  We implement a simple feedback mechanism on a two-mode cavity QED system to
preserve the Zeeman coherence of a ground state superposition that generates
quantum beats on the second-order correlation function. Our investigation
includes theoretical and experimental studies that show how to prevent a shift
away from the Larmor frequency and associated decoherence caused by Rayleigh
scattering. The protocol consists of turning off the drive of the system after
the detection of a first photon and letting it evolve in the dark. Turning the
drive back on after a pre-set time reveals a phase accumulated only from Larmor
precession, with the amplitude of the quantum beat more than a factor of two
larger than with continuous drive.
