Have you ever wondered where the coldest place in the universe is?
Well, you may be surprised to find out that it’s actually in our Milky Way galaxy, more precisely here: https://www.google.dk/maps/@-11.6368098,-83.7133284,23035101m/data=!3m1!1e3?hl=en.
Yes, it is true. The Earth contains the coldest place in the known cosmos. In fact, over the last century, scientists have been able to reach temperatures fractions of Kelvin (K) from absolute zero, the coldest temperature physically possible, equal to 0 K or -273.15 °C. This result is astonishing if you consider that the average temperature of the universe is around 2.7 K. The coldest natural temperature is found in the Boomerang Nebula of the constellation Centaurus, and has been estimated to be around 1 K.
How is it possible to reach such low temperatures artificially?
The journey for the development of cryogenic temperature started almost 150 years ago , when Carl von Linde patented equipment to liquefy air, which has a boiling point around 80 K. The process was rather simple, making use of pure thermodynamic laws and exploiting the Joule–Thomson expansion process. By adiabatically (i.e. by avoiding heat exchange with the external environment) lowering the pressure of the gas through its expansion, the increase in intermolecular distance caused a lowering of the kinetic energy of the atoms therefore the temperature of the gas. The cooled gas was subsequently reintegrated in the cycle, undergoing the process until all the gas reached the liquid state. Liquefaction of air was subsequently followed by nitrogen and oxygen.
About 30 years later, Dewar managed to liquefy hydrogen, exploiting a similar process to the one previously described. However, this turned out to be more complex problem, since the inversion temperature of hydrogen (and helium) is below 50 K at 1 atmosphere, whereas for air, nitrogen and oxygen is well above 500 K. The inversion temperature is the temperature at which the Joule-Thomson process start to be effective and lead to gas cooling. Therefore, the Linde cycle needs to be started already at very low temperature. Dewar succeeded in his purpose using liquid nitrogen (77 K) to precool the hydrogen, before starting the liquefaction process. However, in order to do that, he had to invent a new instrument which allowed him to thermally decouple the hydrogen from room temperature, the Dewar flask: a vacuum-insulated, silver-plated glass container. Sounds familiar? Yes, it is exactly what we use every day to keep our coffee warm, a Thermos!
In 1908 Kamerlingh Onnes, the “father of low temperature physics”, managed to successfully cool down 4He to its boiling point, reaching the record temperature of 4.2 K and opening the few Kelvin temperature range to science. He was awarded with the Nobel prize in physics in 1913 “for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium”. Eventually, Onnes realized that by increasing the evaporation rate of the liquid, it was possible to reduce its temperature even further. In 1920 he successfully cooled down a bath of 4He to 0.8 K, by pumping away the helium vapor from the chamber. However, since the vapor pressure decreases exponentially with the temperature, this method starts to be inefficient very soon.
At this point in history, the technique of gas liquefaction reached its bottleneck, and fundamentally different technologies were required. One solution was offered from adiabatic electronic demagnetization techniques, which was proposed in the late ‘20s, and developed further during mid 20th century, culminating in the development of adiabatic nuclear magnetization technique which allowed to reach 100 microkelvin at the end of the ‘60s. The main idea behind this technique is to adiabatically disorder magnetic domains in materials by removing the magnetizing field. The process of inducing disorder requires energy, which is absorbed from the external environment. However, as the name suggests, this technique requires working with magnetic materials which are uncommon, and at the same time it doesn’t allow to maintain such a low temperature over long periods.
Another approach started to be developed during the second half of the last century, and it dealt with the dilution of 3He (a stable isotope of helium) in 4He, which allowed to implement continuous refrigeration methods down to the millikelvin regime. The first prototype was experimentally realized in 1964 in the Kamerlingh Onnes Laboratorium at Leiden University, in the Netherlands. When the mixture of 3He and 4He is cooled to sufficiently low temperature, like that in a low-pressure liquid 4He bath, the mixture will tend to separate into two liquid phases, one which is pure 3He and a the other which is mostly 4He with a small amount of 3He, the proportion of which is fixed at equilibrium. By evaporating 3He from its diluted phase, the ratio of the mixture is altered, and it tries to get back to equilibrium by moving 3He from the pure to the dilute phase. This is a endoenergetic process, which absorbs energy from the external environment as heat, leading to the cooling of the system. The equipment that exploits this process is known as dilution refrigerator, which is the main apparatus for low temperature physics experiments, such as the computation with spin qubits, which Stephan and Yanick discussed in a previous blog post.
Towards the end of the 20th century, the novel method of atomic laser cooling proved to be successful in reaching even lower temperatures. This technique reduces the average velocity of a few atoms, by using fast energy pulses at precise frequencies, which can be only provided by lasers. Smaller average velocities mean smaller kinetic energy, corresponding to a reduced temperature of the atoms (actually this technique is much more complicated, so for more information I suggest this reading ). Applying this procedure, allowed researchers in 2015 to lower the temperature of few rubidium atoms down 50 picokelvin, that is 50 trillionths of a degree above absolute zero! 
Eventually, after this long story, I am pretty sure you are wondering, why are scientist interested in reaching low temperature?
One important phenomenon that only happens at low temperature is superconductivity. In 1911 Onnes observed that the resistance of mercury dropped to zero as the temperature reached 4.2 K, the first superconducting transition, and the first superconductor. This discovery that revolutionized the field of condensed matter physics as well as leading to powerful technologies such as maglev trains and magnetic resonance imaging (MRI). However, even 100 years after the discovery of the first superconductor, scientists are still discovering new materials that turn superconducting, and no general theory able to fully describe this phenomenon exists yet.
By Fabio Ansaloni, PhD student at the Centre for quantum devices group at the University of Copenhagen, Copenhagen, Denmark.
 F. Pobell, Matter and Methods at Low Temperatures, springer (2007)
 M.A. Kasevich et al., Phys. Rev. Lett. 114, 143004 (2015)
 W.D Philips, Laser Cooling and Trapping of Neutral Atoms, Nobel Lecture (1998)