Special Physics Seminar

Precision measurements aiding the search for dark matter and gravitational waves

Nancy Aggarwal

Northwestern University

Low-energy experiments have recently emerged as the next testbeds to investigate big physics questions like (a) Is there physics beyond the standard-model? (b) Do gravitons exist? What is the nature of gravitational forces between quantum systems? (c) What is dark matter made up of? (d) What happened before the dark ages? In this talk, I will focus on precision measurement experiments searching for gravitational waves (GWs) and axions.

Squeezed states are special quantum states of light which have been engineered to redistribute the uncertainty from one observable into the complementary non-commuting observable.  GW observatories like LIGO and VIRGO now deploy squeezed states in regular operation to detect GWs below the standard quantum limit (SQL). The SQL arises from both the photon shot noise as well as from quantum radiation pressure noise. This radiation-pressure interaction can also be used to generate squeezed states, i.e., optomechanical squeezing, but is often challenging to achieve due to presence of thermal motion. I will show recent measurements of optomechanical squeezing in the audio-frequency band at room temperature and describe the design considerations that enabled this squeezer.

In the second part of my talk, I will describe another precision-measurement experiment (ARIADNE) to look for spin-dependent forces mediated by the axion. The axion is a solution to the strong-CP problem (i.e., unnaturally small dipole moment of the neutron) and also happens to be an excellent dark matter candidate. To solve the strong-CP problem, the QCD axion must mediate forces between nucleons. We will test for one such force in the form of a monopole-dipole coupling by looking for interaction between an unpolarized tungsten source mass and highly polarized He-3 gas. This interaction can be quantified as being equivalent to measuring an effective magnetic field of 10^-20 T at 100 Hz. I will describe the experiment’s concept, a subset of the technical challenges that must be overcome to look for this effect, and recent progress towards realizing the experiment.

While these experiments are well-motivated to answer fundamental science questions in the short term, they also pave the way for future, even more challenging precision measurement experiments. I will show snapshot ideas of a few such future experiments.