Session Q05: Atom Interferometry I

Probing gravity with trapped atoms: the optical lattice atom interferometer

Atom interferometers are quantum mechanical devices sensitive to gravitational and inertial forces, with applications in fundamental physics and inertial sensing in the field. 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 mechanical and acoustic vibrations. Our experiment probes gravitational potentials by holding, rather than dropping, atoms. We realize an interrogation time of up to 25 seconds by suspending the spatially separated atomic wave packets in an optical lattice mode-filtered by an optical cavity. This trapped geometry suppresses phase variance due to vibrations by three to four orders of magnitude, overcoming the dominant noise source in atom-interferometric gravimeters. We describe recent progress in characterizing and reducing decoherence of the interferometer. An upgraded optical lattice interferometer experiment is currently being commissioned, with the goal of increased sensitivity to gravity.   

Observation of a gravitational Aharonov-Bohm effect

Gravity curves space and time.  This can lead to proper time differences between freely falling, nonlocal trajectories.  A spatial superposition of a massive particle is predicted to be sensitive to this effect. We measure the gravitational phase shift induced in a matter-wave interferometer by a kg-scale source mass close to one of the wave packets.  Deflections of each interferometer arm due to the source mass are independently measured. The phase shift deviates from the deflection-induced phase contribution, as predicted by quantum mechanics. In addition, the observed scaling of the phase shift is consistent with Heisenberg's error-disturbance relation.  These results show that gravity creates Aharonov-Bohm phase shifts analogous to those produced by electromagnetic interactions.    

401 $\hbar k$ Atom Interferometry with Floquet Atom Optics

To overcome efficiency limitations due to differential Doppler shifts in large momentum transfer atom interferometry, we implement a periodic atom-light coupling to realize Floquet atom optics. Using the strontium $^1S_0$-$^3P_1$ transition, we use this technique to demonstrate state-of-the-art atom interferometers with momentum separation of 401 $\hbar k$. These Floquet atom optics enable pulse efficiencies greater than 99$\%$ for all Doppler detunings, even under strong driving where the detuning is on the order of the Rabi frequency, and result in symmetric evolution of the two arms of the interferometer. This technique is generally applicable to systems with multiple states with discrete detunings, such as multi-isotope interferometers. Furthermore, this work paves the way for even more sensitive clock atom interferometry with larger momentum transfer, a key ingredient in proposals for gravitational wave detection and dark matter searches.   

Towards levitated, macroscopic-scale atom interferometry with strontium for precision gravity gradiometry

Light-pulse atom interferometry is a versatile and powerful tool for conducting precise measurements of fundamental constants, testing general relativity, searching for signatures of new physics, and investigating quantum mechanics on a macroscopic scale. For atom interferometry, pulses of light are used to create the atom optics equivalents of beam-splitters and mirrors. Recent advances in atomic clocks have illustrated the advantages of using strontium, an alkali-earth atom, over the typically used alkali atoms due to its decreased sensitivity to backgrounds such as magnetic fields. We present progress toward the realization of a two-meter atomic fountain at Northwestern University that will be used to develop atom interferometry with large spacetime areas and long interrogation times by levitating the atoms using optical lattices. Initial interferometry will be performed using sequential Bragg transitions for the atom optics pulses. Large spatial separations are enabled in part by spectral engineering of the atom optics beams to compensate for intensity-dependent phase shifts. The two-meter fountain will be used for precision gravitational measurements such as a measurement of the gravitational constant G and for a precise test of the inverse-square law for gravity.   

Quantum clock interferometry and violations of the equivalence principle

Atom interferometers and atomic cocks are both based on the interference of different degrees of freedom: the atom's motion or their internal state. A combination of the two gives rise to quantum clock interferometry. While clocks are naturally susceptible to violations of the gravitational redshift, atom interferometers constitute a sensor for violations of the universality of free fall. However, quantum clock interferometry has been shown to only test the redshift in specific configurations. In our contribution, we explore the origin of a sensitivity to gravitational redshift violations in both interference experiments and characterize their underlying limitations: The action of trapping potential leads to such a sensitivity of atomic clocks. Due to the linear nature of momentum kicks, Bragg-based interferometers on the other hand are insensitive. However, modified light-pulse configurations can mimic such tests. Internal transitions during the interferometer sequence provide an additional lever to generate redshift-sensitive quantum sensors.   

Coupling of gravity and dilaton backgrounds to light fields and atoms

The search for violations of the Einstein equivalence principle and dark matter is a driving force for atom interferometery. A scalar, light dilaton field constitutes such a basic but consistent extension to known physics. While recent works focus on the coupling of matter to gravity and dilaton fields, we include the propagation of the light essential to manipulate the atoms. In particular, we derive modified Maxwell equations including an expanded gravitational and dilaton field. We show that to leading order the dilaton has no influence on the phase of the electromagnetic field, and only modifies the wave vector via gravity. We transfer this result to various classes of atom interferometers and show that the coupling to the dilaton field is solely given via the atom's mass, whereas the modified light propagation also enters via gravity.   

MAGIS-100 Environmental Noise and Gravity Gradient Noise

To inform research and development for the design and commissioning of the MAGIS-100 atom interferometer at Fermilab a through understanding of the noise sources associated with the environment is required. Considerations of temperature, humidity, and vibration fluctuations arise when moving from the controlled environment of a laboratory to an underground access shaft. To better understand these effects we surveyed the environment measuring the temperature above and below ground as well as measuring and analyzing the vibration spectrum of the installation site. We also studied the seismic vibration effect of gravity gradient noise -- a limiting noise source for our science goals of ultralight scalar dark matter searches and gravitational wave measurements -- which was modeled using input from a low-noise seismometer at multiple locations, and devised a potential mitigation scheme studied through stocastic simulation.   

Methods for atom interferometry with dual-species BEC in space

Atom interferometry is a promising tool for precise measurements, e.g. for measurements of the gravitational and fine structure constant or quantum tests of the weak equivalence principle. As the sensitivity scales with the squared interrogation time, conducting these experiments in microgravity is of great interest. Same holds for using Bose-Einstein-Condensates (BEC) because of the lower expansion velocity. The sounding rocket mission MAIUS-1 demonstrated the first creation of a BEC and matter wave interferences in space [1,2]. With the follow-up missions MAIUS-2 and -3, we extend the apparatus by another species to perform atom interferometry with Rb-87 and K-41 paving the way for implementing and testing the methods of dual-species interferometers on board of space stations or satellites.

In this contribution, the manipulation of BECs using Raman double-diffraction processes and their application to form (asymmetric) Mach-Zehnder-type interferometers, e.g. for inertial sensing, are presented for a compact, robust, and autonomously operating setup that generates Rb-87 and K-41 BECs with a high repetition rate.

[1] D. Becker, et al., Nature 562, 391-395 (2018).

[2] M.D. Lachmann, H. Ahlers, et al., Ultracold atom interferometry in space. Nat Commun 12, 1317 (2021).   

QMATCH: Quantum Measurement of Atomic Charge

The electric charge neutrality of atoms and bulk matter is an experimental fact established through classical measurements at the 10^-21e/nucleon level.  I will present the design of a new experiment, QMATCH: Quantum Measurement of Atomic Charge, which will use atom interferometry to test the charge neutrality of atoms at an unprecedented level using the scalar Aharonov-Bohm effect.  The estimated sensitivity per shot is expected to exceed current bounds by more that a factor of ten.  I will discuss sources of systematic error and the differential measurement strategies we will use to control them.  I will also describe the design and construction of the high voltage in-vacuum electrodes needed for the experiment.