A research team from the University of Toronto, led by CQIQC member Aephraim M. Steinberg, has published findings that have prompted significant interest —and some confusion— across the global press and audiences fascinated by scientific breakthroughs in quantum sciences. The experiments, conducted by Daniela Angulo alongside Kyle Thompson, Vida-Michelle Nixon, and Andy Jiao, with theoretical support from Howard M. Wiseman of Griffith University, do not claim that photons travel backward in time or that time machines are on the horizon, as some press outlets have written. Instead, they offer new insights into a long-known phenomenon in quantum physics: negative group delay.

The study, originally released in September 2024 as a preprint on arXiv, has since been covered in multiple languages, including English, French, and Spanish, by outlets ranging from Phys.org to the BBC and National Geographic France. Yet despite the attention, Steinberg is careful to point out that many popular interpretations have misrepresented the actual findings. To this respect, he mentioned that what the team has done in their experiments has proved a bit hard to explain: “It is obvious that this work caught the imagination of journalists and their readers because of the phrase ‘negative time,’ but in particular because of ways that phrase could be interpreted which do not actually have anything to do with our experiment.”

The team’s work builds on the well-established theoretical idea that photons, when interacting with specific materials at certain frequencies, seem to exit such materials before entering them. Steinberg and his collaborators, however, took a different approach: “What we are actually studying is what happens when a single photon travels through a cloud of atoms,” he explained. “Physicists often describe light slowing down in matter as photons being temporarily ‘trapped’ inside atoms. But this had never been directly measured.” The team devised a method to measure the amount of time atoms spend in excited states as photons pass through. “We also realized that if we tuned our light just to the atomic resonance, something would seem screwy about the above story: right on resonance, the ‘average delay’ for a transmitted photon is actually negative.  That has been known theoretically for over a century, and the fact that it is an accurate description of the propagation of a light pulse has been understood and experimentally tested in multiple situations since the 1980s –but our understanding of it amounts to suggesting that this negative ‘time delay’ is a bit of an illusion, some funny reshaping of a probability distribution, but not a physical duration,” Steinberg explains. 

The work of the team in the lab got surprising results: “even when this ‘group delay’ is negative, it remains a good description of our measurement: even though we are not measuring the time at which photons leave the medium, but instead a probe of how much time atoms spend excited, we get a negative number.” However, to avoid any misinterpretations related to negative time, Steinberg quickly clarifies: “of course it makes no sense to claim that an atom is ‘spending minus five minutes’ doing something, but we find that the physical effect one can use as an indirect measurement of this time actually picks up a minus sign.” This suggests that negative group delay reflects something physically measurable that could describe “other down-to-earth physical effects.” As Steinberg puts it, “we need to take these negative quantities more seriously, and think about what other physical effects they may govern, and what that means. But sadly, not that we can go back in time and fix the mistakes we made in the past!”

While pleased by the international attention, the team remains cautious about sensational headlines: “Sometimes the distinctions we make in science can be a bit subtle, and it is difficult to maintain precision while also being accessible,” said Steinberg. “We regret when articles appear which make it seem as though we are either claiming credit for work others did many years ago, or suggesting that ‘physics is broken.’” Still, he emphasizes that the effort to engage the public is worthwhile. “We are very grateful to the journalists who have put so much effort into getting the story right, [providing a balance] between accessibility and precision and hope that most of the scientifically interested public will end up seeing some of the more accurate articles written about this!”