High-fidelity dispersive spin readout in a scalable unit cell of silicon quantum dots*

Planar MOS multi-gate technology is one of the leading approaches to silicon-based quantum computing. For readout of spin qubits, dispersive sensing offers the potential of scalable unit cells by avoiding the need for multiple charge reservoirs. So far, demonstrations of planar MOS quantum dots have been restricted to architectures where sensors are co-linear with the qubit array, limiting scalability. Achieving readout fidelity at the level of control operations has also remained challenging. In this work, we address both limitations: we demonstrate single-shot spin readout with fidelity above 99.9% measured in 400 us in a planar MOS quantum dot array fabricated using a 300mm wafer process. We use a single electron box (SEB) to measure the two-electron spin state of a double quantum dot using Pauli spin blockade. The sensor and qubit dots are placed in parallel channels of a bilinear array of quantum dots, forming a compact unit cell. The high fidelity is achieved thanks to the tunability of the structure that allows (i) optimization of the tunnel rate of the SEB for enhanced signal and (ii) tuning of the coupling between the double quantum dots using a J-gate, leading to an enhancement of the singlet-triplet relaxation time from 4 us to 0.5 s. Overall, this work demonstrates sensing in a compact unit cell with state-of-the art fidelity, providing a path to scalable high-connectivity bilinear qubit arrays.