In Homer’s Odyssey, Odysseus completes an extraordinary journey from Troy to his home in Ithaca. Along the way, he visits distant lands and spends years with the nymph Calypso on her island.
We can imagine Penelope asking about that particular part of his journey. Odysseus might have answered: “It was nothing – in fact, it was less than nothing. I spent negative five years with Calypso. How else could I have returned home after only ten years? If you doubt me, ask her.”
Quantum particles can be just as elusive as Odysseus. In an experiment published in Physical Review Letters, we showed that a photon’s arrival time can suggest it spent a negative amount of time interacting with other particles. When those particles are measured directly, they appear to confirm the same result.
How photons interact with atoms
Our experiment studied photons – quantum particles of light – as they attempted an unlikely journey straight through a cloud of rubidium atoms.
Rubidium atoms have a specific “resonance” with the photons. This means a photon’s energy can be temporarily transferred to an atom, creating an atomic excitation. The photon can therefore appear to “dwell” inside the atomic cloud before being released.
For this resonant interaction to work efficiently, the photon must have a precisely defined energy that matches the energy needed to excite a rubidium atom.
However, a version of Heisenberg’s famous uncertainty principle tells us that a well-defined photon energy comes with uncertainty in its timing. The photon must occupy a relatively long light pulse. As a result, we cannot know exactly when the photon enters the cloud, although we can determine its average entry time.
When a photon enters the cloud, the most likely outcome is that its energy will be transferred to the atoms and later re-emitted as a photon traveling in a random direction. The photon is then scattered and does not complete its journey to the other side of the cloud – its quantum Ithaca.
Why photons can appear to travel for negative time
When a photon does pass straight through the atomic cloud, something unexpected occurs. From the photon’s average entry time, we can calculate when it should arrive at the far side if it travels at the speed of light, as photons normally do.
Yet the photon arrives much earlier than this expected time. It arrives so early that the result appears to show the photon spending a negative amount of time inside the cloud – emerging, on average, before it entered.
This unusual effect has been known for decades and was observed in a 1993 experiment. For many years, however, physicists were reluctant to interpret this negative time as a real physical effect.
One possible explanation is that only the very front of the long light pulse passes through the cloud, while the rest of the pulse is scattered. The photons that survive would therefore arrive earlier than expected, creating the appearance of a negative transit time.
Measuring the photon’s dwell time
Aephraim Steinberg, one of the authors of the 1993 study, was not convinced that negative time could be dismissed as an artifact. In his laboratory at the University of Toronto, he wanted to measure how long the photon’s energy actually remained in the rubidium atoms as an excitation. After an initial experiment with inconclusive results, he asked me, as a quantum theorist, to help determine what the experiment should reveal.
In practice, measuring the atoms’ dwell time means continuously probing them while the photon moves through the cloud. The goal is to determine whether the photon’s energy is temporarily stored in the atoms. But quantum measurements introduce an important complication: measuring a system inevitably disturbs it.
If we precisely measured, at every moment, whether the photon was interacting with the atoms, we would prevent that interaction from taking place. It would be like watching Calypso so closely that she could never get hold of Odysseus – or vice versa. This is an example of the well-known quantum Zeno effect, in which frequent measurements inhibit a quantum process.
Our experiment reveals measurable negative time
The solution was to use a very imprecise, but carefully calibrated, measurement. This approach minimized the disturbance caused by observing the atoms. We sent a weak laser beam, separate from the single-photon pulse, through the rubidium cloud and measured tiny changes in the beam’s phase. These changes revealed whether the atoms had been excited.
A single experiment provided only a rough estimate of whether the photon had dwelt in the atoms. But by averaging the results of millions of trials, we obtained a precise measurement of the average dwell time.
Remarkably, when the photon passed straight through the cloud, the weakly measured dwell time was exactly equal to the negative time inferred from the photons’ average arrival time. Before this research, no one had expected two times measured through entirely different methods to agree so precisely.
Most importantly, the negative result from the weak measurement cannot be explained simply by saying that the front edge of the photon’s pulse was the only part to pass through the cloud. The atoms themselves display a measurable response consistent with the negative dwell time.
Does this mean that a time machine could be on the horizon?
Sadly, no. The experiment is fully consistent with standard quantum physics and does not allow information or matter to travel backward in time.
It does show, however, that negative photon dwell time is not merely an experimental artifact. Although counterintuitive, it produces a measurable effect in the atomic cloud through which the photon travels. Like Odysseus’ journey, the continuing exploration of quantum physics still has many strange and surprising destinations.
Source: www.sciencedaily.com