  Current fluctuations related to the discreteness of charge passing through
small constrictions are termed shot noise. This unavoidable noise provides both
advantages - being a direct measurement of the transmitted particles' charge,
and disadvantages - a main noise source in nanoscale devices operating at low
temperature. While better understanding of shot noise is desired, the technical
difficulties in measuring it result in relatively few experimental works,
especially in single-atom structures. Here we describe a local shot-noise
measurement apparatus, and demonstrate successful noise measurements through
single-atom junctions. Our apparatus, based on a scanning tunneling microscope
operates at liquid helium temperatures. It includes a broadband commercial
amplifier mounted in close proximity to the tunnel junction, thus reducing both
thermal noise and the input capacitance that limit traditional noise
measurements. The full capabilities of the microscope are maintained in the
modified system and a quick transition between different measurement modes is
possible.
