  Microorganisms such as bacteria are active matters which consume chemical
energy and generate their unique run-and-tumble motion. A swarm of such
microorganisms provide a nonequilibrium active environment whose noise
characteristics are different from those of thermal equilibrium reservoirs. One
important difference is a finite persistence time, which is considerably large
compared to that of the equilibrium noise, that is, the active noise is
colored. Here, we study a mesoscopic energy-harvesting device (engine) with
active reservoirs harnessing this noise nature. For a simple linear model, we
analytically show that the engine efficiency can surpass the conventional
Carnot bound, thus the power-efficiency tradeoff constraint is released, and
the efficiency at the maximum power can overcome the Curzon-Ahlborn efficiency.
We find that the supremacy of the active engine critically depends on the
time-scale symmetry of two active reservoirs.
