In this work, CNT quantum dots have been fabricated simply by depositing metallic contacts on top of an individual single-wall CNT. A whole CNT between the contacts behaves as a single quantum dot, or single electron transistor with a heavily doped substrate as a back gate. At liquid He temperature, the THz wave was irradiated to the CNT-QD and we have observed, for the first time in QDs, new side peaks that originate from a THz-photon absorption, which indicates a quantum response of the CNT-QD to the THz wave.

The fabrication of an antenna structure, which the present devices do not have, will enhance a coupling efficiency. The present finding may lead to a new type of the THz detector with a spectroscopic mean and an ultra-high sensitivity. Our next step is to modify the present device to coupled QDs, where the THz-photon absorption and emission occur between discrete quantum levels in each dot. Then, the quantum detection may be possible at higher temperature.