Marina Maciel Ansanelli probes the reality underlying quantum theory

There is a Richard Feynman quote about quantum mechanics that Marina Maciel Ansanelli strongly disagrees with.

At a lecture given at Cornell University in 1964, after explaining that at least a few scientists now understood the Theory of Relativity, Feynman joked that “on the other hand, I think I can safely say that nobody understands quantum mechanics.” In the recorded lecture, you can hear the crowd laugh, agreeing with Feynman.

Ansanelli, a PhD candidate at Perimeter Institute who studies quantum foundations, says Feynman’s quote gives physicists an easy out, allowing them to use quantum theory in their work without having to delve into the still murky waters of how the theory works and what it says about our world.

“We know how to use the machinery of the theory as a recipe to make predictions about what we are going to see, but we do not know what the theory says about reality,” Ansanelli explains. It is exactly this quest to understand the underlying reality of quantum theory that brought Ansanelli to Perimeter and drives her research.

When Ansanelli began studying quantum theory as an undergraduate at the University of São Paulo in Brazil, the general approach to the theory in the classroom was “‘this is a very confusing theory, and you are going to learn how to do the calculations to make predictions, but do not ask why this is like this.’”

For Ansanelli, however, this was not enough. Even as a child, she was never satisfied with answers that expected her to just memorize statements and facts. “I remember saying, ‘why is it this? Why is it that? Why is it not this?’” she says. In many ways, going into theoretical physics was inevitable. “This is where I could see this forever quest to know why things are the way they are.”

Ansanelli works in an area of theoretical physics called quantum foundations, which seeks to understand quantum theory, including the fundamental aspects that distinguish quantum physics from classical physics. Within quantum foundations, some researchers investigate the advantage gap that is observed when we look at a problem using a classical approach versus a quantum approach.

To explain this gap, Ansanelli uses Bell’s theorem, which was proposed by the physicist John Stewart Bell in 1964. In very simplified terms, Bell’s theorem showed that a simple task could only be solved 75 percent of the time when using a classical strategy, but when a quantum strategy is used, it could be solved successfully 85 percent of the time (different formulations of Bell’s theorem arrive at different percentages).

“There are tasks you can perform better in the quantum world,” Ansanelli says, adding that there are a number of real-world applications where this advantage gap comes into play. Quantum computers, for instance, use principles of quantum mechanics to perform certain calculations orders of magnitude faster than classic computers.

Returning to Bell’s theorem, Ansanelli explains that in the 2010s, researchers at Perimeter Institute realized that the theorem could be reformulated as a statement for causal inference, which investigates the cause behind an observed behaviour. If you take the simple task from Bell’s theorem and use the most natural classical causal structure, you can only explain the 75 percent value, but it is impossible to explain the 85 percent value.

“This shows us that, if we want to causally explain the predictions of quantum theory, we need to either change the causal structure or change our whole framework of causality,” Ansanelli says.

The result is quantum causal inference, an emerging field in quantum foundations that is the focus of Ansanelli’s work. Much like quantum versus classic computers, quantum causal inference also has certain advantages over classical causal inference, allowing researchers to identify causes and interpret experimental results that are impossible to explain with classical causal inference.

Ansanelli, however, notes that the benefits of quantum causal inference go beyond its potential applications: it can also help physicists probe the “why” behind quantum’s strange behaviours. “Understanding quantum causal inference brings us one step closer to understanding what quantum theory says about reality.”

Ansanelli first learned about quantum foundations at Perimeter in 2019, when she participated in the PSI Start Program, a summer school for undergraduates who are interested in applying to Perimeter’s master’s program. It was during the second half of this two-week program that Ansanelli learned about the basic concepts of quantum foundations, and quickly fell in love with the field. “I was like ‘wow, I need to do this. There is no way I am doing anything else.’”

Falling for quantum foundations was inevitable, Ansanelli explains. “It is basically the area where you ask all those questions that, during undergrad, people said ‘this is complicated, let’s just sweep this under the rug.’”

Ansanelli came back to Perimeter for her master’s in 2020, and she has continued at Perimeter for her PhD with Robert Spekkens, a leading quantum foundations researcher, as her advisor. While her undergraduate physics experience was focused on coursework and getting a solid understanding of what’s already known in physics, as a PhD researcher she now has a chance to ask all those “why” questions about quantum theory that will push the field forward.

“I always thought about research as asking every question possible until you understand everything about a specific problem,” she says.

There are still physicists that adopt an approach that Ansanelli calls “shut up and calculate,” emphasizing using quantum theory to explore and push forward technological advances, and leaving foundational questions by the wayside. But for her, that is impossible, not just because of her obsession with “why,” but also because understanding how quantum theory works can only speed up future development in the field and its potential applications. “If you understand how reality works, this can lead us to progress in the long run.”