A Chip-Scale 2-Photon Rubidium Atomic Clock

Our vision is to deliver a chip-scale two-photon Rb optical atomic clock, frequency reference and low phase-noise oscillator by combining narrow linewidth lasers using III-V gain chips with silicon nitride external gratings locked to integrated Rb MEMS cells and down-converted to a 10 GHz output signal using a silicon nitride microring frequency comb driven by a III-V pulsed, mode-locked laser. A silicon nitride photonic integrated circuit (PIC) platform will be used for the heterogeneous integrated of all the photonic and MEMS vacuum components required for the timing systems. An analogy is Harrison's pocket watch, H4, that won the Longitude Prize in 1773 as the small size reduced the uncertainties from temperature and acceleration drifts on navy ships.
Through comparison with the literature and back of the envelope calculations we estimate a performance of 100 fs/(Hz^0.5) and 1 fs accuracy for the proposed clock with a physics package of around 40 x 30 x 5 mm in size. This corresponds to an uncertainty of 2 ns after the Blackett review 72 hour hold-over period requirement for critical national infrastructure. For a position uncertainty where only timing errors dominate, this results in a position uncertainty < 1 m. Our aim is for a clock with comparable performance to a Microchip MHM-2020 Active Hydrogen Maser but x10,000 smaller, x250 lower mass and x150 lower power. Active hydrogen masers are 100 times more stable than Cs atomic clocks at measuring times of 7 days and x100,000 more accurate at 1 day than a commercial CSAC (Chip Scale Atomic Clock).