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Theoretical and Natural Science

Possible Improvements in the Precision of the Atom-interferometric Equivalence Principle Test by Reducing AC Stark Shift Uncertainty

Xinyi Liu

Published May 25, 2023• 0 citations
Publisher

Abstract

We discussed possible improvement methods for a dual-species atom interferometry test of the weak equivalence principle (WEP) at the 10^(-12) level. The original research tested the WEP by comparing the acceleration difference of the free-fallen rubidium isotopes atom clouds. Based on the percent uncertainty presented from this test, AC stark shift is the most significant obstacle to improving precision. This article will discuss a precision-improving method to the original test by suppressing the background AC stark shift by centering the interferometry lasers to their "magic frequency." The expected improved precision of the uncertainty level is below the 10^(-18) level. Due to the limitation of instruments, we have yet to test this improving method in the actual measurement. Still, we analyzed the possible difficulties in practical usage and discussed solving strategies that correspond to the problems.

DOI
10.54254/2753-8818/5/20230576

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We discussed possible improvement methods for a dual-species atom interferometry test of the weak equivalence principle (WEP) at the 10^(-12) level. The original research tested the WEP by comparing the acceleration difference of the free-fallen rubidium isotopes atom clouds. Based on the percent uncertainty presented from this test, AC stark shift is the most significant obstacle to improving precision. This article will discuss a precision-improving method to the original test by suppressing the background AC stark shift by centering the interferometry lasers to their "magic frequency." The expected improved precision of the uncertainty level is below the 10^(-18) level. Due to the limitation of instruments, we have yet to test this improving method in the actual measurement. Still, we analyzed the possible difficulties in practical usage and discussed solving strategies that correspond to the problems.
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The Weak Equivalence Principle (WEP) is a central pillar of general relativity. Its precise test with quantum systems in space offers a unique window onto new physics. Here we report the first in-orbit quantum test of the WEP. A dual-species (85Rb/87Rb) atom interferometer is realized aboard the China Space Station. Methods of platform motion suppression, fluorescence detection switching, and two-photon detuning switching are developed to eliminate phase noise and improve measurement accuracy. A test uncertainty of 2.8*10-8 is obtained from 280 days of WEP test data, and a test result of (-3.1+/-4.6)*10-7 is achieved after error estimation. This improves prior atom-interferometric WEP tests in microgravity by three orders of magnitude. This work paves the way for space-borne quantum inertial sensors and their application to future fundamental physics in space.
The equivalence principle is one of the two basic assumptions of general relativity. It is the extension of weak equivalence principle. At present, a lot of experiments have tested the weak equivalence principle within a certain accuracy. However, the new theories that unify the gravity and the standard model require the weak equivalence principle to be broken, so the highly accurate test of the weak equivalence principle has important scientific significance. The test of the weak equivalence principle using microscopic particles complements and extends that using macroscopic objects. In this paper, the principle of the atomic interferometer is introduced, and the history and status quo of experimental study on weak equivalence principle of microscopic particles using atomic interferometer are reviewed. The precision of experiments using different-mass atoms is improved from 10-7 to 10-8, the precision of experiment using different spin-orientation atoms reaches the level of 10-7, and the precision of quantum test using superposition-state atoms reaches the level of 10-9. The key problems in the weak equivalence principle test using dual-species atom interferometers are summarized. Advances have been made in vibration noise suppression, frequency shift and phase noise suppression of Raman laser, four-wave double-diffraction Raman-transition atom interference, signal detection and data processing. The development of long-baseline atom interferometers for improving the free evolution time of atoms has progressed. The precision of demonstration experiment of weak equivalence principle test using atom interferometers in weightlessness reaches 10-4 level. The space plan for atom interferometer based weak equivalence principle test is also gradually implemented. The test precision of microparticles' weak equivalence principle using long-baseline atom interferometers or space atom interferometers is expected to reach the level of 10-15-10-17 in the future.
In the atom-interferometric test of the weak equivalence principle (WEP) with multicomponent atomic gases, the difference in the center-of-mass positions between different components, as well as the difference in the center-of-mass velocities, leads to systematic uncertainties. An effective way to reduce these systematic uncertainties is to suppress such differences at the preparation stage of atomic sources. In this work, we propose an efficient strategy for preparing two-component gases with a perfectly spatial overlap of the center-of-mass positions and that of the velocities, according to a scheme of the time-optimized atomic lensing mechanism. The key of the time-optimized atomic lensing lies in a two-step quench process of the trap frequency, which is accessible in free-fall experiments by appropriately manipulating the trapping frequency and the center position of the trap. By taking a dual-species rubidium atomic gas as an example, our calculations indicate that the differences in the center-of-mass positions and velocities between 85Rb^{85}\mathrm{Rb}85Rb and 87Rb^{87}\mathrm{Rb}87Rb atoms are simultaneously eliminated, which provides an optimized starting point to test the WEP. An estimation of the systematic uncertainties is also presented. Our method can serve as a potential protocol for the preparation of atom sources in tests of the WEP with multicomponent atomic gases.
In an ideal test of the equivalence principle, the test masses fall in a common inertial frame. A real experiment is affected by gravity gradients, which introduce systematic errors by coupling to initial kinematic differences between the test masses. Here we demonstrate a method that reduces the sensitivity of a dual-species atom interferometer to initial kinematics by using a frequency shift of the mirror pulse to create an effective inertial frame for both atomic species. Using this method, we suppress the gravity-gradient-induced dependence of the differential phase on initial kinematic differences by 2 orders of magnitude and precisely measure these differences. We realize a relative precision of Δg/g≈6×10^{-11} per shot, which improves on the best previous result for a dual-species atom interferometer by more than 3 orders of magnitude. By reducing gravity gradient systematic errors to one part in 10^{13}, these results pave the way for an atomic test of the equivalence principle at an accuracy comparable with state-of-the-art classical tests.