A group of physicists at the Hong Kong University of Science and Technology (HKUST) say they've proved that single photons cannot travel faster than the speed of light in vacuum, effectively putting the kibosh on the idea of time travel.
While it might seem obvious that light can't travel faster than light, scientists have in fact been arguing about the matter for a decade, since faster-than-light propagation of optical pulses was discovered in certain media.
Even when it was realised that this was simply a visual effect, hope remained that a single photon might be able to exceed the speed of light.
But Professor Shengwang Du's study, he says, demonstrates that a single photon obeys the 'traffic law' of the universe, just like electromagnetic waves.
It's the first to experimentally show that optical precursors exist at the single-photon level, and that they are the fastest part of the single-photon wave packet even in a so called 'superluminal' medium.
"The results add to our understanding of how a single photon moves. They also confirm the upper bound on how fast information travels with light," says Professor Du.
"By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon. Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information."
The team measured the ultimate speed of a single photon with controllable waveforms. They used a demonstration which required not only producing single photons, but separating the optical precursor - the wave-like propagation at the front of an optical pulse - from the rest of the photon wave packet.
To do this, they generated a pair of photons, and then passed one of them through a group of laser-cooled rubidium atoms with an effect called electromagnetically induced transparency (EIT). For the first time, they successfully observed optical precursors of a single photon.
And the team found that, as the fastest part of a single photon, the precursor wave front always travels at the speed of light in vacuum. The main wave packet of the single photon travels no faster than the speed of light in vacuum in any dispersive medium, and can be delayed up to 500 nanoseconds in a slow light medium.
Even in a superluminal medium where the group velocity is faster than the speed of light in vacuum, the main part of the single photon can't travel faster than its precursor.