At the beginning of 2020, in the COLD lab (CryOgenic Laboratory for Detectors) the commissioning of a new cryostat with dilution refrigerator was successfully completed.
This apparatus is able to cool down to -273,14 °C, just ten thousandths of a degree above the absolute zero. It is the coldest temperature ever achieved at Frascati Laboratories.
The ability to reach such a low temperature is made possible by the peculiar behavior of a mixture of Helium-3 and Helium-4, two helium isotopes, in the liquid state. Pre-cooled at low temperature with other methods, the mixture separates spontaneously into two phases due to the different quantum behavior of the two isotopes: the lighter Helium-3 floats on Helium-4. By triggering the mixing (dilution effect) of Helium-3 into Helium-4 with a suitable pumping system, the liquid is cooled, allowing to reach just above absolute zero.
Achieving these temperatures is necessary to detect weak and rare signals such as those due to the presence of dark matter in our galaxy in experiments like QUAX. The COLD lab research group is in fact developing superconducting detectors capable to detect single microwave photons that must operate in absolute “silence”, which can be obtained only a few thousandths of a degree above absolute zero.