Measurement-free error correction with coherent ancillas

We show that coherent error correction with redundant syndrome extraction is feasible even when realistic error models for the ancilla qubits are used. This is done by means of a fault-tolerant circuit that has a favorable error threshold for a conservative error model. It is optimal when Toffoli gates are available, as they are in many implementations. The possibility of coherent error correction is important for those quantum computing platforms in which measurements in the course of the computation are undesirable.Comment: 3 figure

Spatial noise correlations in a Si/SiGe two-qubit device from Bell state coherences

We study spatial noise correlations in a Si/SiGe two-qubit device with integrated micromagnets. Our method relies on the concept of decoherence-free subspaces, whereby we measure the coherence time for two different Bell states, designed to be sensitive only to either correlated or anticorrelated noise, respectively. From these measurements we find weak correlations in low-frequency noise acting on the two qubits, while no correlations could be detected in high-frequency noise. We expect nuclear spin noise to have an uncorrelated nature. A theoretical model and numerical simulations give further insight into the additive effect of multiple independent (anti)correlated noise sources with an asymmetric effect on the two qubits as can result from charge noise. Such a scenario in combination with nuclear spins is plausible given the data and the known decoherence mechanisms. This work is highly relevant for the design of optimized quantum error correction codes for spin qubits in quantum dot arrays, as well as for optimizing the design of future quantum dot arrays.QCD/Vandersypen LabQuTechQN/Vandersypen La