Radio-frequency reflectometry of a quantum dot using an ultra-low-noise SQUID amplifier*

Fault-tolerant spin-based quantum computers will require fast and accurate qubit readout. This can be achieved using radio-frequency reflectometry given sufficient sensitivity to the change in quantum capacitance associated with the qubit states. Here, we demonstrate a 23-fold improvement in capacitance sensitivity by supplementing a cryogenic semiconductor amplifier with a SQUID preamplifier. The SQUID amplifier operates at a frequency near 200 MHz and achieves a noise temperature below 550 mK when integrated into a reflectometry circuit, which is within a factor 115 of the quantum limit. It enables a record sensitivity to capacitance of 0.07 aFHz^-0.5 and a sensitivity to oscillating charge of 5.9 x 10^-24CHz^-0.5. We use this circuit to measure the stability diagram of a gate-defined quantum dot, and show that the sensitivity should be sufficient for single-shot readout of a singlet-triplet qubit in GaAs without a charge sensor.