The interplay between quantum mechanics and gravity is almost as mysterious today as it was half a century ago despite remarkable progress in either field. Putting aside exotic topics like quantum theories of gravity, even a purely classical quantity, such as Newton’s gravitational constant (or “big G”), has not been measured to a precision and accuracy close to that of other fundamental constants (e.g. speed of light, fine structure constant, etc.). Moreover, until recently [1], most measurements of big G have been classical in nature. Therefore, using a quantum sensor, such as an atomic gradiometer, to perform precision measurements of big G not only adds another (quantum) dimension to said measurement, but also opens a window into studying other fundamental classical gravitational phenomena, like measuring gravitational waves [2]. Furthermore, due to the quantum nature of the sensor apparatus, the interplay of gravity and quantum mechanics can be explored by studying effects such as gravitational decoherence [3]. In this talk, I will discuss ongoing progress on the construction of a 2 meter tall atom interferometer using Strontium. By virtue of the bosonic isotope (Strontium-88) having a zero nuclear spin, the demands on magnetic shielding of the apparatus are relaxed compared to (say) alkali atoms. Once operational over the 2 meter baseline, this atom interferometer will be used to perform short-distance tests (10 centimeters to 1 meter) of gravity using source (or proof) masses in the 100 kilogram range or higher. The distance between the gravitational sensor (i.e. atom interferometer) and the proof masses can be varied using appropriate translation stages. This measurement configuration will allow us to perform precision measurements of big G. [1] Rosi et al. (2014). Nature 510, 518-521. [2] Abe et al. (2021) Quantum Science and Technology 6, 044003. [3] Roura (2020). Physical Review X 10, 021014.
A point source interferometer (PSI) is a device where atoms are split and recombined by applying a temporal sequence of Raman pulses. During the pulse sequence, an initially trapped cloud of cold atoms is released and allowed to expand, behaving approximately as a point source. The PSI can work as a sensitive multi-axes gyroscope that can automatically filter out the signal from accelerations. The phase shift arising from rotations is proportional to the momentum transferred to each atom from the Raman pulses. Therefore, by increasing the momentum transfer, it should be possibly to enhance the sensitivity of the PSI. Here, we investigate the degree of enhancement in sensitivity that could be achieved by augmenting the PSI with large momentum transfer (LMT) employing a sequence of many Raman pulses with alternating directions. Contrary to typical approaches used for describing a PSI, we employ a model under which the motion of the center of mass of each atom is described quantum mechanically. We show how increasing Doppler shifts lead to imperfections, thereby limiting the visibility of the signal fringes, and identify ways to suppress this effect by increasing the effective, two-photon Rabi frequencies of the Raman pulses. Considering the effect of spontaneous emission, we show that for a given value of the one-photon Rabi frequency, there is an optimum value for the number of pulses employed, beyond which the net enhancement in sensitivity begins to decrease. For a one-photon Rabi frequency of 200 MHz, for example, the peak value of the factor of enhancement in sensitivity is ~39, for a momentum transfer that is ~69 times as large as that for a conventional PSI. We also find that this peak value scales as the one-photon Rabi frequency to the power of 4/5.
Atom interferometers in which atoms are delocalized over macroscopic distances can provide excellent sensitivity for gravitational measurements. I will describe progress on the design and construction of a two-meter-tall strontium atomic fountain that aims to leverage such atom interferometers to search for new interactions beyond the standard model via tests of the gravitational inverse square law and to make a new measurement of Newton’s gravitational constant. Moreover, I will discuss progress on MAGIS-100, a 100-meter-tall atom interferometer being built at Fermilab to serve as a prototype gravitational wave detector in the 0.3 – 3 Hz range and to search for ultralight dark matter.
"Laser wavefront perturbations in extreme momentum transfer atom interferometers: effects and mitigation strategies" was recorded at Photonics West 2020 in San Francisco, California.
Light-pulse atom interferometry—which uses optical pulses to split, recombine, and interfere quantum mechanical atomic matter waves—is a sensitive method for measuring inertial forces, making it a valuable tool for a broad set of applications and fundamental physics tests. The sensitivity of an atom interferometer scales with its enclosed spacetime area, which is proportional to the product of the maximum spatial separation reached between the two interferometer paths and the interferometer duration. Motivated by this scaling, we have realized atom interferometers that cover macroscopic scales in space (tens of centimeters) and in time (multiple seconds). I will present experimental results from the implementation of these large area interferometers as high-precision gravitational sensors. Subsequently, I will discuss a new experimental effort to use such gravitational sensors to look for new particles beyond the standard model, including light moduli associated with the compactified extra dimensions that arise in string theory, by searching for deviations from the gravitational inverse square law with improved sensitivity at the length scale of 10 cm to 1 m. This experiment could also provide a new measurement of Newton’s gravitational constant. In addition, large area atom interferometers using atom optics based on single-photon transitions on the clock transition of strontium have the potential to be excellent gravitational wave detectors in the frequency band from 300 mHz to 3 Hz, which is intermediate between the LIGO detector and the planned LISA detector. I will describe ongoing technology development efforts for an atomic gravitational wave detector.
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