Alain Aspect, a distinguished physicist from the Institut d’Optique Graduate School in France, presented the 41st annual Hanan Rosenthal Memorial Lecture on Monday afternoon.

Speaking to a crowd of over 150 at Sloane Physics Laboratory, Aspect explained his work in quantum optics, the field of physics that studies light particles and their interactions with matter. The world-renowned physicist detailed the decadeslong debate between physicists Niels Bohr and Albert Einstein over the interpretation of quantum mechanics — that quantum mechanics might provide an incomplete description for our physical system and that there exist unknown, “hidden” variables. In the talk, Aspect described his own famous Bell test experiments, which have been influential in resolving the historical debate in Bohr’s favor.

“Professor Aspect’s talk was an inspiration to me because of his crystal-clear explanations, both of the fundamental theory behind a Bell inequality test as well as the experimental steps their lab needed to take to realize it,” Chunyang Ding ’19 said. “He brilliantly captured the Einstein-Bohr debates that led to the initial disputes while laying out how he was able to overcome technical details to actually measure the effect.”

The lecture series honors Hanan Rosenthal, a Yale physics instructor who died in 1971 at the age of 27. Historically, the venue of the talk has alternated between Yale and Columbia — where Rosenthal attended graduate school — but Yale has consistently hosted the lecture in recent years.

Each year, the physics department faculty invites a distinguished leader in Rosenthal’s field of study, atomic physics, said physics professor Nir Navon, who who is currently organizing the Rosenthal Lectures. Past speakers include several Nobel laureates and Newton Medal winners, such as physicists Norman Ramsey, David Wineland and Anton Zeilinger.

“I believe this talk will help me develop new perspectives that I might not have thought of before,” said Shruti Puri, a postdoctoral associate at the Yale Quantum Institute. She added that Aspect’s work serves as the foundation for her research in quantum computing.

Aspect began his lecture by describing the emergence of the field of quantum information and noting that the conceptual discussion of Bohr and Einstein’s debate has spurred a second “quantum revolution.”

He then explained the debate over the completeness of quantum mechanics theory in more detail. This debate has long been thought to exist merely philosophically, but in the mid-1960s, John Bell showed that it was not just a philosophical dispute but, in fact, could be empirically tested.

While Einstein believed there had to be hidden variables not explained by quantum mechanics, Bohr believed otherwise and defended quantum theory as a complete theory.

“Bohr had a deep appreciation that if you accept the complexity of quantum mechanics, you will run into some contradictions,” Aspect said.

The majority of physicists at the time, however, didn’t pay attention to this debate, Aspect said, likely because the argument wasn’t a debate over the validity of quantum predictions, only their interpretation.

Aspect went on to detail Bell’s work to settle the debate experimentally. Bell discovered his now-famous Bell’s theorem, which states that no theory of hidden variables can reproduce all the predictions of quantum mechanics, and Bell’s inequalities, which characterize measurements on pairs of particles that have interacted and separated.

“He showed that you can test ideas that people thought were not testable scientifically, and the beauty of a talk like this is to see how somebody believed in an idea at a time when very few people did — and studied it mostly for the fun of it,” Navon said. “You can achieve beautiful things with hard work if you persevere for long enough, and occasionally, you end up doing things that end up in textbooks and become part of the history of physics.”

Aspect’s own experiments — the “Bell test experiments” — used Bell’s theories and were the first to unambiguously disprove Einstein’s thoery. The tests were one of the hallmark experiments of quantum optics, Navon said.

Aspect also gave a separate talk on Tuesday, speaking to a smaller audience of about 45 professors, postdoctoral associates and students in the physics department.

The first Rosenthal Lecture, in 1973, featured Ugo Fano, an Italian-American physicist who was a pioneer in the study of radiation’s effects on matter.

Amy Xiong | amy.xiong@yale.edu

Correction, Feb. 7: Due to an editing error, the original version of this article said that Nir Navon organized the Rosenthal Lectures in previous years. In fact, Navon is currently organizing the lectures.