  We explore the phase diagram of two-flavor QCD at imaginary values of baryon
and isospin chemical potentials analyzing the thermodynamic potential of QCD
analytically and that of the Polyakov-loop extended Nambu-Jona-Lasinio (PNJL)
model numerically. QCD has no pion condensation at imaginary baryon and isospin
chemical potentials, and therefore has discrete symmetries that are not present
at real baryon and isospin chemical potentials. The PNJL model possesses all
the discrete symmetries. The PNJL model can reproduce qualitatively lattice QCD
data presented very lately.
