Microwave phase detection at the level of 10(-11) rad.

We report on a noise measurement system with the highest spectral resolution ever achieved in the microwave domain. It is capable of detecting the phase fluctuations in rms amplitude of 2x10(-11) rad/sqrt Hz at Fourier frequencies above a few kilohertz. Such precision allows the study of intrinsic fluctuations in various microwave components and materials, as well as precise tests of fundamental physics. Employing this system we discovered a previously unknown phenomenon of down-conversion of pump oscillator phase noise into the low-frequency voltage fluctuations.