  We demonstrate that a quantum absorption refrigerator can be realized from
the smallest quantum system, a qubit, by coupling it in a non-additive (strong)
manner to three heat baths. This function is un-attainable for the qubit model
under the weak system-bath coupling limit, when the dissipation is additive. In
an optimal design, the reservoirs are engineered and characterized by a single
frequency component. We obtain then closed expressions for the cooling window
and refrigeration efficiency, as well as bounds for the maximal cooling
efficiency and the efficiency at maximal power. Our results agree with
macroscopic designs and with three-level models for quantum absorption
refrigerators, which are based on the weak system-bath coupling assumption.
Beyond the optimal limit, we show with analytic calculations and numerical
simulations that the cooling efficiency varies in a non-universal manner with
model parameters. Our work demonstrates that strongly-coupled quantum machines
can exhibit function that is un-attainable under the weak system-bath coupling
assumption.
