For 10 long years, a debate has been raging about the speed of a single photon. Finally, a team led by HKUST assistant professor of physics, Prof Shengwang Du was able to lay the doubts to rest: single photons cannot travel faster than the speed of light in vacuum, reaffirming Einstein’s theory that nothing travels faster than the speed of light.

The findings of this basic research were published in Physical Review Letters recently. Besides Prof Du, the team consists of three postgraduate students Shanchao Zhang, Jiefei Chen and Chang Liu, and chair professors Michael Loy and George K L Wong. This is a highly productive team, this being the fifth time in the last two years that it has published its optics research results in the same journal, one of the most prestigious in physics. What's more, their papers were selected as editors' suggestions for reading. At the same time, it also appeared under a different title "Single Photons Obey the Speed Limits" in American Physical Society’s Physics Synopsis.

Photons are the fundamental quanta of light. This study shows that a single photon obeys the traffic law of the universe just as Einstein has claimed. Our team’s contribution is that it is 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.

The significance of these findings is that they add to our understanding of how a single photon moves, according to the team leader Prof Du. They also confirm the upper bound on how fast information travels with light. By showing that single photons cannot travel faster than the speed of light, this study brings closure to the debate on the true speed of information carried by a single photon. Its findings will likely enable scientists to have a better idea on the transmission of quantum information.

This is basic research at its highest, among others ongoing at the university, pushing the frontiers of knowledge, while our other scientists are busy achieving their breakthroughs in applied research.