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Future quantum computers capable of solving relevant problems will require a large number of qubits that can be operated reliably1. However, the requirements of having a large qubit count and operating with high fidelity are typically conflicting. Spins in semiconductor quantum dots show long-term promise2,3 but demonstrations so far use between one and four qubits and typically optimize the fidelity of either single- or two-qubit operations, or initialization and readout4-11. Here, we increase the number of qubits and simultaneously achieve respectable fidelities for universal operation, state preparation and measurement. We design, fabricate and operate a six-qubit processor with a focus on careful Hamiltonian engineering, on a high level of abstraction to program the quantum circuits, and on efficient background calibration, all of which are essential to achieve high fidelities on this extended system. State preparation combines initialization by measurement and real-time feedback with quantum-non-demolition measurements. These advances will enable testing of increasingly meaningful quantum protocols and constitute a major stepping stone towards large-scale quantum computers.
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On the path topractical large-scale quantum computation, electron spin qubits in semiconductor quantum dots12 show promise because of their inherent potential for scaling through their small size13,14, long-lived coherence4 and compatibility with advanced semiconductor manufacturing techniques15. Nevertheless, spin qubits currently lag behind in scale when compared to superconducting, trapped ions and photonic platforms, which have demonstrated control of several dozen qubits16-18. By comparison, using semiconductor spin qubits, partial19 and universal11 control of up to four qubits was achieved and entanglement of up to three qubits was quantified910,20. In a six-dot linear array, two qubits encoded in the state of three spins each were operated21 and spin exchange oscillations in a 3x3 array have been reported22.
Furthermore, the experience with other qubit platforms shows that, in scaling up, maintaining the quality of the control requires substantial effort, particularly, for instance, to deal with the denser motional spectrum in trapped ions23, to avert crosstalk in superconducting circuits24 or to avoid increased losses in photonic circuits25. For small semiconductor spin qubit systems, state-of-the-art single-qubit gate fidelities exceed 99.9%5,26,27 and two-qubit gates well above 99% fidelity have been demonstrated recently6-8,10. Most quantum-dot-based demonstrations suffer from low initialization or readout fidelities, with typical visibilities of no more than 60-75%,...