  We present two different techniques for achieving low resistance ($<$20 n$\rm
\Omega$) contacts between copper and aluminium at cryogenic temperatures. The
best method is based on gold plating of the surfaces in an e-beam evaporator
immediately after Ar plasma etching in the same apparatus, yielding resistances
as low as 3 n$\rm \Omega$ that are stable over time. The second approach
involves inserting indium in the Al/Cu joint. For both methods, we believe key
elements are surface polishing, total removal of the aluminum oxide surface
layer, and temporary application of large (typ. 11 kN) compression forces. We
believe the values for gold plated contacts are the lowest ever reported for a
Cu/Al joint of a few $\rm cm^{2}$. This technology could simplify the
construction of thermal links for advanced cryogenics applications, in
particular that of extremely low resistance heat switches for nuclear
demagnetization refrigerators.
