New quantum cooling technology opens the way to ultra-cold temperatures

Everything stands still, nothing moves: it seems as if time has stood still. This is roughly how you could imagine a world at -273.15 degrees Celsius - a temperature that, according to thermodynamics, can never be reached. Nevertheless, physicists are competing to get as close as possible to absolute zero, as the quantum mechanical nature of particle systems is clearly evident in these ultra-cold regions. The current record is 38 picokelvins, i.e. just 38 trillionths of a degree above zero. To achieve this, experts have previously used lasers and magnetic fields to slow down atoms and thus cool them down. However, researchers led by Ju Li from the Massachusetts Institute of Technology in Cambridge have now developed a completely new cooling method that could cool particles even further in the future.

As they report in a paper that will soon be published in the journal "Physical Review Letters", an unusual quantum mechanical property could be used to cool particles faster and more strongly than before. Li and his team have investigated so-called non-Hermitian systems, i.e. quantum systems that are not closed and therefore exchange energy with their environment. As the energy in them is not conserved, surprising phenomena occur: Among other things, strange, uneven distributions of particles arise.

In the non-Hermitian skin effect, for example, the probability of finding a particle at one end of a rod is extremely high, while the probability at the other end is almost zero. Experts usually pay most attention to the high peak, Li and his colleagues write in their paper, but the potential of the other "suppressed" end has received little attention so far. What is particularly interesting is that this skin effect can be transferred not only to real particles such as electrons, but also to excitations that merely behave like particles, such as heat-mediated vibrations.

The MIT team therefore calculated what happens when the non-Hermitian skin effect is applied to such oscillations: According to this, the heat of the system would accumulate at one end of the rod, while the other end would cool down rapidly. By applying this method to systems that are already cooled - for example by lasers - the researchers hope to achieve even lower temperatures close to zero in the future.

The work of Li's team is only theoretical so far. However, physicist Uroš Delić from the University of Vienna, who was involved in the employees' work, is already experimenting with non-Hermitian systems. As he explains in an article in "New Scientist", he is currently trying to implement the new cooling method in the laboratory.

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