  Mechanical resonators based on low-dimensional materials provide a unique
platform for exploring a broad range of physical phenomena. The mechanical
vibrational states are indeed extremely sensitive to charges, spins, photons,
and adsorbed masses. However, the roadblock is often the readout of the
resonator, since the detection of the vibrational states becomes increasingly
difficult for smaller resonators. Here, we report an unprecedentedly sensitive
method to detect nanotube resonators with effective masses in the 10^-20 kg
range. We use the beam of an electron microscope to resolve the mechanical
fluctuations of a nanotube in real-time for the first time. We obtain full
access to the thermally-driven Brownian motion of the resonator, both in space
and time domains. Our results establish the viability of carbon nanotube
resonator technology at room temperature and pave the way towards the
observation of novel thermodynamics regimes and quantum effects in
nano-mechanics.
