  We present an experimental realization of a Coulomb blockade refrigerator
(CBR) based on a single - electron transistor (SET). In the present structure,
the SET island is interrupted by a superconducting inclusion to permit charge
transport while preventing heat flow. At certain values of the bias and gate
voltages, the current through the SET cools one of the junctions. The
measurements follow theoretical model down to about 80 mK, which was the base
temperature of the current measurements. The observed cooling increases rapidly
with decreasing temperature in agreement with the theory, reaching about 15 mK
drop at the base temperature. CBR appears as a promising electronic cooler at
temperatures well below 100 mK.
