Session G07: Tests and Measurements of Gravity

Live

The universal law of gravitation has undergone stringent tests for many decades over a significant range of length scales, from atomic to planetary. Of particular interest is the short distance regime, where modifications to Newtonian gravity may arise from axion-like particles or extra dimensions. We have constructed an ultra-sensitive force sensor based on optically-levitated microspheres with a force sensitivity of $10^{-17}$ N/$\sqrt{\rm Hz}$ for the purpose of investigating non-Newtonian forces in the 1-100 $\mu$m range. Microspheres interact with a variable-density attractor mass made by alternating silicon and gold segments with periodicity of 50 $\mu$m. The attractor can be located as close as 10 $\mu$m from a microsphere. I describe the characterization of this system, its sensitivity, and some preliminary results. Further technological developments to reduce background are expected to provide orders of magnitude improvement in the sensitivity, probing beyond current constraints on non-Newtonian interactions.   

Live

Under conditions where the total angular momentum of a ferromagnet is dominated by its intrinsic spin, the ferromagnet is predicted to behave as a gyroscope [Kimball, Sushkov, and Budker, Phys. Rev. Lett. {\textbf{116}}, 190801 (2016)]. If such a ferromagnetic gyroscope (FG) can be sufficiently isolated from the environment, it has the potential to measure spin-dependent interactions with a sensitivity far surpassing that of other systems [Band, Avishai, and Shnirman, Phys. Rev. Lett. {\textbf{121}}, 160801 (2018)]. The high sensitivity is the result of rapid averaging of quantum noise. We propose to use a mm-scale FG in orbit around the Earth to investigate physics at the intersection between quantum mechanics and general relativity by measuring relativistic frame dragging (the Lense-Thirring effect) with intrinsic spin. The behavior of intrinsic spin in spacetime dragged by a massive rotating body is an experimentally open question, hence the results of such a measurement may have important theoretical consequences.   

Live

Precision frequency metrology is a powerful tool to set stringent limits on physics beyond the Standard Model such as a possible mass-dependent fifth force. Thanks to rapid advances on both the experimental and theoretical fronts, ultracold diatomic molecules are promising systems to probe short-range gravity. In this work, we study $^{88}$Sr$_2$ molecules, which we produce by photoassociation and trap in an optical lattice. Using several quantum-optical techniques such as Autler-Townes spectroscopy, we have mapped out parts of the molecular potentials. This allowed us to find magic wavelengths for the optical lattice, where the dynamic polarizabilities of two vibrational states are equalized. Thereby, inhomogeneous line broadening was reduced so that our molecular clock achieved a Q-value of $\nu/\delta \nu = 8 \cdot 10^{11}$. Currently, we are working on improving the setup to achieve even lower linewidths, toward a goal of probing the full ground state molecular potential of several isotopologues of Sr$_2$. These measurements should allow us to set new limits on the existence of a mass-dependent fifth force.   

Live

Here we present an atom interferometer which will measure vertical gravity gradients using a single laser diode, while mounted on an 6-rotor unmanned aerial vehicle (UAV). We will measure gravity gradients by ellipse-fitting the phases from two identical, vertically displaced interferometers in the same vacuum chamber with common Raman lasers and trapping beam. Each interferometer will have a baseline of $T=150$ ms, displaced $1$ m relative to each other. We propose a novel mechanism to generate two magneto-optical traps (MOTs) with a single incoming beam using a diffraction grating inspired by previous grating MOTs. The entire science payload, including vacuum chamber, optics, and all control electronics, is budgeted to weigh $<70$ kg, and the aircraft is rated to carry $110$ kg for 30 minutes.   

Live

The question of which atom-interferometer geometries can be used to test the universality of the gravitational redshift (UGR) is at the center of a long-standing debate. We compare in this contribution classical UGR tests based on the synchronization of two atomic clocks with atom interferometers relying on i) quantum clock interference [1] or ii) internal state transitions during the interferometer [2,3]. For this purpose, we introduce a dilaton model which consistently parametrizes violations of the Einstein equivalence principle. Based on this model, we derive the corresponding phase shifts for atomic clocks and atom interferometers and study their differences. Consequently, we identify a large class of atom-interferometer geometries which measure violations of UGR.\\ {[1] Nat. Commun. \textbf{2}, 505 (2011)}$~~~~~$ [2] arXiv:1810.06744 (2018)\\ {[3] arXiv:2001.09754 (2020)}   

Live

Atom interferometers are powerful tools for fundamental physics and inertial sensing in the field. However, their performance is currently limited by the interrogation time available to freely falling atoms in Earth’s gravitational field, as well as noise due to vibrations. Our experiment probes gravitational potentials by holding, rather than dropping, atoms. We realize an interrogation time of 20 seconds by suspending the spatially separated atomic wave packets in an optical lattice. This record coherence is enabled by the smooth lattice wave fronts, which are mode-filtered by an optical cavity. This trapped geometry suppresses phase variance due to vibrations by three to four orders of magnitude, overcoming a dominant noise source in atom-interferometric gravimeters. We describe recent progress in characterizing and reducing dephasing of the interferometer, with the goal of increased spatial separation between the interferometer wave packets, as well as prospects for improved detection using the coupling of the atoms to the optical cavity.   

Live

Studies of fundamental physics, using light-pulse atom interferometers (AIs), have been proposed for performing tests of the equivalence principle, dark matter searches, and gravitational wave detection. Moreover, such AIs enable tabletop tests of quantum mechanics over macroscopic length scales where gravitational curvature is non-negligible. For example, meter-scale quantum superposition has been demonstrated in a 10 m $^{87}$Rb atomic fountain [1]. Building on techniques developed in [1], we are constructing a $^{87}$Sr gravity gradiometer, with the goal of performing gravitational measurements of well-controlled terrestrial sources. The detector and source(s) in our experimental setup are a stationary 2 m $^{87}$Sr AI and high-purity mobile proof mass(es) respectively. The advantages of this apparatus compared to $^{87}$Rb are: (a) lower sensitivity of $^{87}$Sr to ambient magnetic fields, (b) compact vacuum system allowing up to 10 cm separation between the proof mass and the AI. In this talk, we will provide an overview the design and progress on implementation of the various sub-systems of our $^{87}$Sr AI such as lasers, atom sources, ultrahigh vacuum setup, and the atom imaging scheme.\\[1] Kovachy et al. Nature 528, 530(2015); Asenbaum et al. PRL 118, 183602 (2017).   

Not Participating

Recently, quantum-clock interferometry [1] was suggested as a means to measure relativistic effects on the center-of-mass motion in superpositions of internal states. Such proposals combine high-precision quantum metrological techniques to study the fundamental interconnections between relativity and quantum mechanics. Complementary to tests of relativity by the comparison of atomic clocks/frequency standards, quantum-clock interferometry allows for the test of special- and general-relativistic effects with a single but delocalized quantum object. However, the sensitivity with respect to relativistic effects crucially depends [2,3] on the geometry and the specific details of the beamsplitting processes. In our contribution we investigate, elaborate and clarify the link between typical beamsplitting processes in the proposed schemes and the sensitivity to relativistic effects.\\[0.25ex] {\bfseries References} - {\bfseries [1]} A.~Roura,~arXiv 1810.06744, (2018), {\bfseries [2]} S.~Loriani, A.~Friedrich et al.,~Sci.~Adv.~Vol.~5,~no.~10,~eaax8966 (2019),~ {\bfseries [3]} C.~Ufrecht, F.~D.~Pumpo, A.~Friedrich, et al.,~arXiv 2001.09754 (2020)   

Not Participating

The equivalence principle, which states that gravitational effects cannot be observed in any local experiment, is the foundation of our understanding of gravity. Its validity has been tested to high precision with torsion balances, lunar laser ranging, and a free-fall measurement in space. Here we present the results of an atom-interferometric test of the equivalence principle between $^{85}$Rb and $^{87}$Rb. We demonstrate a relative precision of $2 \times 10^{-11}$ per shot by using a long drift time $T \sim 1$ s and a large momentum transfer of up to 12 $\hbar k$. Our results provide general constraints on extensions of the Standard Model and on alternative theories of gravity.