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, we measure single-qubit gate fidelities of~99%, and through self-consistent quantum measurement, state, and process tomography, we measure 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.npj Quantum Information (2017) 3:3 ; doi:10.1038/s41534-016-0003-1
INTRODUCTIONThe quantum phase coherence of isolated spins in semiconductors 1-7 can persist for long times, reaching tens of milliseconds for electron spins 8 and tens of minutes for nuclear spins. 9 Such long coherence times enable single-qubit gate fidelities exceeding the threshold for fault-tolerant quantum computing 8 and make spins promising qubits. However, entangling spins is difficult because magnetic interactions between spins are weak. For electrons, this challenge can be met by exploiting the charge of the electron for electric-dipole 10 or gate-controlled exchange coupling 2 between spins. In these methods, however, the qubit energy depends on electric fields, and charge noise in the host material limits singlequbit coherence.11 Charge noise also affects other qubit platforms. For example, heating due to charge noise is a limiting factor in the coherence of trapped ion qubits, 12 and the transmon superconducting qubit was designed to suppress noise from charge fluctuations in superconducting islands.13 Strategies such as composite pulses, 14, 15 dynamical decoupling, 11 and sweet-spot operation [16][17][18] have been developed to mitigate the effects of charge noise.In this work, we present a technique to suppress decoherence caused by charge noise. The key idea is to apply a large transverse qubit energy splitting that does not depend on electric fields and, therefore, suppresses the effects of charge fluctuations. We implement this scheme with two singlet-triplet qubits, each of which consists of two electrons in a double-quantum-dot.