Quantum All-Star: Mikael Backlund
What is the most accurate measurement a scientist can possibly make?
Even if an experiment is perfectly designed and perfect equipment is used, a researcher’s ability to take measurements is restricted by quantum uncertainty. Knowing one characteristic of a system with high precision implies that knowledge of other characteristics is lost, putting fundamental limitations on experimental accuracy.
IQUIST’s Mikael Backlund wants to know exactly what those limitations are. A professor of chemistry, he aims to push optical microscopy and magnetic sensing to the highest levels of accuracy allowed by quantum physics. If he succeeds, the resulting techniques would allow individual molecules and atoms to be characterized, giving unprecedented insights into chemical and biological processes. To understand how precise these techniques can be, Backlund uses the methods of quantum information science to put limits on their accuracy, and then he designs experiments to achieve those accuracies.
He explained that this is a natural extension of the theoretical analyses now central to classical microscopy. “Over the past 20 or so years, we’ve realized that statistics and information theory can tell you what the best way to detect light with a microscope is,” he said. “But you can take a step removed from that and ask, ‘How can I optimize the light before deciding how I’m going to detect it?’ And then it basically becomes a quantum information problem.”
An experimental chemist doing theoretical physics
Backlund is trained as a physical chemist, having earned a bachelor’s degree from the University of California, Berkeley and a doctorate from Stanford University. However, he has found himself drifting towards more physics-related topics over the course of his research career. He recalls that the drift began during his graduate work in a single-molecule microscopy research group.
“It was chemistry, but there was also lots of physics and lots of opportunities to do math, both pen-and-paper and computational,” he said. “But it was especially interesting because only half the group members were chemists, and the rest were people from physics, electrical engineering, and applied math. A lot of my training came from people with those backgrounds.”
He began performing research to place limitations on how accurately single molecules can be resolved using classical microscopy. Then, when he was a postdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics, other researchers published a paper demonstrating how quantum information theory could be applied to surpass the classical limitations of optical microscopes. Backlund then began to incorporate quantum techniques into his own calculations. During this time, he also gained experience in experimental quantum sensing.
These new areas of interest led him to the University of Illinois Urbana-Champaign because it was “the best place to do the kind of science [he] wanted to.” In addition to a chemistry department world-renowned for physical chemistry research, the university had IQUIST, affording him opportunities to collaborate with physicists, engineers, and mathematicians as he applied quantum information science to improve chemistry experiments.
Resolving single molecules: quantum 2.0
Backlund’s research has two main thrusts. The first is in single-molecule microscopy—the use of light emitted by molecules to determine their characteristics. He and his research group are building on his graduate work in classical microscopy techniques to incorporate the quantum nature of light.
“It’s like quantum 1.0 versus quantum 2.0,” Backlund said. “Before, we were doing quantum in the sense that we were looking at light from the quantum transitions in molecules, but we didn’t think about the light’s quantum features.”
His research group is experimental, but he and his students often conduct theoretical analyses to support their single-molecule microscopy experiments. Most recently, Backlund and graduate student Cheyenne Mitchell published an analysis describing how accurately two molecules can be distinguished based on their fluorescence lifetimes—how long they emit light after being injected with energy. They will follow up with a second analysis that shows how the standard quantum information technique of multi-photon interferometry can be used to super-resolve the two lifetimes, making the method “less quantum-inspired and more just quantum,” according to Backlund.
NV diamonds: The best of both worlds
The second thrust is in magnetic resonance imaging with nitrogen vacancy diamond—a chemically modified version of diamond in which two carbon atoms are replaced with a nitrogen atom and a gap, or “vacancy,” in the crystal structure. Such “NV” diamonds act as quantum systems capable of detecting magnetic fields with extraordinarily high precision, allowing molecular information to be resolved by the magnetic behaviors of atomic nuclei.
“It would take the best of both worlds, and it would be great for materials and molecules where you want to know the atomic-scale detail and there’s enough variation that the interesting stuff gets lost in averaging.”
--Prof. Mikael Backlund
The second thrust is in magnetic resonance imaging with nitrogen vacancy diamond—a chemically modified version of diamond in which two carbon atoms are replaced with a nitrogen atom and a gap, or “vacancy,” in the crystal structure. Such “NV” diamonds act as quantum systems capable of detecting magnetic fields with extraordinarily high precision, allowing molecular information to be resolved by the magnetic behaviors of atomic nuclei.
“NV diamond sensing would triangulate the capabilities of these two really important measurement tools,” Backlund said. “It would take the best of both worlds, and it would be great for materials and molecules where you want to know the atomic-scale detail and there’s enough variation that the interesting stuff gets lost in averaging.”
As one example, his group is currently using NV diamond sensing to study a phenomenon that has attracted attention from physical chemists for some time: phase transitions in thin polymer films. The physical properties of these materials display nanoscale variations, and they are often missed by existing experimental techniques. Backlund’s group is working to leverage the sensitivity of NV diamond sensors to parse this inherent inhomogeneity.
Don't be afraid to try something new
Looking back on his research career, Backlund believes it was his broad exposure to several different areas that allowed him to bring fresh perspectives to both physical chemistry and quantum information science. Moreover, he believes his decision to try something outside his field led to an interesting and unique career direction.
“My advice to people thinking of trying a new research area is to not be afraid,” he said. “People should be encouraged to jump into new areas because that’s how you keep science interesting. You need people with different perspectives jumping into a new field and applying their mode of thinking. In my experience, it’s hard, but it pays off.”
He believes that quantum information science is an especially exciting area to jump into. “Quantum information science is just so inherently weird and interesting that it’s very fun to think about on a daily basis,” he said. “Also, there’s a lot of excitement around it that lends energy to feed from. It’s nice to be anchored to an active community that’s not just here, but across all research institutions.”
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This story was published June 6, 2023.