| Summary: | Temperatures below 1 mK on-chip hold great potential for quantum physics but present a great challenge due to the lack of suitable thermometry and the detrimental pulse-tube vibrations of cryogen-free refrigerators. Here, we solve the pulse-tube problem using a rigidly wired metallic sample holder, which provides a microkelvin environment with low heat leaks despite the vibrations. Further, we demonstrate an improved type of temperature sensor, the gate Coulomb blockade thermometer (gCBT), employing a gate metallization covering the entire device. This immunizes against nanofabrication imperfections and uncontrollable offset charges, and extends the range to lower temperatures compared to a junction CBT with the same island capacitance, here down to ≈160 μK for a 10% accuracy. Using on- and off-chip cooling, we demonstrate electronic temperatures as low as 224 ± 7 μK, remaining below 300 μK for 27 hours, thus providing time for experiments. Finally, we give an outlook for cooling below 50 μK for a future generation of microkelvin transport experiments.
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