  The frequency noise power spectral density of a free-running quantum cascade
laser frequency comb is investigated. A plateau is observed at high
frequencies, attributed to the quantum noise limit set by the Schawlow-Townes
formula for the total laser power on all comb lines. In our experiment, a
linewidth of 292 Hz is measured for a total power of 25 mW. This result proves
that the four-wave mixing process, responsible for the comb operation,
effectively correlates the quantum noise of the individual comb lines.
