Very tiny, very cool

Annoyingly tiny fridges may not be restricted to hotels or dorm rooms much longer. A new study proposes a way to construct the smallest refrigerator yet, based on just a few particles and capable of cooling to near absolute zero. 

The study, which will appear in an upcoming issue of Physical Review Letters, pushes the limits of how small a cooling device can get and still remain functional.

When thermodynamics was first invented, it was applied to big, steam engine sorts of things, says physicist Tony Short of the University of Cambridge in England, who was not involved in the study. The fact that you can bring the ideas all the way down to individual quantum systems of tiny dimensions and the same basic ideas still work is quite nice.

Study coauthors Noah Linden, Sandu Popescu and Paul Skrzypczyk, all of the University of Bristol in England, propose a cooling scheme that relies on three linked qubits particles that can exist in one of two states. Two of these qubits make up the refrigerator and would be held in two different heat baths: one very hot and one near room temperature. The third qubit is the object to be cooled. Because these qubits share a quantum connection, they can influence one another. So, as the hottest qubit absorbs energy from its toasty bath, it triggers the tepid qubit to siphon energy off the third qubit, cooling it. This extra energy dissipates off the second qubit in the same way the coil at the back of a refrigerator in the kitchen emits heat. 

In their calculations, the physicists found that as the bath of the hottest qubit got hotter, the cooling ability of the fridge got better. And in principle, as long as the heat bath stays hot, the system can run forever. "Once you set it up, it just sits there, gently cooling away," Linden says.

Other small systems have been created, but this is the first that doesnt rely on external mechanisms, such as sophisticated lasers. The whole guts of the fridge, its all accounted for and not hidden in some macroscopic object which is really doing the work, Linden says.

Linden and his team also propose an even smaller system, in which a single particle with three distinct levels of quantum information, called a qutrit, acts as the refrigerator. We believe this is the smallest possible thing you can call a fridge, Linden says.

Physicist Nicolas Gisin of the University of Geneva says the new study is extremely elegant. It opens a totally new avenue for interesting questions, combining thermodynamics and quantum information science in a very original way.

The researchers plan to collaborate with other groups to settle on an exact blueprint for the minifridge and to build it. In the future, a tiny fridge might be used to slow or speed up reactions between proteins in cells by cooling precise parts, or to keep delicate components in quantum computers frigid.

A particularly fascinating question is whether such fridges might already exist in nature, Gisin says. For instance, a sun-drenched plant could have molecules with one end sitting directly in a natural thermal bath, allowing a tiny refrigerator to cool the other end.

Linden and his colleagues also find that idea exciting, but hes careful to point out that so far, its just an idea. We dont want to claim that we know of a place where this happens, he says. But it would be great if someone came up with a molecule and said, Look at this. Doesnt it have the characteristics you need? Wed be really, really happy if that happened.