In our familiar realm of causality, we anticipate that responses invariably follow an action rather than the other way around. This expectation is why the recent discussions in the media regarding scientists having identified ‘negative time’ with photons being discharged prior to the sample being struck by source photons generated such excitement. Did these scientists genuinely just revolutionize our fundamental notions of (quantum) physics? As it turns out, not exactly.
A significant portion of the misunderstanding arises from the reality that photons are not tiny spheres that ricochet around, but rather manifestations of (electromagnetic) energy. Consequently, their resultant interactions with matter (i.e., collections of atoms) are vastly more complex, often leading to the photonic energy being absorbed by an atom, elevating the energy state of its electron(s) before potentially being re-emitted as the excited electrons revert to a lower orbit.
This dwell time prior to re-emission is what many find perplexing, as in our classical comprehension we would anticipate this to be a highly deterministic process, whereas in a quantum context, it most certainly is not.
This aspect is emphasized in the Scientific American article on the matter, particularly regarding quantum probability. Within this framework, it is feasible that re-emissions might occur before the atomic excitation has completely subsided. It was this original 2022 discovery that was recently re-evaluated, with the results corroborated.
As bewildering as this may appear, the authors of the recent study emphasize that the essence of the matter lies in the so-called ‘group delay’ of the initial pulse as it stimulates the cloud of rubidium atoms. If one were to regard this pulse as distinct quanta of photon particles, it could appear to disrupt causality; however, as a wave function within quantum theory, this is entirely valid. Phenomena such as the rubidium atoms being excited despite photons traversing the cloud, and emitting a photon prior to the electrons returning to their ground state may not seem logical, but we also need to examine how and what we are measuring.
In summary, causality remains intact, and the realm of quantum physics has its own intuitive logic, albeit in peculiar manners. Research such as this enhances our fundamental comprehension of photonics and associated domains, none of which pertain to time travel.
Experimental configuration and observed optical depth. (Credit: Josiah Sinclair et al., PRX Quantum, 2022)