  We propose and demonstrate a method for measuring the time evolution of the
off-diagonal elements $\rho_{n,n+k}(t)$ of the reduced density matrix obtained
from the quantum theory of the laser. The decay rates of the off-diagonal
matrix element $\rho_{n,n+k}(t)$ (k=2,3) are measured for the first time and
compared with that of $\rho_{n,n+1}(t)$, which corresponds to the linewidth of
the laser. The experimental results agree with the quantum theory of the laser.
