Scottish Scientists Slow down Light Particles - Photons

The speed of light is a limit, not a constant, that's what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently.

Scientists already knew light could be slowed temporarily. Photons change speeds as they pass through glass or water, but when they exit the other side and return to a vacuum (like outer space) they speed back up.

In a new experiment at the University of Glasgow, however, scientists were able to permanently manipulate light's speed by passing photons through a device that alters their structure. The device, created in collaboration with researchers at Heriot-Watt University in Edinburgh, is a filter of sorts that the scientists refer to as a mask.

"That mask looks a little bit like a bull's-eye target," researcher Miles Padgett told reporters. "And that mask patterns the light beam, and we show that it's the patterning of the light beam that slows it down.

"But once that pattern has been imposed, even now the light is no longer in the mask, it's just propagating in free space, the speed is still slow," Padgett added.

In other words, the beam of light is reorganized in a way that slows down each individual photon. When tested in a vacuum next to a regular light beam.

Photons that had been filtered through mask were milliseconds behind in a sprint to the end of the vacuum racetrack.

Researchers, whose latest work was published this week in the journal Science Express, say the findings prove the speed of light is not an absolute, more like a ceiling.

Miles Padgett

The work was carried out by a team from the University of Glasgow’s Optics Group, led by Professor Miles Padgett, working with theoretical physicists led by Stephen Barnett, in partnership with Professor Daniele Faccio from Heriot-Watt’s Institute of Photonics and Quantum Sciences.

Daniele Faccio

Professor Faccio said, “The speed of light is a universal constant and plays a central role in our understanding of the Universe and Einstein's theory of relativity."

"The exciting discovery here is that this speed is the true speed of light only for plane waves, that is waves that are perfectly flat."

"In everyday situations however, we interact with light that is not a plane wave but has some kind of structure on it."

"The presence of this structure (think of the light beam emitted from a laser pointer) forces the light to actually move slower."

"There are lots of technicalities involved in the actual experiments used to measure this slow-down, but the result is widely applicable. A very appropriate discovery for the 2015 international year of light".

Professor Padgett added, “It might seem surprising that light can be made to travel more slowly like this, but the effect has a solid theoretical foundation and we’re confident that our observations are correct.

“The results give us a new way to think about the properties of light and we’re keen to continue exploring the potential of this discovery in future applications."

"We expect that the effect will be applicable to any wave theory, so a similar slowing could well be created in sound waves, for example.”

More Information
Spatially structured photons that travel in free space slower than the speed of light - Science Magazine January 22 2015 - Science DOI: 10.1126/science.aaa3035

Scottish scientists create single-pixel camera for 3D images

A projector displays patterned light on the face on a mannequin. These patterns are used to produce a 2D image. A 3D image is then formed using a technique called "shape from shade"

Scientists in Glasgow have discovered a low-cost way to create 3D images.

Their system uses detectors which have a single pixel to sense light instead of the millions of pixels used in the imaging sensors of digital cameras.

The detectors can "see" frequencies beyond visible light, which researchers say could open up new uses for 3D imaging in medicine and geography.

They said the single-pixel detectors cost "a few pounds" compared to current systems, which cost "thousands".

It is hoped that the system's ability to senses wavelengths far beyond the capability of digital cameras and its low cost, could make it a valuable tool for a wide range of industries.

Researchers said possible uses could range from locating oil to helping doctors find tumours.

Miles Padgett

Crossword patterns
Prof Miles Padgett led the team at University of Glasgow's School of Physics and Astronomy, which developed the technique.

He said: "Single-pixel detectors in four different locations are used to detect light from a data projector, which illuminates objects with a rapidly-shifting sequence of black-and-white patterns similar to crossword puzzles.

"When more of the white squares of these patterns overlap with the object, the intensity of the light reflected back to the detectors is higher.

"A series of projected patterns and the reflected intensities are used in a computer algorithm to produce a 2D image."

"A 3D image was then created by combining images from the four detectors using a well-known technique called "shape from shade"."

This 3D computational imaging, or ghost imaging produces detailed images of objects in just a few seconds.

Conventional 3D imaging systems uses multiple digital camera sensors to produce a 3D image from 2D information. Careful calibration is required to ensure the multi-megapixel images align correctly.

Beyond the visible
Prof Padgett continued: "Our single-pixel system creates images with a similar degree of accuracy without the need for such detailed calibration."

Baoqing Sun

Lead author on the paper Baoqing Sun said: "It might seem a bit counter-intuitive to think that more information can be captured from a detector which uses just a single pixel rather than the multi-megapixel detectors found in conventional digital cameras.

"However, digital camera sensors have a very limited sensitivity beyond the spectrum of visible light, whereas a single-pixel detector can easily be made to capture information far beyond the visible, reaching wavelengths from X-ray to TeraHertz."

More Information:
The team's paper, 3D Computational Imaging with Single-Pixel Detectors, is published in the Science 17 May 2013: Vol. 340 no. 6134 pp. 844-847 DOI:10.1126/science.1234454.