Detection of gravitational waves and dark matter using atom interferometry

by Dr Tejas Deshpande (Northwestern)

Wednesday 14 Sept 2022, 15:00 → 16:00 Europe/London

Barkla lecture theatre (Liverpool Physics)

Precision measurements have recently garnered a lot of interest in studies of fundamental physics. Some of the reasons for this interest are the successful terrestrial detection of gravitational waves (GWs) and developments in the optical atomic clock (OAC) technology, to name a few. Contrary to the traditional paradigm of exploring new physics at high energies (> 10 TeV), precision measurements instead focus on studying low-energy signatures of phenomena which lie beyond the scope of the standard model of particle physics. Some examples of such novel physics include exploring: (1) microscopic properties of dark matter (DM) and energy, (2), quantum mechanics (QM) at macroscopic scales, (3) gravity at microscopic scales, and (4) a combination of (2) and (3).

In this talk, I will present our work on the construction of a 100 meter tall atom interferometer (AI) prototype, currently under construction at Fermilab, which will serve as a testbed for potential future kilometer-scale GW and DM detectors. Crudely, the operating principle of this AI, in the GW (and scalar DM) detection mode, can be thought of as using a pair of vertically separated OACs (Strontium atoms) to measure changes in the light propagation time, Δt = ΔL/c, due to GW-induced strain, across a baseline of length L (0.1-1 kilometer), where c is the speed of light. This instrument, known as Matter-wave Atomic Gradiometer Interferometric Sensor (MAGIS), derives its sensitivity to GWs in its target frequency band (0.03-3 Hz) due to the fact that the atoms, which comprise the OAC, are under free fall; thus effectively decoupling them from seismic noise. Furthermore, the long interrogation times (> 8 s) associated with the long baseline, L, allow exploration of scalar and vector DM, with Compton frequencies in the 0.03-3 Hz range, to unprecedented sensitivity levels. My talk will focus on our work, at Northwestern University (NU), in developing a versatile laser system capable of operating MAGIS in the GW and DM detection modes. I will present the design and performance of our dual-Titanium:Sapphire laser system with the following features: (1) frequency-agile (2 GHz/s) coherent laser power combination up to 7 Watts (GW and scalar DM modes), (2) independent frequency jumps (per laser) on the order of 30 GHz/s (vector DM mode), and (3) Hz-level linewidth derived from a frequency comb locked to an ultrastable cavity. Moreover, I will discuss techniques such as fiber noise cancellation, free-space laser transport, and laser wavefront aberration mitigation, which will minimize the susceptibility of the laser beam, as seen by the atoms, to environmental disturbances and system imperfections.

I will conclude the talk with a brief discussion of two other experiments involving precision measurements with AI and optical interferometry currently under construction at NU. The former involves a 2 meter tall AI which will be used to perform tests of gravity and QM on length scales of the order of 10 centimeters to a meter. The immediate goals of this quantum sensor will be performing tests of the gravitational inverse square law and measurement of Newton’s gravitational constant (“big G”). The latter involves measurement of differential length change between two ultrastable tabletop cryogenic optical cavities to detect ultralight scalar DM by studying variation of fundamental “constants” like the electron mass.