When it comes to refrigerators, size matters. Who hasn‘t at least once in their life wished for a bigger fridge’ However, who can say they‘ve wished for the extreme opposite ’ the smallest conceivable one? But this is exactly what experts in quantum mechanics from the University of Bristol have done.It’s not a question of engineering, of how small can one build a refrigerator, but about the fundamental limitations that Nature may impose on the size of refrigerators. Is there a minimum size below which no refrigerator can work?The Bristol team - Professor Noah Linden , Professor Sandu Popescu and Paul Skrzypczyk - found there is no minimum size, and, using quantum mechanics, designed what is arguably the smallest possible refrigerator. It works extremely well too: it can cool as close as you like towards absolute zero.One model refrigerator is made from just three two-level quantum systems - the simplest possible physical systems, known as qubits. Two of the qubits make up the refrigerator - one in a hot heat bath, the other in a heat bath at ‘room temperature’; the third is the object to be cooled. In addition to their interaction with their local heat baths, the qubits interact with each other. As the hot qubit absorbs energy from its bath, it causes the tepid qubit to siphon energy from the third qubit, thus cooling it.According to Nicolas Gisin, a theoretical and experimental physicist at the University of Geneva, who was not involved with the work, this result is ’extremely elegant. It opens a totally new avenue for interesting questions, combining thermodynamics and quantum information science in a very original way.’And an article about the work in the online news section of Science quotes physicist David Wineland from the U.S. National Institute of Standards and Technology in Boulder, Colorado, saying he believes such schemes can actually be implemented using ‘trapped ions’.So while the main motivation behind this research was understanding fundamental limitations of Nature and not possible applications, as high-tech devices get smaller and smaller - nano-technology, quantum computers and so on - the smallest possible refrigerator may still find its way into your home.The findings are reported in a forthcoming issue of Physical Review Letters .