We report observations of the Schawlow-Townes noise limit in a cryogenic sapphire secondary frequency standard. The effect causes a fundamental limit to the frequency stability, and was measured through the novel excitation of a bimodal maser oscillation of a Whispering Gallery doublet at 12.04 GHz. The beat frequency of 10 kHz between the oscillations enabled a sensitive probe for this measurement of fractional frequency instability of ${10}^{-14}{\tau }^{-1/2}$ with only 0.5 pW of output power.

• Received 20 March 2008

DOI:https://doi.org/10.1103/PhysRevLett.100.233901