  Illumination of atoms by resonant lasers can pump electrons into a coherent
superposition of hyperfine levels which can no longer absorb the light. Such
superposition is known as dark state, because fluorescent light emission is
then suppressed. Here we report an all-electric analogue of this destructive
interference effect in a carbon nanotube quantum dot. The dark states are a
coherent superposition of valley (angular momentum) states which are decoupled
from either the drain or the source leads. Their emergence is visible in
asymmetric current-voltage characteristics, with missing current steps and
current suppression which depend on the polarity of the applied source-drain
bias. Our results demonstrate for the first time coherent-population trapping
by all-electric means in an artificial atom.
