The frontiers of quantum electronics have been linked to the discovery of new refrigeration methods since the discovery of superconductivity at a temperature around $4,$K, enabled by the liquefaction of helium. Since then, the advances in cryogenics led to discoveries such as the quantum Hall effect and new technologies like superconducting and semiconductor quantum bits. Presently, nanoelectronic devices typically reach electron temperatures around $10,$mK to $100,$mK by commercially available dilution refrigerators. However, cooling electrons via the encompassing lattice vibrations, or phonons, becomes inefficient at low temperatures. Further progress towards lower temperatures requires new cooling methods for electrons on the nanoscale, such as direct cooling with nuclear spins, which themselves can be brought to microkelvin temperatures by adiabatic demagnetization. Here, we introduce indium as a nuclear refrigerant for nanoelectronics and demonstrate that solely on-chip cooling of electrons is possible down to $3.2pm0.1,$mK, limited by the heat leak via the electrical connections of the device.
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