Dark states and transport spectroscopy in high purity carbon nanotubes

Aalto Quantum Physics Seminar (Nanotalo). Speaker: Prof. Christoph Strunk (Regensburg University, Germany).

Illumination of atoms having a lambda-shaped level structure with two laser frequencies can trap electrons into a coherent superposition of two nearly degenerate orbital states, an effect called coherent population trapping in quantum optics. These states can be strongly decoupled from the dynamics and suppress fluorescent light emission, and are called "dark states". We measure an all-electric analogue of this effect in the transport through a carbon nanotube quantum dot. It is visible through distinct features in the I-V characteristics, which is robust over several of shells. Missing resonant lines and negative differential conductance result from the destructive interference of two transmission channels. 
In the second part of the talk quantum transport through CNT-quantum dots in a high magnetic field is discussed. We find that in finite length CNTs, uniquely combining bipartite hexagonal lattice and cylindrical topology, a magnetic field along the nanotube axis can tune the wave function of a single trapped electron all the way from a “half-wave resonator” behavior, i.e., a traditional “quantum box”, to a “quarterwave resonator”, where an antinode at one end occurs. This explains a peculiar dependence of the conductance on the magnetic field.