The darkness in the night sky has its temperature of about 3 degrees Kelvin (-270 degrees Celsius) above the absolute zero temperature. Therefore, the temperature below 3 degrees Kelvin only exists as a proof of interectual activities in the Universe. Nowadays, low temperature physicists have developed temperatures even below millidegrees Kelvin (0.001 degrees Kelvin).
Helium gas is liquefied below 4.2 degrees Kelvin under atmospheric pressure. As it is cooled down further, liquid helium enters a new phase without viscosity, so-called superfluid phase. Two isotopes of helium are commonly known, namely, helium-3 and helium-4. While helium-4 shows the properties of superfluidity below 2.2 degrees Kelvin, helium-3 shows the similar properties only below 1 millidegrees Kelvin. This difference is one of the macroscopic manifestations of thier quantum natures. It is necessary to cool down the system to very low tempearatures in order to observe such unusual quantum properties.
In order to understand the nature in such an intermediate region between the Newtonian classical world and quantum one, we are carrying out studies in the tempearature region of microdegrees Kelvin in RIKEN.