By Manon Bischoff, Jeanna Bryner | Scientific American
Quantum physicists are familiar with wonky, seemingly nonsensical phenomena: atoms and molecules sometimes act as particles, sometimes as waves; particles can be connected to one another by a “spooky action at a distance,” even over great distances; and quantum objects can detach themselves from their properties like the Cheshire Cat from Alice’s Adventures in Wonderland detaches itself from its grin. Now researchers led by Daniela Angulo of the University of Toronto have revealed another oddball quantum outcome: photons, wave-particles of light, can spend a negative amount of time zipping through a cloud of chilled atoms. In other words, photons can seem to exit a material before entering it.
“It took a positive amount of time, but our experiment observing that photons can make atoms seem to spend a *negative* amount of time in the excited state is up!” wrote Aephraim Steinberg, a physicist at the University of Toronto, in a post on X (formerly Twitter) about the new study, which was uploaded to the preprint server arXiv.org on September 5 and has not yet been peer-reviewed.
The idea for this work emerged in 2017. At the time, Steinberg and a lab colleague, then doctoral student Josiah Sinclair, were interested in the interaction of light and matter, specifically a phenomenon called atomic excitation: when photons pass through a medium and get absorbed, electrons swirling around atoms in that medium jump to higher energy levels. When these excited electrons lapse to their original state, they release that absorbed energy as reemitted photons, introducing a time delay in the light’s observed transit time through the medium.
AI Generated Overview:
Negative time is a phenomenon in quantum physics that occurs when photons interact with atoms in a way that makes it seem like the photons spent a negative amount of time exciting the atoms:
- Explanation In a quantum clock that measures how long atoms spend in an excited state, the clock hand would move backward instead of forward. This is because the time in which the photons were absorbed by the atoms is negative.
- Significance This phenomenon challenges traditional ideas of light-matter interactions and suggests that negative group delays have real physical significance. It could also have implications for improving quantum memory and communication systems.
- Experiment Researchers from the University of Toronto and Griffith University used a method called the cross-Kerr effect to measure atomic excitation times and confirm the existence of negative time. They observed that when photons were beamed into a cloud of atoms, they appeared to exit the medium before entering it.
