Etienne Batori, Christoph Affolderbach, Matthieu Pellaton, Florian Gruet, Laboratoire Temps-Fréquence, University of Neuchâtel, Switzerland; Yuanyan Su, Microwave and Antennas Group, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland; Maddalena Violetti, Microwave and Antennas Group, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland & Fondazione Toscana Life Sciences, Italy; Anja K. Skrivervik, Microwave and Antennas Group, EPFL, Switzerland; Gaetano Mileti, Laboratoire Temps-Fréquence, University of Neuchâtel, Switzerland

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Summary We present the latest results of our micro-POP (pulsed optically pumped) DR (double resonance) clock in a 51 mm3 natural Rb micro fabricated vapor cell. The Ramsey signal shows contrast > 5% and a central fringe width of 1.4 kHz. As expected by the interrogation method, the clock shows significantly smaller light shift coefficients (more than 2 orders of magnitude) compared to the continuous mode, which is limited by the said effect. The current short-term stability is of the order of 1e-11 at one second, mainly limited by technical detection noise. The one-day stability is limited at a few 1e-12. The main limitations to the clock stability will be discussed. Motivation The principal mid and long (1 day) term limitation of continuous, double resonance optically pumped frequency standards is the intensity light shift. The pulsed optically pumped (POP) double resonance scheme tackles this problem. Indeed, as the light is switched off during the entire microwave interrogation time, a strongly reduced light shift is theoretically expected. There resides the main advantage of this technology compared to the CPT micro clocks, which still exhibit non-zero light shift coefficients. The potential limitation to the implementation of the micro-POP scheme in a small vapor cell is also evaluated. Indeed, the low volume of the cell and the consequently higher operating temperatures required induce higher relaxation rates for the atoms and limit the Ramsey cycle’s length. Consequently, relaxation rates must be measured in order to quantify the limitations on the setup. Setup We currently use a frequency-stabilized laser head enclosing an acousto-optical modulator as the optical switch. The microwave field pulses are sustained by a sub-wavelength sized micro-loop-gap resonator (u-LGR) of less than 1cm3 of volume. Results Various relaxation processes on the Rb atomic ground state in the small mm-scale vapor cell used in our study limit the useful Ramsey time and overall cycle time of the Ramsey interrogation sequence employed. The relaxation rates of our cell are estimated with the Franzen method. They are measured of the order of gamma1 ? 5.1 kHz and gamma2 ? 4.5 kHz. This results in an upper limit for the Ramsey time of around 0.4 ms. The Ramsey fringes measured on our clock setup are fully compatible with the theoretical framework, showing high signal contrast > 5% and linewidth of 1.4 kHz. The short-term performances of the clock are on the level of 1e-11, mainly limited by the relative intensity noise (RIN) of the laser. We will present a detailed long-term time scale analysis of the typical limitation factor found in the literature: frequency and intensity light shift, microwave power shift, barometric effect, cavity pulling, temperature shift and frequency drift. As expected, the intensity light shift coefficient of the pulsed clock is significantly smaller as compared to the same setup operated in continuous DR regime. We will discuss the main limiting effects to the long-term stability of our clock and prospects for further stability improvement. The results strongly suggest the possibility of realizing a miniature microfabricated POP DR clock with a below 1e-12 stability at one day.