Quantum physics often challenges our understanding of reality, and a new study led by Daniela Angulo from the University of Toronto adds another mind-bending discovery to the mix: photons, the wave-particles of light, can seem to pass through a cloud of atoms in “negative” time, effectively appearing to exit a material before they even enter it.

Aephraim Steinberg, a physicist from the same university, shared this surprising result on X (formerly Twitter), after the team’s findings were uploaded to arXiv.org in early September. While not yet peer-reviewed, the study reveals that photons interacting with a cloud of ultracold rubidium atoms can appear to spend a negative amount of time in the excited state—a phenomenon that defies conventional expectations.

The origins of this research date back to 2017 when Steinberg and Josiah Sinclair, a doctoral student at the time, began investigating how light interacts with matter through atomic excitation. When photons pass through a material and are absorbed, they cause electrons in the atoms to jump to higher energy levels. As the electrons return to their original state, they reemit the absorbed energy as new photons, introducing a delay. The goal of their research was to measure this delay—known as “group delay”—and determine whether it depended on whether the photon had been absorbed or had simply passed through.

What the researchers found was unexpected. During their experiments, photons sometimes passed through the atoms without being absorbed, yet the atoms still entered an excited state, behaving as though they had absorbed the light. Even more puzzling was the finding that when photons were absorbed, they appeared to be reemitted almost instantly, as though the light exited the material before the atomic excitation was over. This led the team to the conclusion that the photons could have a “negative” transit time.

To understand this bizarre behavior, the researchers turned to quantum theory. Collaborating with Howard Wiseman, a quantum physicist from Griffith University, they developed a framework showing that the time photons spend as atomic excitations can vary. In some cases, the photon’s transit through the medium is instantaneous, and in others, it can seem to complete its journey before the atomic excitation has finished, creating a negative time value. In quantum mechanics, events like absorption and reemission do not occur in fixed intervals but rather within a probabilistic range of times.

Sinclair, now at MIT, was taken aback by the results. He explained that photons interacting with atoms can exist in superpositions of different states, meaning they can interact and not interact with atoms at the same time. This strange superposition allows for the possibility of a “negative” time measurement when observing how long photons spend in an excited state.

The follow-up experiment, led by Angulo and praised by Steinberg, involved measuring the difference between photons that were absorbed and those that passed through the cloud unscathed. The results revealed that photons traveled faster when they excited the atoms than when they left them untouched. This apparent faster-than-light behavior does not violate Einstein’s theory of relativity, as no information is being transmitted faster than light.

“A negative time delay might seem paradoxical,” Sinclair explained, “but in this case, it’s like a quantum clock running backward under certain conditions.” While this doesn’t alter our fundamental understanding of time, it demonstrates once again that quantum mechanics holds many surprises yet to be uncovered.

The results of this experiment open new questions about how photons behave in absorptive media and challenge our existing interpretations of group delay in optics. As Steinberg put it, the findings provide a fascinating look into the strange, counterintuitive nature of the quantum world.

By Impact Lab