  Electron spins in semiconductors are promising qubits because their long coherence times enable nearly 10^9 coherent quantum gate operations. However, developing a scalable high-fidelity two-qubit gate remains challenging. Here, we demonstrate an entangling gate between two double-quantum-dot spin qubits in GaAs by using a magnetic field gradient between the two dots in each qubit to suppress decoherence due to charge noise. When the magnetic gradient dominates the voltage-controlled exchange interaction between electrons, qubit coherence times increase by an order of magnitude. Using randomized benchmarking and self-consistent quantum measurement, state, and process tomography, we measure single-qubit gate fidelities of approximately 99% and an entangling gate fidelity of 90%. In the future, operating double quantum dot spin qubits with large gradients in nuclear-spin-free materials, such as Si, should enable a two-qubit gate fidelity surpassing the threshold for fault-tolerant quantum information processing. 
John M. Nichol, Lucas A. Orona, Shannon P. Harvey, Saeed Fallahi,   Geoffrey C. Gardner, Michael J. Manfra, Amir Yacoby
cond-mat.mes-hall quant-ph