  Future quantum computers capable of solving relevant problems will require a
large number of qubits that can be operated reliably. 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 promise 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
readout. 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 allow for testing of
increasingly meaningful quantum protocols and constitute a major stepping stone
towards large-scale quantum computers.
