Cover image courtesy of ESA: https://www.esa.int/ESA_Multimedia/Images/2007/07/The_Bullet_Cluster2
Article by Olivia Fann and Fernanda Sophia Morais Laroca
On October 17, 2023, the bottom floor of the Science Center was filled with professors and students enjoying refreshments and conversation. It was time for the weekly Physics Colloquium, a public talk given by a visiting scholar on a topic relevant to their research. For this Colloquium, Professor Alex Sushkov gave a talk entitled “The Quantum Limits of Precision Measurements for Fundamental Science.”
Sushkov is an Associate Professor of Physics at Boston University, where his lab develops new quantum tools for precision measurement to address key problems in fundamental and applied science. This includes using precise nuclear magnetic resonance (NMR) techniques to search for dark matter, the subject of his presentation.
What Even is Dark Matter?
Sushkov began with some background. Dark matter makes up 27% of our universe, and it was theorized in the 1930s to explain gravitational forces not accounted for by “normal” matter observed by scientists. Problems that give evidence for the existence of dark matter include galaxy rotation curves, or the observation that past a certain distance threshold, stars rotate around the center of a galaxy at a nearly flat velocity regardless of their distance from the center, as well as observations of the Bullet Cluster.
There are two main particles theorized to make up dark matter: weakly interacting massive particles (WIMPs), which would create a particle-like dark matter, or axions, which would create a wave-like dark matter. Sushkov’s talk focused on the latter.
Axions & the Strong CP Problem
Axions are extremely light theoretical particles proposed to solve the strong-CP problem of quantum chromodynamics. The strong-CP problem, fundamentally, is a question within particle physics of why the electric dipole moment — a measure of a system’s overall polarity — of a neutron is far smaller than what calculation would suggest. The existence of axions would both solve this problem and define dark matter as being made up of axion-like particles.
CASPEr: The Physics Detector, Not the Friendly Ghost
Using nuclear magnetic resonance, Sushkov’s lab built the Cosmic Axion Spin Precession Experiment (CASPEr) to detect axions. The instrument’s sensitivity is limited by noise, keeping it far from the predicted range necessary to detect the axion. Sushkov’s lab has found several ways to mitigate noise. For example, adding a capacitor, a device which stores electrical energy, decreases magnetic field noise; and regulating temperature decreases thermal noise. Currently, CASPEr sensitivity is limited by quantum spin projection noise.
Applications of His Work
While CASPEr was created to research dark matter, Sushkov also highlighted the ways his research could impact our daily lives; specifically, a quantum-limited NMR like CASPEr could provide an improved way to approach magnetic resonance imaging (MRI).
For instance, MRI machines, widely used in medical applications, work by generating a strong magnetic field that spins the protons in our bodies out of equilibrium into a state in which they can be imaged. Since these pulses must be at a safe speed for patients and the magnets required are expensive, current MRI technology is not particularly accessible. Sushkov suggests that NMR technology, which doesn’t require a magnet or pulses, would be faster and cheaper.
Sushkov’s innovative research into dark matter and precision measurement not only expands our understanding of the universe, but also underscores the impact of technological innovation in scientific research. While it may seem like dark matter and its precise measurements are things that only astronomers should care about, these studies can actually lead to revolutions in medicine and other fields that impact our daily lives.