  We demonstrate a microwave power-to-electrical energy conversion in a
resonator-coupled double quantum dot. The system, operated as a photodiode,
converts individual microwave photons to electrons tunneling through the double
dot, resulting in an electrical current flowing against the applied voltage
bias at input powers down to 1 femto-watt. The device attains a maximum power
harvesting efficiency of 2%, with the photon-to-electron conversion efficiency
reaching 12% in the single photon absorption regime. We find that the power
conversion depends on thermal effects showing that thermodynamics plays a
crucial role in the single photon energy conversion.
