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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Optical Metrology

Gravimetric Atom Interferometer GAIN

B. Leykauf, A. Stiekel, C. Freier, V. Schkolnik, M. Krutzik and A. Peters

Matter wave interferometers with cold atoms use light-pulses to coherently manipulate atomic wave packets and have become versatile tools for precision measurements of inertial forces and physical constants as well as for testing fundamental physics. The Gravimetric Atom Interferometer GAIN uses beam splitter and mirror pulses realized by stimulated Raman transitions between the two hyperfine ground states of 87Rb in an atomic fountain to measure the gravitational acceleration g.

GAIN is a mobile experiment allowing the transport to sites of interest and has demonstrated long-term measurements of local gravity with an unprecedented stability of less than 0.5 nm/s² and an accuracy competitive with other state-of-the-art absolute gravimeters. On this page, you will find an overview of atom interferometry in general and our Berlin gravimeter.

 

Our gravimeter during a mobile measurement campaign at the space observatory in Onsala, Sweden.

Gravity data obtained with GAIN at the Onsala Space Observatory. Data points are averaged over one hour.

Atom Interferometry for Inertial Sensing

Our atom interferometer uses light-pulses that act as beam splitters and mirrors for atoms.

 

 

Principle of a light-pulse atom interferometer.

 

First, a sample of atoms is subjected to a so-called π/2 pulse, creating a coherent superposition of the effective two-level system that will spatially separate due to momentum transferred from the light field to the atoms. A π pulse applied after a time T mirrors the internal and momentum state of the atoms so the two trajectories will spatially overlap when a second π/2 pulse closes the interferometer. During the interaction with the laser, the light field’s local phase at each atom’s position will be imprinted onto its atomic wave function. The phase shift between both interferometer arms is ΔΦ = ϕ1 – 2 ϕ2 + ϕ3 = kgT², where k the length of the light field’s effective wave vector. This phase shift in turn is encoded in the atoms’ state population P|F=2〉= [1 + cos(ΔΦ)]/2 which can be read out via fluorescence detection.
 

Quantum Sensor GAIN

The central part of our gravimeter is the physics package shown schematically in the figure below. It consists of an ultra-high vacuum chamber and a vibration isolation platform.

 

(a) Schematic of the physics package. (b) Top view of the fluorescence detection.
(c) Active vibration isolation platform for the Raman mirror acting as a reference
plane for the measurement.

 

Inside the physics package, a sample of some 108 87Rb are trapped and cooled in a magneto-optical trap within 600 ms. These atoms are launch upwards on a parabolic flight while cooling them further to about 2 μK using the moving molasses technique.

During the velocity- and state-selection sequence, a narrow vertical velocity class of atoms in a magnetically insensitive sub-state are selected and the other atoms are removed from the cloud using “blow-away” pulses. Once the atoms reach the magnetically shielded interferometer zone, a light-pulse atom interferometer sequence of π/2 – π – π/2 with a pulse separation time of T = 260 ms is performed. Finally, about 105 atoms are state-sensitively detected and a value for the gravitational acceleration is determined from the populations of the two hyperfine ground states.

The interferometer uses Doppler-sensitive two-photon transitions. The necessary counter-propagating light beams are realized by retro-reflecting the laser light entering from the top off a mirror below the chamber. Since vibrations of the mirror will enter the measurement as spurious acceleration signals, the mirror is placed on actively stabilized vibration isolation platform. A piezo-driven tip/tilt stage is furthermore used to actively stabilize the measurement axis formed by the Raman beams and to compensate the Coriolis effect by counter-rotating the mirror against Earth’s rotation.

Control electronics and computer system are housed in two transportable racks. They also contain the in-house built laser system. It’s modular and rugged design allows the transport of the instruments with only small readjustments necessary after setup at a new location. 

 

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