Light-pulse atom interferometry in microgravity

We describe the operation of a light pulse interferometer using cold Rb atoms in reduced gravity. Using a series of two Raman transitions induced by light pulses, we have obtained Ramsey fringes in the low gravity environment achieved during parabolic flights. With our compact apparatus, we have operated in a regime which is not accessible on ground. In the much lower gravity environment and lower vibration level of a satellite, our cold atom interferometer could measure accelerations with a sensitivity orders of magnitude better than the best ground based accelerometers and close to proven spaced-based ones. PACS. PACS-key discribing text of that key – PACS-key discribing text of that key Atom interferometry is one of the most promising candidates for ultra-accurate measurements of gravito-inertial forces [1], with both fundamental [2,3,4,5] and practical (navigation or geodesy) applications. Atom interferometry is most often performed by applying successive coherent beam-splitting and -recombining processes separated by an interrogation time T to a set of particles [6]. Understanding matter waves interferences phenomena follows from the analogy with optical interferometry [7,8]: the incoming wave is separated into two wavepackets by a first beam-splitter; each wave then propagates during a time T along a different path and accumulates a different phase; the two wavepackets are finally recombined by a last beam-splitter. To observe the interferences, one measures the two output-channels complementary probability amplitudes which are sine functions of the accumulated phase difference ∆φ. This phase difference increases with the paths length, i.e. with the time T between the beamsplitting pulses. When used as inertial sensors [9,10], the atoms are usually left free to evolve during the interrogation time T so that the interferometer is only sensitive to gravitoinertial effects. In particular, one avoids residual trapping fields that would induce inhomogeneities or fluctuations and would affect the atomic signal. The interrogation time T is consequently limited by, on the one hand, the free expansion of the atomic cloud, and, on the other hand, the free fall of the atomic cloud. The limitation of expansion is alleviated by the use of ultracold gases [11,12], Send offprint requests to: Fig. 1. Top: The atom interferometer assembled in the Airbus. The main rack on the left houses the laser sources and the control electronics. The rack on the front right contains the uninterruptable power-supply, the electrical panel and the high-power laser part. The rack on the back right hosts atomoptics part of the experiment. Bottom: the architecture of the atom interferometer. 2 G. Stern et al.: Light-pulse atom interferometry in microgravity but, due to free-fall distance, long-interrogation-time experiments require tall vacuum chambers [13]. Laboratory experiments are typically limited to about 300 ms of free fall with a 1 m-tall apparatus if the atoms are simply released, or more by launching them upward as in atomic fountains. This can be increased on much larger apparatuses : a 10 m-high atom interferometer is currently under construction at Stanford [3], giving access to 1.4 seconds of interrogation time. Free-fall heights of more than 100 m, corresponding to durations of about 5 seconds are also available in a drop tower (ZARM Bremen, Germany [14]). Another solution consists in performing the experiments in microgravity achieved during parabolic flights provided by an aeroplane, as for the PHARAO prototype [15]. In the I.C.E. (Interférometrie Cohérente pour l’Espace) collaboration [16,17] that we present here, we are conducting cold-atom interferometry experiments in such an airplane (the A-300 0-G Airbus of NOVESPACE), which carries out ballistic flights. Microgravity is obtained via 20 seconds-long parabolas by steering the plane to cancel drag and follow free fall. The residual acceleration is on the order of 10g. With 90 parabolas per flight session, we have access to 30 minutes total of reduced gravity. In this letter, we present a first validation of our 0-g setup by obtaining Ramsey fringes with copropagating Raman transitions during parabolas, the interrogation times being longer than those we could obtain on Earth with the same configuration. Transferring a laboratory-bound cold atoms interferometer into an automated experiment suitable for microgravity use poses many technical challenges [14,16,18, 19]. We assembled a prototype atomic source suitable for inertial-sensing in an airplane from the I.C.E. collaboration components [16] (see Fig. 1). The atom interferometer is made of 4 elements: a vacuum chamber with optics; lasers sources for cooling and coherent manipulation of atoms; a stable oscillator (in our case a hyperfrequency source at about 6.8 GHz [16]) which is a frequency reference for the Raman lasers; and an autonomous real-time controller for the experimental sequence and data calculations. For the interferometric measurement, we prepare clouds of cold Rb in a Magneto-Optical Trap (MOT) and release them for interrogation during their free fall. Moving away from extended-cavity-laser-diode-based systems, as developed in the PHARAO project [20], we have designed laser sources at 780 nm for cooling and coherently manipulating the atoms that rely on telecom technologies and second harmonic generation [21,22]. This allows to use fiber-optics components and offers a reliable, robust and compact system, quite insensitive to the environmental perturbations encountered in the airplane. These novel laser sources are very similar to the ones described in details in [23] so we limit here to outlining the successful design. A first reference DFB 1560 nm pigtailed laser diode (linewidth ∼ 1 MHz) is frequency doubled in a PPLN waveguide and locked on a Rb transition through a saturated absorption setup (see Fig. 3). A slave DFB 1560 nm pigtailed diode, similar to the first one, is locked to the first laser at a frequency difference monitored Fig. 2. Spectral density of the phase noise of the quartz recorded by comparison with other ultra-stable quartz oscillators phase-locked on H-Maser of the SYRTE. through the beat note signal, as measured by a fibered fast photodiode. The frequency offset can be adjusted so that the slave DFB is red detuned from the resonance of the F = 2 → F = 3 transition of Rb with a detuning ranging from 0 to 1.1 GHz. A 1560 nm fibered phase modulator is then used to generate two sidebands ∼ 6.8 GHz apart. One of these sidebands acts either as the repumping laser during the cooling phase, or as the second Raman laser during coherent manipulation of the atoms, depending on the applied frequency. The microwave reference has been simplified compared to [16] in order to make it more reliable in the plane environment. It’s based on a direct multiplication of a 10 MHz quartz oscillator to 6.8 GHz without any intermediate oscillator or phase lock loop. The ultra-stable quartz has been chosen to be a good compromise to achieve low phase noise at low and high frequencies simultaneously (see Fig. 2), as in [15]. The multiplication is done in three steps: a first multiplication by 10 to 100 MHz (commercial Wenzel system), then multiplication by 2 and finally to 6.8 GHz by a comb generator (non-linear transmission line, Wenzel model 7100). Two direct digital synthesis (DDS) are used to adjust the cooling/repumping frequency difference and the Raman beams frequency difference respectively. After amplification through a 5W Erbium-Doped Fiber Amplifier (EDFA), the slave laser is frequency doubled in free space with a double-pass in a 4 cm bulk PPLN crystal. We typically obtain ∼ 300 mW at 780 nm. A 80 MHz Acousto-Optical Modulator (AOM) is used to switch between the MOT configuration (in which the non diffracted order of the AOM is used) and the Raman configuration (in which the first diffraction order is used, see Fig. 3). The use of the first order of the AOM for the Raman beam enables to create ultra-short pulses of light (10 μs typically). Additional mechanical shutters ensure a total extinction of the beams. Two optical fibers finally bring the MOT and the Raman beams to the science chamber. The fibers deliver the light to the vacuum-chamber module [17]. The MOT fiber is sent to a 1-to-3 fiber beamG. Stern et al.: Light-pulse atom interferometry in microgravity 3 splitter 1 which delivers three beams which are then retroreflected and produce the MOT. The circularly-polarized Raman beam has a 1 inch diameter and is aligned with the horizontal plane. A 300 mG horizontal magnetic field is aligned with the Raman beam to raise the Zeeman degeneracy of the hyperfine sub-levels. The intensity of the lasers can be up to 20 times the saturation intensity of rubidium, which allows for short Raman pulses with weak velocity selection. The Raman detuning is about 700 MHz. The effective Rabi pulsation Ωeff is about 2π × 12.5 kHz. Finally, a magnetic shield around the science chamber prevents from changes of the Earth’s magnetic field directions during parabolas [17]. The science chamber in which we operate our atom interferometer is shown in Fig. 4. We load about 10 atoms in the MOT from a Rubidium vapor in 500 ms. We release the atoms from the MOT and further cool them down below 100 μK during a brief phase of optical mo1 From Schäfter und Kirchhoff : http://www.sukhamburg.de/ PM S la ve la se r M a st e r la se r