Canadian researchers from the University of Toronto, led by Aephraim Steinberg, in collaboration with theorist Howard Wiseman from Griffith University in Australia, have experimentally confirmed a paradoxical quantum phenomenon. They found that photons successfully passing through a cloud of atoms cause, on average, a negative excitation time within those atoms.
The Core Mechanics of the Experiment
Researchers sent extremely weak pulses of resonant light—essentially individual photons—through a cloud of cold rubidium atoms. At this specific frequency, atoms are expected to absorb and re-emit photons, temporarily entering an excited state.
Most of these photons are scattered during the process. However, a small portion passes directly through. The central question for the researchers was exactly how much "time" these transmitted photons spent inside the atoms as excitations.
Standard measurement techniques would have destroyed the effect due to the quantum Zeno effect. Therefore, the team utilized weak measurements: they passed another very faint probe laser through the cloud and determined the average atomic excitation by measuring a tiny shift in its phase. This approach barely disturbs the process, though it requires a massive amount of statistical data.
After approximately tens of millions of cycles—representing dozens of measurement hours—and post-selection to isolate only those cases where a photon was actually detected after transit, the scientists obtained a clear result.
Defining "Negative Time" in Simple Terms
In this context, "negative time" refers to the average duration the atoms spent in an excited state specifically because of their interaction with the photons that successfully passed through.
The result was negative, measuring approximately -0.8 times the standard positive excitation duration for narrow-band pulses.
This does not imply any of the following:
- photons traveled back in time;
- causality was violated;
- atoms were excited before the photon arrived in a traditional sense.
What the result actually signifies is this:
Previously, negative group delay—where a light pulse's peak exits a medium sooner than it should—was often explained simply as pulse reshaping, where the medium filters the back end so only the leading edge passes. This made it seem like a mathematical artifact rather than a true physical interaction time.
In this new experiment, scientists essentially asked the atoms themselves how long they were excited by the transmitted photon.
The atoms responded: negative time. This answer matched the negative group delay measured by the arrival of the photons exactly.
Thus, negative time is not merely an illusion of pulse shape. It has a direct physical manifestation in the state of matter itself. In quantum mechanics, using weak values, the average interaction time for successfully transmitted photons can indeed be negative due to the interference of probability amplitudes.
To explain the core concept simply: in the quantum world, a photon behaves not like a solid ball, but like a wave of probability. As the quantum wave moves through the cloud of atoms, it interacts with them to create an interference effect where waves are added or subtracted. When a photon successfully passes through the medium, its wave properties combine in such a way that the mathematical probability of an atom being in an excited state becomes negative.
"Negative time" in this sense is a specific quantum value showing how wave interference subtracts from the interaction time for particles that successfully transit.
Why This Discovery Matters
This confirms that negative group delay is a real physical property of quantum light and matter, not just a mathematical trick. The effect was theoretically predicted long ago and observed in a 1993 photon tunneling experiment—also involving Steinberg—but its physical meaning was highly questioned at the time.
Now, there is less doubt: the atoms "feel" this negative time.
The researchers emphasize that this is not a time machine. This is a feature of standard quantum physics observed through the lens of weak measurements and post-selection. In the complete picture, which includes scattered photons, causality and the overall positivity of time are maintained.
The team plans to study the scattered photons in greater detail to understand how positive and negative time compensate for each other within the overall statistics.
