Synopsis: Single photons obey the speed limits

Observations of single photons traveling through cold atomic gases confirm that information can only travel as fast as the speed of light.
Synopsis figure
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


Announcements

More Announcements »

Subject Areas

Quantum InformationOptics

Previous Synopsis

Superconductivity

Nesting questions

Read More »

Next Synopsis

Particles and Fields

Two is good, four is better

Read More »

Related Articles

Viewpoint: Cavity with Iron Nuclei Slows Down X Rays
Optics

Viewpoint: Cavity with Iron Nuclei Slows Down X Rays

Slow light effects have been measured for x rays using a cavity filled with iron nuclei, where the speed of light was reduced by a factor of 10,000. Read More »

Synopsis: Nanofiber Optical Memory
Quantum Information

Synopsis: Nanofiber Optical Memory

Light signals propagating down an ultrathin fiber can be temporarily stored in a cloud of cold atoms surrounding the fiber. Read More »

Synopsis: Zooming in on Failures
Optics

Synopsis: Zooming in on Failures

A near-infrared microscopy technique can detect defects in electronic devices with a resolution better than the diffraction limit of light. Read More »

More Articles