Crazy, Cool and Weird Stuff

Among the many questions that scientists ask in our science world, there is this one question whether or not something can exceed the speed of light which is about 299.79 million m/s. Physicist Professor Shengwang Du from Hong Kong University of Science and Technology with his team, investigated and published about their study on the maximum speed that photons can achieved.

Light can be described as a packet of photons.

In the attempt to answer this question, Du and his team must first isolate and produce single photons. As photons usually exists and travel in group, the question was more addressed to whether or not single photons can exceed the speed of light as the group photons, can't. In this experiment also, Du and his team need to separate the optical precursor, the wave-like propagation exists at the front of optical pulse. This is because previous experiments in macroscopic level suggest that the optical precursor is the fastest part in the propagation of the optical pulse. And this study was the first study conducted to prove that optical precursor exists at even in the single photons level and is also the fastest part in single-photon wave packet.

The simple understanding about the method used in the experiment, as explained in here, is as follows:

"In order to separate the optical precursor from the rest of the photon, the scientists generated a pair of photons, and then passed one of the photons through a group of cold rubidium atoms, while using an electro-optic modulator to shape the photon’s waveform. The atoms had an effect called electromagnetically induced transparency (EIT), which enabled the scientists to separate the single-photon precursors from the main wave packet. As the optical precursor and main wave packet traveled through a second group of rubidium atoms, the scientists took measurements on the speed of the two photon components"


The experiment found out that the optical precursor wave front in single photons always travel at the speed of light whereas the main wave packet travels with the maximum speed of c and can be delayed up to 500 ns in slow light medium where it travels below c. Du explained that in slow light medium, the main packet always follows the group velocity and as the density increases, the group velocity decreases.

Simple understanding about photons.

Source: As edited from http://www.physorg.com/news/2011-06-photons.html