Universal control of a six-qubit quantum processor in silicon.

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.

Author Correction: Rapid gate-based spin read-out in silicon using an on-chip resonator.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Rapid gate-based spin read-out in silicon using an on-chip resonator.

Silicon spin qubits are one of the leading platforms for quantum computation1,2. As with any qubit implementation, a crucial requirement is the ability to measure individual quantum states rapidly and with high fidelity. Since the signal from a single electron spin is minute, the different spin states are converted to different charge states3,4. Charge detection, so far, has mostly relied on external electrometers5-7, which hinders scaling to two-dimensional spin qubit arrays2,8,9. Alternatively, gate-based dispersive read-out based on off-chip lumped element resonators has been demonstrated10-13, but integration times of 0.2-2 ms were required to achieve single-shot read-out14-16. Here, we connect an on-chip superconducting resonant circuit to two of the gates that confine electrons in a double quantum dot. Measurement of the power transmitted through a feedline coupled to the resonator probes the charge susceptibility, distinguishing whether or not an electron can oscillate between the dots in response to the probe power. With this approach, we achieve a signal-to-noise ratio of about six within an integration time of only 1 µs. Using Pauli's exclusion principle for spin-to-charge conversion, we demonstrate single-shot read-out of a two-electron spin state with an average fidelity of >98% in 6 µs. This result may form the basis of frequency-multiplexed read-out in dense spin qubit systems without external electrometers, therefore simplifying the system architecture.