Early experiments In 1904, Dutch scientist
Heike Kamerlingh Onnes created a special lab in
Leiden in the Netherlands with the aim of producing
liquid helium. In 1908, he managed to lower the temperature to less than , which is four degrees above
absolute zero. Only in this exceptionally cold state will helium liquefy; the boiling point of helium being . Kamerlingh Onnes received a
Nobel Prize in Physics for his achievement. Onnes' method relied upon depressurising the subject gases, causing them to cool by
adiabatic cooling. This follows from the first law of thermodynamics; \Delta U = \Delta Q - \Delta W where
U =
internal energy,
Q =
heat added to the system,
W =
work done by the system. Consider a gas in a box of set volume. If the pressure in the box is higher than atmospheric pressure, then upon opening the gas will do work on the surrounding atmosphere to expand. As this expansion is
adiabatic and the gas has done work \Delta Q = 0 \Delta W > 0 \Rightarrow \Delta U Now as the internal energy has decreased, so has the temperature.
Modern experiments As of November 2000,
nuclear spin temperatures below 100 pK were reported for an experiment at the
Helsinki University of Technology Low Temperature Lab. However, this was the temperature of one particular type of motion—a quantum property called nuclear spin—not the overall average thermodynamic temperature for all possible degrees of freedom. At such low temperatures, the concept of "
temperature" becomes multifaceted since molecular motion cannot be assumed to average out across degrees of freedom. The corresponding peak emission will be in radio waves, rather than in the familiar infrared, so it is very inefficiently absorbed by neighboring atoms, making it difficult to reach thermal equilibrium. The Low Temperature Laboratory recorded a record low temperature of 100
pK, or 1.0 × 10−10 K in 1999. The current apparatus for achieving low temperatures has two stages. The first uses a
helium dilution refrigerator to get to temperatures of millikelvins, then the next stage uses
adiabatic nuclear demagnetisation to reach picokelvins. Extremely low temperatures are useful for observation of
quantum mechanical phases of matter such as
superfluids and
Bose–Einstein condensates, which would be disrupted by thermal motions. == See also ==