  We use time-resolved charge detection techniques to investigate
single-electron tunneling in semiconductor quantum dots. The ability to detect
individual charges in real-time makes it possible to count electrons one-by-one
as they pass through the structure. The setup can thus be used as a
high-precision current meter for measuring ultra-low currents, with resolution
several orders of magnitude better than that of conventional current meters.
  In addition to measuring the average current, the counting procedure also
makes it possible to investigate correlations between charge carriers. In
quantum dots, we find that the strong Coulomb interaction makes electrons try
to avoid each other. This leads to electron anti-bunching, giving stronger
correlations and reduced noise compared to a current carried by statistically
independent electrons.
  The charge detector is implemented by monitoring changes in conductance in a
near-by capacitively coupled quantum point contact. We find that the quantum
point contact not only serves as a detector but also causes a back-action onto
the measured device. Electron scattering in the quantum point contact leads to
emission of microwave radiation. The radiation is found to induce an electronic
transition between two quantum dots, similar to the absorption of light in real
atoms and molecules. Using a charge detector to probe the electron transitions,
we can relate a single-electron tunneling event to the absorption of a single
photon. Moreover, since the energy levels of the double quantum dot can be
tuned by external gate voltages, we use the device as a frequency-selective
single-photon detector operating at microwave energies.
