Attosecond timing jitter in millimeter waves via Kerr optical frequency division

Millimeter-wave oscillators are essential for communication, radar, spectroscopy, and emerging photonic-electronic systems, yet their spectral purity remains constrained by noise processes intrinsic to direct generation at the carrier frequency. Here we demonstrate a fundamentally different approach that circumvents these limits by optically dividing a multi-terahertz reference to the millimeter-wave domain using Kerr soliton dynamics. A 3.3 THz dual-wavelength Brillouin laser injection-locks a chip-scale Kerr microcomb, enforcing exact optical frequency division to a 300 GHz repetition rate without electronic multiplication or feedback control. The resulting oscillator enters a previously inaccessible noise regime, exhibiting single-sideband phase noise of -152dBc/Hz at a 1 MHz offset, corresponding to an unprecedented timing noise of 18 zs.Hz^{-1/2} for a directly measured photodetected microwave or millimeter-wave signal. Integration of the measured spectrum yields an rms timing jitter of 135 attoseconds from 1 kHz to 1 MHz. To directly resolve this performance, we implement cross-correlation phase-noise metrology at 300 GHz using independent photonic local oscillators. These results establish Kerr optical frequency division as a general and scalable route to millimeter-wave and sub-terahertz carriers whose coherence is no longer constrained by direct-generation limits, opening a new operating regime for photonic oscillators.