  Rectification of thermal fluctuations in mesoscopic conductors is the key
idea of today's attempts to build nanoscale thermoelectric energy harvesters in
order to convert heat into a useful electric power. So far, most concepts make
use of the Seebeck effect in a two-terminal geometry where heat and charge are
both carried by the same particles. Here, we experimentally demonstrate the
working principle of a new kind of energy harvester, proposed recently using
two capacitively coupled quantum dots. We show that due to its novel
three-terminal design which spatially separates the heat reservoir from the
conductor circuit, the directions of charge and heat flow become decoupled in
our device. This enables us to manipulate the direction of the generated charge
current by means of external gate voltages while leaving the direction of heat
flow unaffected. Our results pave the way for a new generation of
multi-terminal, highly efficient nanoscale heat engines.
