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Semiconductor spin qubits are promising candidates for quantum computation because of their slow decoherence and potential for scalability. All fundamental single qubit operations have been demonstrated for GaAs-based spin qubits, but they suffer from decoherence due to hyperfine coupling to nuclear spins. We have developed effective techniques to mitigate this decoherence channel for two-electron spin qubits in double quantum dots. By repeatedly inverting the qubit in a way that decouples its evolution from the fluctuations of the nuclear spin bath, its coherence time can be extended to more than 200 ms, two orders of magnitude longer than previously shown. Alternatively, operating the qubit as a feedback loop that controls the nuclear bath also enhances the coherence time and enables universal single qubit control with greatly improved gate fidelities. Having achieved good single qubit control, a fundamental milestones is to couple qubits in order to generate entanglement. We demonstrate a protocol to implement two-qubit gates using the Coulomb interaction between adjacent double dots while decoupling both qubits from slow electrical fluctuations. These results indicate that Bell states can be created with sufficient fidelity to demonstrate entanglement.

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Hendrik Bluhm, RWTH Aachen
Seminarraum Theoretische Physik
Contact: not specified