Cascade-Based Fast, High-Fidelity and Scalable Spin Readout

Spin-qubits based on gate-defined semiconductor quantum dots are a promising platform for quantum computation and simulation. An important advantage of quantum dots is their small footprint. The dot pitch is about 100 nm, hence 100 million dots fit on 1 mm². A problem is that qubit readout with charge sensing based on capacitive coupling only enables to sense nearby quantum dots and placing charge sensors within the quantum dot array hosting the qubits is detrimental for connectivity. Here, we report on cascade-based fast, high-fidelity and scalable spin readout. The cascade consists of an initial charge transition, far away from the sensor, and subsequent charge transitions induced by Coulomb repulsion, with the final transition nearby the sensor. Combined with spin-to-charge conversion a cascade enables the readout of charge and spin occupation of quantum dots remote from the charge sensor. We demonstrate fast and high-fidelity spin readout by performing Pauli spin blockade with a cascade implemented in a quadruple dot with a sensing dot. The cascade-based readout is a promising alternative for readout of large quantum dot arrays compared to gate-dispersive readout or state transfer via either shuttling or logical operations.