Synopsis

Single photons obey the speed limits

Physics 4, s88
Observations of single photons traveling through cold atomic gases confirm that information can only travel as fast as the speed of light.
Credit: Carin Cain

Einstein taught us that the speed of light was the traffic law of the universe—nothing could go faster. The development of media in which atomic gases can slow down or speed up the passage of light pulses initially caused a stir, at least until the difference between phase velocity and group velocity could be carefully explained. But what about the behavior of single photons, the fundamental quanta of light? Reporting in Physical Review Letters, Shanchao Zhang and colleagues at the Hong Kong University of Science and Technology have shown that photons obey the law too.

Zhang et al. study optical precursors, which are signals preceding the main wave packet in a light pulse with a sharply rising leading edge (as in a step function pulse). Past work has shown that even in “superluminal” media where the group velocity may be faster than light speed, the precursor is always in front of the pulse. The authors extend this work to the single-photon level with the help of cold atomic gases: a photon generated in one rubidium gas traverses a second collection of rubidium atoms. With careful use of electromagnetically induced transparency, the researchers can separate the precursor from the main pulse and confirm it travels at the speed of light. The results add to our understanding of how single-photon signals propagate but also confirm the upper bound on how fast information travels. – David Voss


Subject Areas

Quantum InformationOptics

Related Articles

Plasma Gratings for High-Power Lasers
Optics

Plasma Gratings for High-Power Lasers

A compact, high-power laser could be made using gratings made of plasma. Read More »

Motion Synchronization Goes Long Distance
Mechanics

Motion Synchronization Goes Long Distance

Researchers have optically synced the motion of two micrometer-sized objects separated by 5 km, a distance around a hundred million times longer than previous demonstrations. Read More »

A Tiny Photonic Nose Captures Odor Fingerprints
Biological Physics

A Tiny Photonic Nose Captures Odor Fingerprints

A bio-inspired detector the size of a US penny can identify the unique odor profiles of different gases, something that could help in detecting food freshness and product counterfeits and in designing new cosmetics. Read More »

More Articles