  The Baryon Antibaryon Symmetry Experiment (BASE) aims at performing a
stringent test of the combined charge parity and time reversal (CPT) symmetry
by comparing the magnetic moments of the proton and the antiproton with high
precision. Using single particles in a Penning trap, the proton/antiproton
$g$-factors, i.e. the magnetic moment in units of the nuclear magneton, are
determined by measuring the respective ratio of the spin-precession frequency
to the cyclotron frequency. The spin precession frequency is measured by
non-destructive detection of spin quantum transitions using the continuous
Stern-Gerlach effect, and the cyclotron frequency is determined from the
particle's motional eigenfrequencies in the Penning trap using the invariance
theorem. By application of the double Penning-trap method we expect that in our
measurements a fractional precision of $\delta g/g$ 10$^{-9}$ can be achieved.
The successful application of this method to the antiproton will represent a
factor 1000 improvement in the fractional precision of its magnetic moment. The
BASE collaboration has constructed and commissioned a new experiment at the
Antiproton Decelerator (AD) of CERN. This article describes and summarizes the
physical and technical aspects of this new experiment.
