  Recent studies of silicon spin qubits at temperatures above 1 K are
encouraging demonstrations that the cooling requirements for solid-state
quantum computing can be considerably relaxed. However, qubit readout
mechanisms that rely on charge sensing with a single-island single-electron
transistor (SISET) quickly lose sensitivity due to thermal broadening of the
electron distribution in the reservoirs. Here we exploit the tunneling between
two quantised states in a double-island SET (DISET) to demonstrate a charge
sensor with an improvement in signal-to-noise by an order of magnitude compared
to a standard SISET, and a single-shot charge readout fidelity above 99 % up to
8 K at a bandwidth > 100 kHz. These improvements are consistent with our
theoretical modelling of the temperature-dependent current transport for both
types of SETs. With minor additional hardware overheads, these sensors can be
integrated into existing qubit architectures for high fidelity charge readout
at few-kelvin temperatures.
