N. Dimarcq

Six-axis inertial sensor using cold-atom interferometry.

We have developed an atom interferometer providing a full inertial base. This device uses two counterpropagating cold-atom clouds that are launched in strongly curved parabolic trajectories. Three single Raman beam pairs, pulsed in time, are successively applied in three orthogonal directions leading to the measurement of the three axis of rotation and acceleration. In this purpose, we introduce a new atom gyroscope using a butterfly geometry. We discuss the present sensitivity and the possible improvements.

Investigations on continuous and pulsed interrogation for a CPT atomic clock

We investigated the influence of some critical parameters and operating conditions such as cell temperature, laser intensity, and interrogation technique affecting the performances of a gas cell Cs frequency standard based on coherent population trapping (CPT). Thanks to an original experimental setup, the atoms can be trapped in the dark state and interrogated using continuous wave (CW) or pulsed coherent optical radiations. Using a double-lambda scheme, a signal contrast as high as 52% has been measured in the continuous regime for an optimum cell temperature of 35degC. Compared with the conventional continuous CPT interrogation, the pulsed interrogation technique reduces the light shift by a factor of 300 and allowed it to reach high-frequency stability for higher laser intensities. The frequency stability has been measured to be 9 x 10-13 for a 1 s integration time. Main noise contributions limiting the short-term and medium-term frequency stability are reviewed and estimated.

Current Status of a Pulsed CPT Cs Cell Clock

A Raman-Ramsey Cs cell atomic clock is presented. The relaxation times of the population and the hyperfine coherences in the cell are measured. The effect on the central Ramsey fringe amplitude of the critical experimental parameters such as laser intensity, magnetic field, temperature, and Ramsey time is investigated. The existence and impact of the additional Deltam = 2 transitions involved in the pumping scheme are pointed out. Narrow resonance linewidths as low as 33 Hz with reasonable signal-to-noise ratios have been recorded. By removing a frequency drift attributed to the cell, the achieved frequency stability is 7 times 10-13 tau-1/2. The main noise contributions that limit the short-term frequency stability are reviewed and estimated.

Cold atoms in space and atomic clocks: ACES

Abstract In this article, the interest of space environment for cold atoms is outlined. After a brief review of cooling techniques and Bose–Einstein condensation, the case of atomic clocks in microgravity is discussed. The scientific objectives of the European mission ACES are presented. ACES will fly onboard the international space station in 2005–2006.

Cold-Atom Clocks on Earth and in Space

We present recent progress on microwave clocks that make use of lasercooled atoms. With an ultra-stable cryogenic sapphire oscillator as interrogation oscillator, a cesium fountain operates at the quantum projection noise limit. With 6 × 105 detected atoms, the relative frequency stability is 4 × 10−14 τ −1/2, where τ is the integration time in seconds. This stability is comparable to that of hydrogen masers. At τ = 2 × 104 s, the measured stability reaches 6 × 10−16. A 87Rb fountain has also been constructed and the 87Rb ground-state hyperfine energy has been compared to the Cs primary standard with a relative accuracy of 2.5 × 10−15. The 87Rb collisional shift is found to be at least 30 times below that of cesium. We also describe a transportable cesium fountain, which will be used for frequency comparisons with an accuracy of 10−15 or below. Finally, we present the details of a space mission for a cesium standard which has been selected by the European Space Agency (ESA) to fly on the International Space Station in 2003.

Observation of Raman-Ramsey fringes with optical CPT pulses

The Ramsey method has been applied by means of optical coherent population trapping (CPT) pulses through a cesium vapor cell with N/sub 2/ buffer gas at room temperature, using two phase-locked lasers. With this method, CPT resonance spectral widths are no longer limited by optical saturation and collision effects, but only depend on free evolution time between the two pulses. A fringe width below 100 Hz is reported. Experimental Raman-Ramsey fringes are analyzed using the classical wavefunction formalism.

Raman–Ramsey Interaction for Coherent Population Trapping Cs Clock

A gas cell atomic frequency standard based on coherent population trapping (CPT) with Ramsey interrogation pulses is developed. The clock resonance is detected in a Cs cell with buffer gas using two phase-locked diode lasers tuned to the Dl line. A preliminary short-term stability of 3.5 10-12t is reported. The CPT specificity allows a new interrogation-detection sequence, i.e., the continuous pulse train, which, with phase modulation, could lead to better short-term stability compared to usual continuous CPT clocks. The parameters optimizing the stability include a high Rabi frequency, a very short measurement time, and an interrogation time equal to the lifetime of the hyperfine coherence

Reaching the quantum noise limit in a high-sensitivity cold-atom inertial sensor

In our high-precision atom interferometer, the measured atomic phase shift is sensitive to rotations and accelerations of the apparatus, and also to phase fluctuations of the Raman lasers. In this paper we study two principal noise sources affecting the atomic phase shift, induced by optical phase noise and vibrations of the setup. Phase noise is reduced by carrying out a phase lock of the Raman lasers after the amplification stages. We also present a new scheme to reduce noise due to accelerations by using a feed-forward on the phase of the Raman beams. With these methods, it should be possible to reach the range of the atomic quantum projection noise limit, which is about 1m rad rmsfor our experiment, i.e. 30 nrad s −1 Hz −1/2 for a rotation

A limit to the frequency stability of passive frequency standards

It is shown that in passive frequency standards (e.g. cesium, rubidium, stored ion(s)), the noise of the interrogation signal at even multiples of the modulation frequency entails a limitation to the achievable frequency stability. The effect will be significant in laser-pumped future frequency standards, in which an improved signal-to-noise ratio is obtained. The frequency stability capability of future semiconductor-laser-pumped cesium beam tube or rubidium cell frequency standards being on the order of 10/sup -14/ tau /sup -1/2/ to 10/sup -13/ tau /sup -1/2/, the effect constitutes a very serious limitation. The same conclusion applies to microwave and optical frequency standards based on stored ions. It will therefore be necessary to reduce as much as possible the level of frequency or phase noise that the interrogation signal exhibits at Fourier frequencies equal to even multiples of the modulation frequency, unless a different processing of the resonator response can relax the requirement on the spectral purity.<<ETX>>It is shown that in passive frequency standards (e.g. cesium, rubidium, stored ion(s)), the noise of the interrogation signal at even multiples of the modulation frequency entails a limitation to the achievable frequency stability. The effect will be significant in laser-pumped future frequency standards, in which an improved signal-to-noise ratio is obtained. The frequency stability capability of future semiconductor-laser-pumped cesium beam tube or rubidium cell frequency standards being on the order of 10/sup -14/ tau /sup -1/2/ to 10/sup -13/ tau /sup -1/2/, the effect constitutes a very serious limitation. The same conclusion applies to microwave and optical frequency standards based on stored ions. It will therefore be necessary to reduce as much as possible the level of frequency or phase noise that the interrogation signal exhibits at Fourier frequencies equal to even multiples of the modulation frequency, unless a different processing of the resonator response can relax the requirement on the spectral purity. >

Analysis of the noise sources in an optically pumped cesium beam resonator

The main noise sources that affect the clock signal detection in an optically pumped cesium beam resonator are determined and their influence on the clock S/N ratio is evaluated. The following noise sources are taken into account: atomic shot noise, fluorescence photon noise, laser frequency noise, and clock signal detection noise. A theoretical model that predicts the variations of the S/N ratio as a function of different parameters characterizing the atom-laser interaction is developed. The main conclusion concerns the saturation of S/N value for high atomic flux due to laser frequency fluctuations. All the theoretical predictions were experimentally verified. >