  The spin 1/2 entropy of electrons trapped in a quantum dot has previously
been measured with great accuracy, but the protocol used for that measurement
is valid only within a restrictive set of conditions. Here, we demonstrate a
novel entropy measurement protocol that is universal for arbitrary mesoscopic
circuits and apply this new approach to measure the entropy of a quantum dot
hybridized with a reservoir, where Kondo correlations dominate spin physics.
The experimental results match closely to numerical renormalization group (NRG)
calculations for small and intermediate coupling. For the largest couplings
investigated in this work, NRG predicts a suppression of spin entropy at the
charge transition due to the formation of a Kondo singlet, but that suppression
is not observed in the experiment.
