Physicists Have Measured “Negative Time” in Bizarre Quantum Experiment
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Physics<br>Physicists Have Measured “Negative Time” in Bizarre Quantum Experiment<br>By Howard Wiseman, Griffith UniversityMay 26, 20268 Comments6 Mins Read
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A quantum physics experiment showed photons can exhibit negative dwell time while traveling through atoms. Scientists confirmed the phenomenon using weak measurements that minimally disturbed the system. Credit: ShutterstockQuantum physicists have uncovered a bizarre phenomenon in which photons appear to spend a “negative” amount of time interacting with atoms before emerging from a cloud of matter.<br>As Homer tells us, Odysseus made an epic journey, against the odds, from Troy to his home in Ithaca. He visited many lands but mostly dwelt with the nymph Calypso on her island.<br>We can imagine that his wife, Penelope, would have asked him about that particular time. Odysseus might have replied, “It was nothing. In fact, it was less than nothing. Negative five years I dwelt with Calypso. How else could I have arrived home after only ten years? If you don’t believe me, ask her.”<br>Quantum particles, it turns out, are just as wily as Odysseus, as we have shown in an experiment published in Physical Review Letters. Not only can their arrival time suggest that they dwelt with other particles for a negative amount of time, but if one asks those other particles, they will corroborate the story.<br>How Photons Travel Through an Atomic Cloud<br>Our experiment used photons—quantum particles of light—and the against-the-odds journey they must undertake to pass straight through a cloud of rubidium atoms.<br>These atoms have a “resonance” with the photons, meaning the energy of the photon can be transferred temporarily to the atoms as an atomic excitation. This allows the photon to “dwell” in the atomic cloud for a time before being released.<br>For this resonance to be effective, the photon must have a well-defined energy, matching the amount of energy required to put a rubidium atom into an excited state.<br>But, by a form of Heisenberg’s famous uncertainty principle, if the energy of the photon is well defined, then its timing must be uncertain: the pulse of light the photon occupies must have a long duration. This means we can’t know exactly when the photon enters the cloud, but we can know on average when it enters.<br>Why Some Photons Arrive Earlier Than Expected<br>If a photon like this is fired into the cloud, the most likely outcome is that its energy will be transferred to the atoms and then re-emitted as a photon traveling in a random direction. In such cases, the photon is scattered and fails to arrive at its Ithaca.<br>But if the photon does make it straight through, a strange thing happens. Based on the average time when the photon enters the cloud, one can calculate the expected average time it would arrive at the far side of the cloud, assuming it travels at the speed of light (as photons usually do).<br>What one finds is that the photon actually arrives far earlier than that. In fact, it arrives so early it appears to have spent a negative amount of time inside the cloud—to exit, on average, before it enters.<br>This effect has been known for decades and was observed in a 1993 experiment. But physicists had mostly decided not to take this negative time seriously.<br>Scientists Revisit a Long-Standing Quantum Mystery<br>That’s because it can be explained by saying that only the very front of the long-duration pulse makes it straight through the atomic cloud, while the rest is scattered. This leads to a successful (non-scattered) photon arriving earlier than would be naively expected.<br>However, Aephraim Steinberg, one of the authors of that 1993 paper, was not so quick to accept this dismissal of the negative time as an artifact. In his laboratory at the University of Toronto, he wanted to find out what happened if one queried the rubidium atoms in the cloud to find out how long the photon had spent dwelling among them as an excitation. After an initial experiment with inconclusive results, he asked me, as a quantum theorist, for help in working out what to expect.<br>When we talk of querying the atoms, what this means in practice is continuously making a measurement on the atoms while the photon is passing through the cloud, to probe whether the photon’s energy is currently dwelling there. But there is a subtlety here: measurements in quantum physics inevitably disturb the system being measured.<br>Weak Measurements Reveal Negative Dwell Time<br>If we were to make a precise measurement of whether the photon is dwelling in the atoms at each instant of time, we would prevent the atoms from interacting with the photon. It is as if, merely by...