  Proposals for large-scale semiconductor spin-based quantum computers require
high-fidelity single-shot qubit readout to perform error correction and read
out qubit registers at the end of a computation. However, as devices scale to
larger qubit numbers integrating readout sensors into densely packed qubit
chips is a critical challenge. Two promising approaches are minimising the
footprint of the sensors, and extending the range of each sensor to read more
qubits. Here we show high-fidelity single-shot electron spin readout using a
nanoscale single-lead quantum dot (SLQD) sensor that is both compact and
capable of reading multiple qubits. Our gate-based SLQD sensor is deployed in
an all-epitaxial silicon donor spin qubit device, and we demonstrate
single-shot readout of three $^{31}$P donor quantum dot electron spins with a
maximum fidelity of 95%. Importantly in our device the quantum dot confinement
potentials are provided inherently by the donors, removing the need for
additional metallic confinement gates and resulting in strong capacitive
interactions between sensor and donor quantum dots. We observe a $1/d^{1.4}$
scaling of the capacitive coupling between sensor and $^{31}$P dots (where $d$
is the sensor-dot distance), compared to $1/d^{2.5-3.0}$ in gate-defined
quantum dot devices. Due to the small qubit size and strong capacitive
interactions in all-epitaxial donor devices, we estimate a single sensor can
achieve single-shot readout of approximately 15 qubits in a linear array,
compared to 3-4 qubits for a similar sensor in a gate-defined quantum dot
device. Our results highlight the potential for spin qubit devices with
significantly reduced sensor densities.
