Ast, Christian R.Jaeck, BertholdSenkpiel, JacobEltschka, MatthiasEtzkorn, MarkusAnkerhold, JoachimKern, Klaus2016-11-212016-11-212016-11-21201610.1038/ncomms13009https://infoscience.epfl.ch/handle/20.500.14299/131405WOS:000385555600001The tunnelling current in scanning tunnelling spectroscopy (STS) is typically and often implicitly modelled by a continuous and homogeneous charge flow. If the charging energy of a single-charge quantum sufficiently exceeds the thermal energy, however, the granularity of the current becomes non-negligible. In this quantum limit, the capacitance of the tunnel junction mediates an interaction of the tunnelling electrons with the surrounding electromagnetic environment and becomes a source of noise itself, which cannot be neglected in STS. Using a scanning tunnelling microscope operating at 15 mK, we show that we operate in this quantum limit, which determines the ultimate energy resolution in STS. The P(E)-theory describes the probability for a tunnelling electron to exchange energy with the environment and can be regarded as the energy resolution function. We experimentally demonstrate this effect with a superconducting aluminium tip and a superconducting aluminium sample, where it is most pronounced.text::journal::journal article::research article