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Physical Review A

Absolute-phase-shift measurement in a phase-shear-readout atom interferometer

Sitong Yan, Junjie Jiang, Lin Zhou, Yu-Hang Ji, Chuan He, Zhuo Hou, Biao Tang, Xi Chen, Jin Wang, Mingsheng Zhan

Published Dec 19, 2023• 2 citations
Publisher

Abstract

We demonstrate that the shear phase introduced by a tilting Raman beam in a Mach-Zehnder atom interferometer actually shifts the absolute phase, and propose how this shift can be compensated for by reference to a special point of the interference fringe. In the experiment, we obtain shear interference fringes and achieve the extraction of absolute phase corresponding to gravity acceleration. To suppress the impact of position drift on measurement accuracy, we adopt a positive and negative tilt angle alternating measurement (PNTAM) method. After using this PNTAM method, when the position drift leads to a change of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{g}$, the impact of this drift on the absolute measurement value is less than $1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{g}$. This work expands the application range of the phase shear readout based atom interferometer from the current relative or differential gravity measurement to the field of absolute gravity measurement.

DOI
10.1103/physreva.108.063313

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We demonstrate that the shear phase introduced by a tilting Raman beam in a Mach-Zehnder atom interferometer actually shifts the absolute phase, and propose how this shift can be compensated for by reference to a special point of the interference fringe. In the experiment, we obtain shear interference fringes and achieve the extraction of absolute phase corresponding to gravity acceleration. To suppress the impact of position drift on measurement accuracy, we adopt a positive and negative tilt angle alternating measurement (PNTAM) method. After using this PNTAM method, when the position drift leads to a change of 2.2\ifmmode×\else\texttimes\fi10\ensuremath−3g2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{g}2.2\ifmmode×\else\texttimes\fi10\ensuremath−3g, the impact of this drift on the absolute measurement value is less than 1.9\ifmmode×\else\texttimes\fi10\ensuremath−4g1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{g}1.9\ifmmode×\else\texttimes\fi10\ensuremath−4g. This work expands the application range of the phase shear readout based atom interferometer from the current relative or differential gravity measurement to the field of absolute gravity measurement.
The sensitive axis of an atom gravimeter is defined by the direction of the Raman lasers. Any tilt of the Raman lasers with respect to the vertical direction introduces systematic errors in gravity measurements. In this work, we report a determination of the tilt of Raman lasers, where the fringe of the atom interferometer is scanned by varying the tilt, rather than the phase, of the Raman lasers. Unlike the periodic cosine fringes typically used in atom interferometers, the fringe obtained by changing the tilt, referred to as the tilt-scanned fringe, is aperiodic in tilt and exhibits a shift due to the presence of a nonzero tilt. The tilt-scanned fringe is highly sensitive to the tilt-caused shift, enabling convenient and precise determination of the Raman laser tilt in atom gravimeters. We demonstrate that one tilt-scanned fringe, corresponding to a tilt measurement sampling time of 13 s, can determine the tilt with a typical precision of about $30\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{rad}$ in our developed atom gravimeter. The short-term sensitivity and measurement accuracy of the tilt-scanned fringe method were evaluated, demonstrating performance comparable to conventional tilt determination techniques. We expect this method to offer a convenient solution for Raman laser alignment in atom gravimeters and enhance the efficiency of absolute gravity surveys.
The sensitive axes of atom gravimeters are defined by the directions of the respective Raman lasers. Any tilt of the Raman lasers with respect to the vertical direction introduces errors in gravity measurements. In this work, we report a fast determination of the tilt of Raman lasers, where the fringe of the atom interferometer is scanned by varying the tilt, rather than the phase, of the Raman lasers. Unlike the periodic cosine fringes typically used in atom interferometers, the fringe obtained by changing the tilt, referred to as the tilt-scanned fringe, is aperiodic and symmetric with respect to zero tilt. The tilt-scanned fringe is highly sensitive to asymmetries caused by non-zero tilt, enabling fast and precise determination of the Raman laser tilt in atom gravimeters. We demonstrate that one tilt-scanned fringe, corresponding to a measurement cycle time of 13 s, can determine the tilt with a typical precision of about 30 μμμrad in our developed atom gravimeter. Further investigation proves that the tilt-scanned fringe approach shortens the measurement cycle time by over an order of magnitude while keeping comparable precision with conventional tilt determination techniques. The fast tilt determination presented here is significant for the application of atom gravimeters, particularly in absolute gravity surveys.
Gravity gradients represent a major obstacle in high-precision measurements by atom interferometry. Controlling their effects to the required stability and accuracy imposes very stringent requirements on the relative positioning of freely falling atomic clouds, as in the case of precise tests of Einstein's equivalence principle. We demonstrate a new method to exactly compensate the effects introduced by gravity gradients in a Raman-pulse atom interferometer. By shifting the frequency of the Raman lasers during the central π pulse, it is possible to cancel the initial position- and velocity-dependent phase shift produced by gravity gradients. We apply this technique to simultaneous interferometers positioned along the vertical direction and demonstrate a new method for measuring local gravity gradients that does not require precise knowledge of the relative position between the atomic clouds. Based on this method, we also propose an improved scheme to determine the Newtonian gravitational constant G towards the 10 ppm relative uncertainty.
Wavefront distortions are a leading source of systematic uncertainty in light-pulse atom interferometry, limiting absolute measurements of gravitational acceleration at the 30 nm/s^{2} level. Here, we demonstrate in situ spatially resolved measurement of the interferometer phase in a Mach-Zehnder atom interferometer as a tool to characterize and correct wavefront bias. By introducing controllable curvature of the Raman light using an adjustable collimation retro-reflector, we show that the bias due to parabolic wavefront curvature can be measured with 1 mrad uncertainty and that finite-size corrections impact the measured phase curvature. This measurement process could be adopted in optimized atom interferometer gravimeters to reduce wavefront bias uncertainty below the nm/s^{2} level.
We demonstrate a precision measurement of local gravity acceleration g in Wuhan by a compact cold atom interferometer. The atom interferometer is in vertical Mach—Zehnder configuration realized using a π/2 - π - π/2 Raman pulse sequence. Cold atoms were prepared in a magneto-optical trap, launched upward to form an atom fountain, and then coherently manipulated to interfere by stimulated Raman transition. Population signal vs Raman laser phase was recorded as interference fringes, and the local gravity was deduced from the interference signal. We have obtained a resolution of 7 × 10 −9 g after an integration time of 236s under the best vibrational environment conditions. The absolute g value was derived from the chirp rate with a difference of 1.5 × 10 −7 g compared to the gravity reference value. The tidal phenomenon was observed by continuously monitoring the local gravity over 123 h.
We investigated the Raman-pulse-duration effect in a gravity gradiometer with two atom interferometers. Since the two atom clouds in the gradiometer experience different gravitational fields, it is hard to compensate for the Doppler shifts of the two clouds simultaneously by chirping the frequency of a common Raman laser. This leads to an appreciable phase shift. The magnitude of the phase shift relative to the differential phase shift of the two interferometers is in an order of $\ensuremath{\tau}/T$, and cannot be neglected in the precision measurements such as measuring the gravity gradient and the Newtonian gravitational constant.