  The electron waiting time is the time that passes between two subsequent
charge transfers in an electronic conductor. Recently, theories of electron
waiting times have been devised for quantum transport in Coulomb-blockade
structures and for mesoscopic conductors, however, so far a proper description
of a detector has been missing. Here we develop a quantum theory of a waiting
time clock capable of measuring the distribution of waiting times between
electrons above the Fermi sea in a mesoscopic conductor. The detector consists
of a mesoscopic capacitor coupled to a quantum two-level system whose coherent
precession we monitor. Under ideal operating conditions our waiting time clock
recovers the results of earlier theories without a detector. We investigate
possible deviations due to an imperfect waiting time clock. As specific
applications we consider a quantum point contact with a constant voltage and
lorentzian voltage pulses applied to an electrode.
