With the contribution of atomic spectroscopy, MEMS technologies and integrated photonics, miniaturized atomic clocks, sensors and instruments have been developed for around 20 years. This field of research, initiated in 2004 with the demonstration at NIST of the first microwave chip-scale atomic clock (CSAC), has since experienced growing popularity, due to its strategic interest and the plethora of applications (GNSS, secure communications, PNT systems, etc.) it covers. 

Current commercial CSACs lose about 1 microsecond per day, a hundred times less than quartz oscillators, within a comparable volume-consumption budget. However, the frequency noise of their laser (VCSEL) limits their short-term frequency stability while the presence of a buffer gas pressure in their cell induces a collisional shift that compromise their long-term stability.

Recent years have seen a relevant interest towards the development of new-generation miniaturized optical atomic clocks. In this domain, sub-Doppler spectroscopy techniques, based on the interaction of hot alkali atoms contained in a microfabricated vapor cell with two counter-propagating laser fields obtained from a single laser, constitute an attractive approach due to their simplicity and integration potential.

Among various methods, the 2-photon absorption spectroscopy of the Rb atom at 778 nm offers promising features. Using a Rb microcell and an external-cavity diode laser, NIST has demonstrated a compact optical reference with a fractional frequency stability of 1.8x10-13 at 1 s and approaching 10-14 at 100 s. Competitive stability results of 3x10-13 at 1 s and 3x10-14 at 100 s have been recently reported at FEMTO-ST.

In this context, based on the experience acquired at FEMTO-ST, the PhD thesis   targets the development, progress, and metrological characterization of an ultra-stable microcell optical reference based on two-photon transition of Rb atom at 778

nm. A fractional frequency stability in the 10-14 range at 1 s and 1 hour integration time is targeted in a very simple laser system architecture. Such an optical reference might contribute to the positioning or navigation of GNSS-denied vehicles, nano-satellites, ships, and could find interest in the implementation of ambitious space missions that require precise timekeeping.

The short-term frequency stability of the Rb microcell reference is today limited by contributions from the two main blocks (laser and cell) : 1/ the laser frequency noise, which degrades the short-term stability of the atomic reference through an intermodulation effect, 2/ the poor signal-to-noise ratio of the atomic resonance explained by the reduced atom-field interaction length, the insufficient blue photon collection efficiency, and the limited number of Rb atoms involved in the atom-field interaction, as well as the presence of contaminants or residual gases in the microfabricated cell, which causes line broadening (~10-20 MHz/Torr) of the optical atomic resonance. 

These points suggest the first research axis and objectives for the PhD candidate:  1/Implementation of a novel ultra-low frequency noise laser to reduce the intermodulation contribution and improve the reference short-term stability, 2/ Development of original micro-fabricated cell architectures offering increased atomic signal, reduced photon shot noise contribution (optimized fluorescence collection) and improved cell internal atmosphere purity, to access the detection of narrower optical resonances.

Further efforts will be pursued to improve the long-term stability of the reference. For optimized isolation of the atoms to their environment, the new cell will be implemented in a dedicated specially-designed small-size physics package, with optimized thermal, magnetic and mechanical stability. Also, innovative interrogation sequences will be implemented for mitigation of light-shift effects.  

The metrological characterization of the Rb microcell two-photon optical reference stability will be obtained by comparing the laser output signal to a 778 nm reference signal generated from an optical frequency comb locked to an ultra-stable cavity laser, available at FEMTO-ST.

The PhD candidate will work at the interface between the OHMS and MOSAIC groups at FEMTO-ST. The candidate will interact with researchers, engineers, technicians and will benefit from the support and skills of FEMTO-ST internal services (electronics/mechanics/computing), with access to a large number of instruments dedicated to time-frequency metrology and clean-room facilities. The candidate will present his/her work in international conferences and will target high-impact publications.

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - rodolphe.boudot@femto-st.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online  before March 13th, 2026 Midnight Paris time !