A new type of cooling relies on an exotic quantum mechanical property rather than putting objects into cold environments like refrigerators – and it might one day help us chill things to temperatures lower than any we have reached before.
How cold or warm an object is depends on how its constituent atoms are jiggling, with more and faster jiggling corresponding to more heat and a higher temperature. As such, making objects colder involves finding ways to slow that atomic jiggling – which is why researchers don’t necessarily need to use a kitchen-style refrigerator to cool things down. For instance, for decades physicists have been cooling atoms close to the lowest possible temperature – absolute zero – by repeatedly hitting them with laser light to make them stop moving.
at the Massachusetts Institute of Technology and his colleagues have now realised there is a third, completely new cooling method that doesn’t involve either a refrigerator or a laser. The new technique is called non-Hermitian cooling.
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Non-Hermitian systems, like arrays of special resonators or magnets, are susceptible to the effects of their environment and can gain or lose energy through interacting with it.
In studying such systems, the researchers focused on the concept of the “quantum state”, which is a handy mathematical description of what properties a particle can have, how it can move and where it is most likely to be within a system. If the system is a slab of metal, for example, quantum states of electrons are mathematically represented as waves that spread throughout it, indicating that electrons are good at flowing everywhere within the slab.
For a non-Hermitian system, the theoretical picture can be very different, with the quantum states represented as a tall, narrow wave at one end of the system, with a very shallow valley at the other. The shallowness of the valley signifies a suppression effect. This means that a particle – or a heat-carrying jiggle, which behaves a like a particle – is unlikely to find itself at that end of the system.
The researchers mathematically proved that they could capitalise on this suppression effect. They say it is possible to tune the quantum interactions between the two nearly identical components of a simple system so that one corresponds to the “tall wave” end and the other corresponds to the “shallow valley” end. This “shallow valley” component then becomes cold, because any heat-carrying atomic jiggle is very unlikely to ever go there.
Based on these calculations, the team hopes to reach a “better ultimate limit of cooling” – meaning even lower temperatures than physicists have been able to reach before – says at the University of Vienna in Austria and a co-author on the study.
Xu says their calculations assume that the objects being cooled are already fairly cold, perhaps after having been laser cooled. The idea is that the new non-Hermitian method could then cool them even further, which could allow for better quantum devices, or new kinds of physics research projects exploring quantum effects specific to extremely low temperatures.
Delić got involved because he was working on an experiment that turned out to be non-Hermitian. It involves tiny particles that are levitated by light and interact by scattering that light towards each other – Delić thinks those interactions could be tuned exactly right to achieve the non-Hermitian effect and then this new kind of cooling. “The experiment is actually up and running in the lab at the moment,” he says.
at Stockholm University in Sweden says the non-Hermitian effect has already been used in applications like sensors, so leveraging it for another practical use – cooling – is both plausible and exciting. “The impact of this will depend on how powerful implementations will turn out to be. What seems clear already is that the potential is, in principle, huge,” he says.
Journal reference:
Physical Review Letters,
