Mid-circuit Measurement and Sensor-free Feedforward Control in Silicon-MOS Quantum Dots.

The ability to perform mid-circuit measurements is a critical requirement for quantum error correcting protocols, however these pose several unique experimental hurdles. The performance of the mid-circuit measurements are impacted by not only the quality of constituent gate operations, but also the SNR and speed of the ancilla readout sensor, as well as the coherence times of data qubits. As a result, engineering high fidelity mid-circuit measurements is a challenging task, especially in cases where nominal readout times are comparable to the T2 lifetimes. Here, we present and characterise methods of performing mid-circuit measurements in a spin qubit architecture. We extend this work by performing a real-time feedforward control based on the outcome of the mid-circuit measurement, and additionally demonstrate a novel method for sensor-free feedforward phase operations. In this work, a four-qubit Silicon-MOS array is used, which has an RF-SET at each end of the device. This allows for two independent parity PSB measurements per shot, giving separate ancilla and data qubit readout needed for mid-circuit measurements. We study the error mechanisms impacting the mid-circuit measurement through the use of gate-set-tomography to verify that data-qubit decoherence during the ancilla qubit measurement is the dominant error source.