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optical frequency metrology

par Caroline CHAMPENOIS - publié le

Due to their very long confinement times, ranging from seconds to days, and the controlled experimental conditions, laser cooled trapped ions are ideal candidates for many high-precision measurements, like illustrated in frequency metrology. Indeed, the most accurate atomic clocks to date are built with single trapped ions. There, the readout of the internal state of single ions carries uncertainty that requires long integration times for a single-ion clock to reach frequency stability comparable to neutral atom clocks. Compared to large ensembles of cold atoms, this single atom feature is the main limiting parameter in the increase of the apparent signal-to noise ratio in a given timescale.

We have proposed a novel protocol for the interrogation of trapped ions [1], based on the probing of a large cloud of trapped ions and the clever use of a 3-photon coherent population trapping (CPT) to produce a dark resonance. In the level scheme of an alkaline-earth ion, the coherent excitation of three atomic states creates a dark resonance in the THz frequency domain, which can serve as frequency standard. This scheme only implies the visible and near-infrared lasers involved in the probing of the ion, and does not require an additional THz-source. Adapted combination of the involved laser wave-vectors allows canceling Doppler line broadening to first order, and paves the way for the use of clouds containing many ions as a sample.

To produce a dark line, the coherent excitation by three lasers of different wavelengths requires a stable phase relation that can only be achieved by simultaneous locking of the three frequencies to a frequency comb in the near-infrared domain. For calcium ions, the involved wavelengths are 794 nm (which is frequency doubled for ion excitation on the resonance line), 866 nm and 729 nm. The optical frequency comb has to be fully stabilized onto an ultra-stable frequency reference in order to obtain the coherent stabilization of these three lasers. This ultra-stable reference will be provided to PIIM thanks to its involvement in the national equipment Refimeve+, which organizes the distribution of a high-precision metrological signal via the national academic network RENATER.

The core of this project is the ultra-stable Ti:Sa laser developed in the group. To ensure its stability and precision, a Pound-Drever-Hall (PDH) scheme locks the laser frequency onto an eigen-mode of an ultra-stable high-finesse ULE cavity, designed in the group. Optimization of the cavity design has been carried out with a Finite-Elements Method. Cavity has been simulated and realized in an iteration process, leading to an expected relative length variations below 10-14 under the influence of gravity acceleration (1 g). We have optically contacted silicon mirrors on two identical spacers, resulting in finesses of the order of 140,000 and 200,000. The evaluation of the lock performances are underway.

Picture of one of the ULE cavity, seen from above, siting in a clean room and waiting for the mirror to be definitely contacted. The pre-contacting step is characterized by the interference fringes visible on the mirror : the larger the fringes, the more parallel are the surfaces to be contacted. The contact is triggered by a pressure on one side of the mirror. The double flower pot shaped spacer is hold tight by the dark ring.

[1] Champenois, et al., “Terahertz Frequency Standard Based on Three-photon Coherent Population Trapping,” Phys. Rev. Lett. 99, 013001, 2007.

[2] D. Guyomarc’h et al., “Some aspects of simulation and realization of an optical reference cavity”, Phys. Rev. A, 80 (6) : 063802, 2009.