Graphene is a formidable test bed to verify fundamental theory of physics. Hydrodynamics, relativity and superlattice physics can all be explored by observing the behavior of Dirac electrons in graphene, either with electronic transport or various local probes. In this work, we use a carbon nanotube (CNT) as a nanoscale, one-dimensional local probe placed on top of boron-nitride encapsulated graphene. The operation of this hybrid device is based on electrostatic coupling and Coulomb drag , a frictional coupling between electrons in closely spaced but electrically isolated conductors. This phenomenon, caused by electron-electron interactions, can be observed by flowing a current in graphene (resp. CNT) while measuring the voltage drop in the CNT (resp. graphene). The specific geometry and electronic properties of the nanotube allow to reveal in a non-invasive way intrinsic properties of the graphene channel including Fermi velocity, quantum capacitance, lifetime of individual Landau levels, etc... The microscopic effects of electronic interactions in this system are elucidated through three experiments:
(i) Under strong magnetic field, we observe the transition between the compressible and incompressible states in the quantum Hall regime by monitoring the drag signal, sensitive to the local current density in the graphene channel.
(ii) We measure the penetration of electric field through graphene by probing the energy of a single electronic level of the nanotube, providing us a novel insight on the internal interaction strength between charge carriers of graphene.
(iii) Using the nanotube as a gate, we create a local perturbation in the electrostatic landscape of graphene, forming a nanometer-sized interferometer for Dirac fermions. The interference patterns we observe unveil properties, otherwise elusive, of interacting edge states.
This unique carbon nanotube-based probe offers powerful capabilities to study local properties and effect of electronic interactions in graphene and more generally two-dimensional conductors.
 R. V. Gorbachev, A. K. Geim, M. I. Katsnelson, K. S. Novoselov, T. Tudorovskiy, I. V. Grigorieva, A. H. MacDonald, S. V. Morozov, K. Watanabe, T. Taniguchi and L. A. Ponomarenko, Strong Coulomb drag and broken symmetry in double-layer graphene, Nat. Phys. 8, 896 (2012)