One of the prominent challenges of this project is to fabricate the nanomechanical resonator, that is a carbon nanotube cantilever, as a large part of the expected quantum sensitivity performances arises from it. Additionally, despite many attempts, the vibration of carbon nanotube resonators could not be optomechanically detected, which essentially arises from their very tiny size, in the nanometre range (that is 10 millionth of the size of a human hair), making it very difficult to couple to a laser.
For this reason, the project has readily started with developing entirely new, cutting edge nanofabrication techniques, whose purpose is to decorate the carbon nanotube with an optical aggregate which appears much "brighter" in response to a laser beam. The principle of this method relies on the so-called Focused Electron Beam Induced Deposition (FEBID), which consists in aggregating atomic or molecular precursors at the focus of an electron beam.Directing the electron beam at the tip of the carbon nanotube therefore yield to the sought result, in principle. The carbon nanotube however is very tiny, which requires a level of control on the process which had never been reached priori to this project: Indeed, the mass of the optical scatterer shall be small compared to that of the carbon nanotube, which is on the order of the attogram, that is about a hundred thousandth of a billionth of the mass of a single fly. To address this challenge, we have developed a method enabling to detect the mass of the carbon nanotube using the same electron beam as that utilized to grow the optical aggregate. This method is so precise, that we are able to control the deposition process with an accuracy of just a few tens of atoms. We also started tests intending to grow various functions on our nanotubes, including magnetic and dielectric particles.
Because they are so sensitive, the vibrations of this kind of resonators may be strongly affected by the measurement, either using an electron or a laser beam. In fact, the sensitivity is such that it was long expected that their oscillation behaviour would not follow the usual laws of thermodynamics. We have performed a series of experiments in order to address these outstanding questions, and have be able to provide the first experimental evidence of the role of momentum and thermal energy exchange between the measurement probe (electron or laser beam) and the the carbon nanotube. We also confirmed that the very large thermal vibrations of carbon nanotubes set them in a state that strongly differ from larger scale solid state resonators.
