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Physics Today

Atom Interferometer Measures G with Same Accuracy as Optical Devices

Barbara Goss Levi

Published Nov 1, 1999• 6 citations
Publisher

Abstract

The acceleration due to gravity, g, can be measured simply by timing how long it takes an object to fall. One can accomplish this with great precision by orienting an optical interferometer so that one of its arms is vertical. If the mirror in that arm is then allowed to fall pan way, a Doppler shift in the reflected light signals its rate of fall. A similar measurement can now be done just as accurately with falling atoms (whose atomic frequencies are Doppler shifted), thanks to a long-term effort by a group at Stanford University. Achim Peters, Keng Yeow Chung, and Steven Chu recently reported that their atom interferometer has determined g to within three parts per billion.

DOI
10.1063/1.882652

Related papers

Atom Interferometers Prove Their Worth in Atomic Measurements

Barbara Goss Levi

Thanks to the wave nature of matter, it's possible to build the atomic analog of an optical interferometer, although it's extremely difficult, because the de Broglie wavelengths of atoms are typically 10 000 times shorter than those of light. The feat was accomplished four years ago by a number of groups, each with a different scheme for splitting and recombining the matter waves. (See PHYSICS TODAY, July 1991, page 17.) We are now starting to see the fulfillment of the hopes raised by those first atom interferometers as researchers hone their interferometers to yield ever more precise values of fundamental constants or explore fundamental aspects of quantum mechanics such as Berry's phase. For example, Steven Chu and his colleagues at Stanford University have measured the ratio h/m of Planck's constant to the mass of a cesium atom with a precision of one part in 107, although the systematic errors are at the level of 1 part in 106. An experiment to improve the absolute accuracy of the h/m measurement to a few parts in 109 is underway. This ratio is one factor in the determination of the finestructure constant.

Physics Today•Jul 1, 1995•6 citations

Precision atom interferometry

Achim Peters, Keng Yeow Chung, Brent C. Young +2 more

The basic physical principles behind atom interferometers based on optical pulses of light are summarized. This method of atom interferometry is based on measurements in the time and frequency domain and is an inherently precise measurement technique. After a brief discussion of some of the important technical requirements for good fringe accuracy and visibility, we describe an interferometer that has measured the acceleration of an atom due to gravity with a resolution better than one part in 1010. We project that the absolute accuracy of our measurement will be of the order of a few parts in 109. We also describe an interferometer experiment that measures the recoil energy shift of an atom when it absorbs a photon. When combined with the value of the Rydberg constant and the mass ratios MCs/mp and mp/me, one can obtain a value for α, the fine structure constant. Currently, we have an experimental resolution Δα/α ∼ 10-8 after two hours of integration time and are studying the systematic effects that affect the measurement.

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Suspension of Atoms Using Optical Pulses, and Application to Gravimetry

K. J. Hughes, John H. Burke, Charles A. Sackett

Atoms from a (87)Rb condensate are suspended against gravity using repeated reflections from a pulsed optical standing wave. Up to 100 reflections are observed, yielding suspension times of over 100 ms. The local gravitational acceleration can be determined from the pulse rate required to achieve suspension. Further, a gravitationally sensitive atom interferometer was implemented using the suspended atoms. This technique could potentially provide a precision measurement of gravity without requiring the atoms to fall a large distance.

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THE APPLICATION OF ATOM INTERFEROMETER TO THE MEASUREMENT OF GRAVITATIONAL ACCELERATION

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This paper has described and reviewed the pinciples,expermental set up and observational results of applying atom interferometer to measuring the earth's gravitational acceleration.This method was developed by Nobelist Steven Chu,M.Kaservich,A.Peters et al.,who demonstrated a resolution of 10 -10 .The application of this technique to geophysics and the related problems are also discussed.

Geophysical and Geochemical Exploration•Jan 1, 2000•0 citations

An atom interferometer for measuring horizontal accelerations

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Matter-wave interferometry has enabled high precision measurements of inertial forces such as gravity and the Coriolis force. This is facilitated by the long-term stability of the physical properties of atoms and lasers. Recent experiments have demonstrated the operation of portable, robust sensors using atom interferometry. This has potential uses in the context of inertial navigation, where conventional devices suffer from long-term drifts due to bias instability. Furthermore, determining position via dead reckoning requires minimisation of dead time between measurements. This thesis presents the development of an atom interferometer for measuring horizontal accelerations. In this configuration, gravity induces motion across the laser wavefront, which constrains the tolerable level of wavefront distortions. Effective control of the experiment allows the interferometer to be operated at a rate of 4 Hz. A cold ensemble of 10^6 atoms in the same internal state is prepared in 150 ms. The interferometer operates using a sequence of three laser pulses separated by T= 25 ms to achieve sensitivity to horizontal accelerations. Combining this with a classical accelerometer provides a method of correcting for vibration-induced noise, as well as determining the interferometer fringe order. After an integration time of 40 s, the sensitivity to horizontal accelerations is better than 3^-6 m s^-2. Effects which limit this sensitivity are discussed.

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A Transportable Gravity Gradiometer Based on Atom Interferometry

Nan Yu, Robert J. Thompson, James R. Kellogg +3 more

A transportable atom interferometer-based gravity gradiometer has been developed at JPL to carry out measurements of Earth's gravity field at ever finer spatial resolutions, and to facilitate high-resolution monitoring of temporal variations in the gravity field from ground- and flight-based platforms. Existing satellite-based gravity missions such as CHAMP and GRACE measure the gravity field via precise monitoring of the motion of the satellites; i.e. the satellites themselves function as test masses. JPL's quantum gravity gradiometer employs a quantum phase measurement technique, similar to that employed in atomic clocks, made possible by recent advances in laser cooling and manipulation of atoms. This measurement technique is based on atomwave interferometry, and individual laser-cooled atoms are used as drag-free test masses. The quantum gravity gradiometer employs two identical atom interferometers as precision accelerometers to measure the difference in gravitational acceleration between two points (Figure 1). By using the same lasers for the manipulation of atoms in both interferometers, the accelerometers have a common reference frame and non-inertial accelerations are effectively rejected as common mode noise in the differential measurement of the gravity gradient. As a result, the dual atom interferometer-based gravity gradiometer allows gravity measurements on a moving platform, while achieving the same long-term stability of the best atomic clocks. In the laboratory-based prototype (Figure 2), the cesium atoms used in each atom interferometer are initially collected and cooled in two separate magneto-optic traps (MOTs). Each MOT, consisting of three orthogonal pairs of counter-propagating laser beams centered on a quadrupole magnetic field, collects up to 10(exp 9) atoms. These atoms are then launched vertically as in an atom fountain by switching off the magnetic field and introducing a slight frequency shift between pairs of lasers to create a moving rest frame for the trapped atoms. While still in this moving-frame molasses, the laser frequencies are further detuned from the atomic resonance (while maintaining this relative frequency shift) to cool the atom cloud's temperature to 2 K or below, corresponding to an rms velocity of less than 2 cm/s. After launch, the cold atoms undergo further state and velocity selection to prepare for atom interferometry. The atom interferometers are then realized using laser-induced stimulated Raman transitions to perform the necessary manipulations of each atom, and the resulting interferometer phase is measured using laser-induced fluorescence for state-normalized detection. More than 20 laser beams with independent controls of frequency, phase, and intensity are required for this measurement sequence. This instrument can facilitate the study of Earth's gravitational field from surface and air vehicles, as well as from space by allowing gravity mapping from a low-cost, single spacecraft mission. In addition, the operation of atom interferometer-based instruments in space offers greater sensitivity than is possible in terrestrial instruments due to the much longer interrogation times available in the microgravity environment. A space-based quantum gravity gradiometer has the potential to achieve sensitivities similar to the GRACE mission at long spatial wavelengths, and will also have resolution similar to GOCE for measurement at shorter length scales.

•May 1, 2010•2 citations
The acceleration due to gravity, g, can be measured simply by timing how long it takes an object to fall. One can accomplish this with great precision by orienting an optical interferometer so that one of its arms is vertical. If the mirror in that arm is then allowed to fall pan way, a Doppler shift in the reflected light signals its rate of fall. A similar measurement can now be done just as accurately with falling atoms (whose atomic frequencies are Doppler shifted), thanks to a long-term effort by a group at Stanford University. Achim Peters, Keng Yeow Chung, and Steven Chu recently reported that their atom interferometer has determined g to within three parts per billion.
Thanks to the wave nature of matter, it's possible to build the atomic analog of an optical interferometer, although it's extremely difficult, because the de Broglie wavelengths of atoms are typically 10 000 times shorter than those of light. The feat was accomplished four years ago by a number of groups, each with a different scheme for splitting and recombining the matter waves. (See PHYSICS TODAY, July 1991, page 17.) We are now starting to see the fulfillment of the hopes raised by those first atom interferometers as researchers hone their interferometers to yield ever more precise values of fundamental constants or explore fundamental aspects of quantum mechanics such as Berry's phase. For example, Steven Chu and his colleagues at Stanford University have measured the ratio h/m of Planck's constant to the mass of a cesium atom with a precision of one part in 107, although the systematic errors are at the level of 1 part in 106. An experiment to improve the absolute accuracy of the h/m measurement to a few parts in 109 is underway. This ratio is one factor in the determination of the finestructure constant.
The basic physical principles behind atom interferometers based on optical pulses of light are summarized. This method of atom interferometry is based on measurements in the time and frequency domain and is an inherently precise measurement technique. After a brief discussion of some of the important technical requirements for good fringe accuracy and visibility, we describe an interferometer that has measured the acceleration of an atom due to gravity with a resolution better than one part in 1010. We project that the absolute accuracy of our measurement will be of the order of a few parts in 109. We also describe an interferometer experiment that measures the recoil energy shift of an atom when it absorbs a photon. When combined with the value of the Rydberg constant and the mass ratios MCs/mp and mp/me, one can obtain a value for α, the fine structure constant. Currently, we have an experimental resolution Δα/α ∼ 10-8 after two hours of integration time and are studying the systematic effects that affect the measurement.
Atoms from a (87)Rb condensate are suspended against gravity using repeated reflections from a pulsed optical standing wave. Up to 100 reflections are observed, yielding suspension times of over 100 ms. The local gravitational acceleration can be determined from the pulse rate required to achieve suspension. Further, a gravitationally sensitive atom interferometer was implemented using the suspended atoms. This technique could potentially provide a precision measurement of gravity without requiring the atoms to fall a large distance.
Matter-wave interferometry has enabled high precision measurements of inertial forces such as gravity and the Coriolis force. This is facilitated by the long-term stability of the physical properties of atoms and lasers. Recent experiments have demonstrated the operation of portable, robust sensors using atom interferometry. This has potential uses in the context of inertial navigation, where conventional devices suffer from long-term drifts due to bias instability. Furthermore, determining position via dead reckoning requires minimisation of dead time between measurements. This thesis presents the development of an atom interferometer for measuring horizontal accelerations. In this configuration, gravity induces motion across the laser wavefront, which constrains the tolerable level of wavefront distortions. Effective control of the experiment allows the interferometer to be operated at a rate of 4 Hz. A cold ensemble of 10^6 atoms in the same internal state is prepared in 150 ms. The interferometer operates using a sequence of three laser pulses separated by T= 25 ms to achieve sensitivity to horizontal accelerations. Combining this with a classical accelerometer provides a method of correcting for vibration-induced noise, as well as determining the interferometer fringe order. After an integration time of 40 s, the sensitivity to horizontal accelerations is better than 3^-6 m s^-2. Effects which limit this sensitivity are discussed.
A transportable atom interferometer-based gravity gradiometer has been developed at JPL to carry out measurements of Earth's gravity field at ever finer spatial resolutions, and to facilitate high-resolution monitoring of temporal variations in the gravity field from ground- and flight-based platforms. Existing satellite-based gravity missions such as CHAMP and GRACE measure the gravity field via precise monitoring of the motion of the satellites; i.e. the satellites themselves function as test masses. JPL's quantum gravity gradiometer employs a quantum phase measurement technique, similar to that employed in atomic clocks, made possible by recent advances in laser cooling and manipulation of atoms. This measurement technique is based on atomwave interferometry, and individual laser-cooled atoms are used as drag-free test masses. The quantum gravity gradiometer employs two identical atom interferometers as precision accelerometers to measure the difference in gravitational acceleration between two points (Figure 1). By using the same lasers for the manipulation of atoms in both interferometers, the accelerometers have a common reference frame and non-inertial accelerations are effectively rejected as common mode noise in the differential measurement of the gravity gradient. As a result, the dual atom interferometer-based gravity gradiometer allows gravity measurements on a moving platform, while achieving the same long-term stability of the best atomic clocks. In the laboratory-based prototype (Figure 2), the cesium atoms used in each atom interferometer are initially collected and cooled in two separate magneto-optic traps (MOTs). Each MOT, consisting of three orthogonal pairs of counter-propagating laser beams centered on a quadrupole magnetic field, collects up to 10(exp 9) atoms. These atoms are then launched vertically as in an atom fountain by switching off the magnetic field and introducing a slight frequency shift between pairs of lasers to create a moving rest frame for the trapped atoms. While still in this moving-frame molasses, the laser frequencies are further detuned from the atomic resonance (while maintaining this relative frequency shift) to cool the atom cloud's temperature to 2 K or below, corresponding to an rms velocity of less than 2 cm/s. After launch, the cold atoms undergo further state and velocity selection to prepare for atom interferometry. The atom interferometers are then realized using laser-induced stimulated Raman transitions to perform the necessary manipulations of each atom, and the resulting interferometer phase is measured using laser-induced fluorescence for state-normalized detection. More than 20 laser beams with independent controls of frequency, phase, and intensity are required for this measurement sequence. This instrument can facilitate the study of Earth's gravitational field from surface and air vehicles, as well as from space by allowing gravity mapping from a low-cost, single spacecraft mission. In addition, the operation of atom interferometer-based instruments in space offers greater sensitivity than is possible in terrestrial instruments due to the much longer interrogation times available in the microgravity environment. A space-based quantum gravity gradiometer has the potential to achieve sensitivities similar to the GRACE mission at long spatial wavelengths, and will also have resolution similar to GOCE for measurement at shorter length scales.