  Superconducting heat switches with extremely low normal state resistances are
needed for constructing continuous nuclear demagnetization refrigerators with
high cooling power. Aluminum is a suitable superconductor for the heat switch
because of its high Debye temperature and its commercial availability in high
purity. We have constructed a high quality Al heat switch whose design is
significantly different than that of previous heat switches. In order to join
the Al to Cu with low contact resistance, we plasma etched the Al to remove its
oxide layer then immediately deposited Au without breaking the vacuum of the
e-beam evaporator. In the normal state of the heat switch, we measured a
thermal conductance of $8 T$ W/K$^2$ which is equivalent to an electrical
resistance of 3 n$\Omega$ according to the Wiedemann-Franz law. In the
superconducting state we measured a thermal conductance that is $2\times10^6$
times lower than that of the normal state at 50 mK.
