Helium was first liquefied in 1908 by Heike Kamerlingh-Onnes. It has a boiling point of 4.2 kelvins (-268.95°Celsius). It can also be cooled to a lower temperature by pumping. A powerful pump is used to remove the vapour and the liquid evaporates off. Therefore the helium is reduced down to a temperature of around 1.2 K
For a higher price however, helium three can be used. This has two protons, but unlike helium four has 1 neutron rather than two. This enables the temperature to be reduced down to around 0.3 K.

Through the further race to get down as close to absolute zero, (0 K), developments of a dilution refrigerator are in use, enabling temperatures of as low as 0.002 K (2 millikelvin)


and now an update!

I have been reliably informed (I think) by thornton that the lowest temperature on record is ...and I quote...

'with the advent of laser trapping and cooling, the lowest temperature on record is now in the nanokelvin (10^-9 K) range'

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