  We present a novel and simple method of stabilizing the laser phase and
frequency by polarization spectroscopy of an atomic vapor. In analogy to the
Pound-Drever-Hall method, which uses a cavity as a memory of the laser phase,
this method uses atomic coherence (dipole oscillations) as a phase memory of
the transmitting laser field. A preliminary experiment using a distributed
feedback laser diode and a rubidium vapor cell demonstrates a
shot-noise-limited laser linewidth reduction (from 2 MHz to 20 kHz). This
method would improve the performance of gas-cell-based optical atomic clocks
and magnetometers and facilitate laser-cooling experiments using narrow
transitions.
