  Next-generation communications, radar and navigation systems will extend and
exploit the higher bandwidth of the millimeter-wave domain for increased
communication data rates as well as radar with higher sensitivity and increased
spatial resolution. However, realizing these advantages will require the
generation of millimeter-wave signals with low phase noise in simple and
compact form-factors. The rapidly developing field of photonic integration
addresses this challenge and provides a path toward simplified and portable,
low-noise mm-wave generation for these applications. We leverage these advances
by heterodyning two silicon photonic chip lasers, phase-locked to the same
miniature Fabry-Perot (F-P) cavity to demonstrate a simple framework for
generating low-noise millimeter-waves with phase noise below the thermal limit
of the F-P cavity. Specifically, we generate 94.5 GHz and 118.1 GHz
millimeter-wave signals with phase noise of -117 dBc/Hz at 10 kHz offset,
decreasing to -120 dBc/Hz at 40 kHz offset, a record low value for such
photonic devices. We achieve this with existing technologies that can be
integrated into a platform less than $\approx$ 10 mL in volume. Our work
illustrates the significant potential and advantages of low size, weight, and
power (SWaP) photonic-sourced mm-waves for communications and sensing.
