A refrigerator moves heat from cold to hot by doing work on the system. The classical version uses compression, expansion, and contact with thermal reservoirs in a cyclic protocol. The quantum version — demonstrated here in simulation — uses Bose-Einstein condensates as the working fluid, the piston, and the reservoir, with time-dependent potential barriers as the valves.
Simmons, Mayergoyz, and Davis (arXiv 2602.23074, February 2026) implement a three-condensate refrigeration cycle. Three spatially separated BECs are initialized at the same temperature. One serves as the system (the cold side), one as the piston (the mechanical intermediary), and one as the reservoir (the hot side). Potential barriers between them are raised and lowered in sequence, controlling when the condensates can exchange energy and particles.
The compression stroke increases the piston condensate's energy by tightening its confinement. The contact stroke opens the barrier between piston and reservoir, dumping the excess energy. The expansion stroke releases the piston, lowering its temperature below the system's. A second contact stroke opens the barrier between piston and system, extracting heat from the system into the now-cold piston. The cycle repeats.
After one complete cycle, the system condensate's temperature drops by approximately 20%. A second cycle achieves a cumulative 27% reduction. The cooling is real — verified by momentum-space thermometry, which measures the thermal occupation of high-momentum modes. Sound excitations and mass transfer between condensates complicate the dynamics but do not prevent net refrigeration.
The working fluid is quantum. The thermodynamic cycle is classical. The barriers — raised and lowered on microsecond timescales — are the only moving parts. A refrigerator built from nothing but atoms and the walls between them.