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The rule that nothing can travel faster than the speed of light, c, is one of the most fundamental laws of nature. But since this speed limit has only been experimentally demonstrated for information carried by large groups of photons, physicists have recently speculated as to whether single photons and the information carried by them may be able to exceed the speed of light. In a new study, physicists have performed the difficult task of producing single photons with controllable waveforms, and have shown that single photons also obey the speed limit c.
The physicists, led by Professor Shengwang Du from The Hong Kong University of Science and Technology in Hong Kong, China, have published their study on the ultimate speed of a single photon in a recent issue of Physical Review Letters. The results have implications for the maximum speed of information transmission by confirming that single photons obey causality; that is, an effect cannot occur before its cause.
“The greatest significance of our work is that our experimental results bring closure to the debate on the true speed of information carried by a single photon,” Du told PhysOrg.com. “It deepens our understanding of the particle-wave duality of photons and the nature of quantum mechanics. It provides people a clear picture of photons (since the name was invented by Einstein more than 100 years ago) and corrects some ‘wrong’ and confusing pictures from before.”
With recent advances in technology over the past several years, many groups of scientists have been investigating exactly how fast light can travel. Although previous studies have found that the “group velocity” of light can travel faster than c, the “signal velocity” – the speed at which information travels – cannot. In light of this finding, scientists have wondered whether single photons travel at the group velocity or the signal velocity.
To address this question, Du and his coauthors’ demonstration required not only producing single photons, but separating the optical precursor, which is the wave-like propagation at the front of an optical pulse, from the rest of the photon. Previous experiments based on macroscopic electromagnetic wave propagation (involving lots of photons) have shown that the optical precursor is the fastest part in the propagation of an optical pulse. But this study 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.