When Faster Isn't Better
Scientists are manipulating the speed of light to discover new capabilities and devices.
Science fiction heroes zooming faster than the speed of light is the stuff of space-age movies, but slowing down or stopping light’s speed may prove more useful to the military and others. Scientists have found that changing the pace of light brings technologies that were once considered impossible closer to reality.
DARPA is funding teams from academia, government and industry in its Slow Light program. Several techniques can be used to slow, stop and store pulses of light, but each technique has certain requirements. They all rely on setting up a frequency-dependent change in the interaction of light with a material. The change modifies the group velocity—the speed of the pulse through a material.
Though the teams have different projects and objectives, the Slow Light program has two overall goals: to demonstrate tunable delay lines with tunable delay bandwidth products in excess of 3,600 with a signal bandwidth greater than 30 gigahertz and to demonstrate a nondestructive single-photon detector. The program’s two principal thrusts are to develop and demonstrate high-bandwidth, analog-tunable optical delays compatible with modern communications and computational systems and to develop systems with unprecedented low-light response, such as single-photon sources and nondestructive photon detectors (quantum nondemolition detectors).
Howell’s work is determining what effects slow light and stop light enable. “Slow light is using what’s called the group velocity to slow down a pulse of light in a medium,” he explains. “You can think of that as a group of frequencies. Usually you think of a pulse of light consisting of a lot of different frequencies, and you’re trying to slow down that group of frequencies to some new slow value.” Through the process, scientists try to slow down a pulse of light to make it propagate very slowly in a medium so they can pull out its information at a specific time.
This technology could affect fields such as telecommunications and remote sensing. Howell says that the classic problem scientists work on in this area deals with optical routers or buffers. When one router has two simultaneous signals coming in and it can accept only one at a time, it needs to delay one signal, process the first one, and then process the second one.
Another project Howell and his team have been working on involves an interferometer, a device that can measure frequency to sensitive amounts. The problem with interferometer systems, however, is that they have to be extremely long in order to obtain certain spectral measurements. For example, if researchers wanted to distinguish between two very close frequencies of light that could not be seen by the naked eye or with other tools, they would need a more sensitive device to resolve the difference between the frequencies. The longer the path mismatch between one frequency and the other in the interferometer, the more resolution scientists can view. However, achieving that resolution is difficult. “It’s harder and harder because now you need a really long interferometer. You need to make sure beams are preserved,” Howell says. “It’s very difficult to get sensitive measurements.” By using slow light, scientists can capture the information in a more compact device.
While the military often funds projects such as these light experiments under the category of basic research, Howell explains that his work has concrete military applications. Howell and his team’s projects in stop light are enabling them to perform some unique experiments with images. “We’ve been able to take an image and make it propagate really slowly so all images are preserved,” he says. “This could be interesting in remote sensing.” Users may want to preserve an image of something distant and then compare it later with an image of the same distant object to observe any changes. If a tank were moving in the distance, for example, users could hold onto an image of the vehicle without having to process the picture. Digital image processing requires people to gather all the amplitude and phase information about an image in order to compare it to another image at a later time. Howell’s team has figured out how to hold onto the image without changing any of its properties so they can interfere it with a similar image without any reconverting. Interference is a highly sensitive method for detecting change. It provides the most precise means for measuring changes in objects by using all the light coming off the object back to the viewer.
A picture on a regular camera can contain amplitude information, but not phase information. Howell’s research detects phase, which deals with how a signal oscillates. When two signals are in perfect phase, the peaks and troughs of one correspond with the peaks and troughs of another. If personnel are viewing two simultaneous signals with the same frequency and one starts to get out of phase, the viewer will see the same signals, but one will be displaced.
When users can detect an object at one time, they can use phase to interfere its image with a picture from another time in a sensitive manner. This type of technology can be used for radar to capture two images and compare them later.
Howell uses the example of the military trying to monitor tanks several kilometers away. Using his technology, troops could compare two images from different times and notice subtle differences in the phase information that would not be detectable with a camera offering only amplitude information.
The distance the technology covers depends on how long users have to store the information. If they can store it for a millisecond, they would be able to capture an image the distance that light can travel in a millisecond—300 kilometers. Howell’s team has stopped light for more than 100 microseconds, which is more than 30 kilometers. Military and other users would benefit from the stop-light and slow-light enabled technology that requires considerably less space. Take a situation in which the military needs to hold on to an image for 100 microseconds. Without the light manipulations, users would need a camera in which every pixel held on to amplitude and phase information for that length of time. Or, in a free-space delay line, which means users need to keep the image and bounce it back and forth among mirrors, users would need to have a device 30 kilometers long. “It would be enormous,” Howell says.
In his device, however, users could have the same results in only a couple of inches. The new technology uses slow light to compress the pulses and get the image inside the medium. Once it is trapped, stop light is used to store it. According to Howell, his research team is the only one that has been examining how the technology of light stoppage affects images, although other groups are experimenting with slow light and stop light.
The team has encountered some problems with its camera development, however, including cost and bandwidth. Howell recently priced a high-level camera that would offer only amplitude information at a cost of $120,000. The camera he is developing would take images in both amplitude and phase, but it would not have the bandwidth of a camera displaying just amplitude. To produce images with both types of information, the camera has to operate in the hyperfine ground state and coherences of the rubidium frequency, near infrared.
Howell’s camera technology works by putting light into a rubidium vapor and coherently absorbing it. The light is converted into a spin wave in the medium, and the stored spin wave is maintained. When users need the information, they read it out. The information is stored in a pump beam. Users turn off the pump beam while the image is inside. The light is converted to the atomic spin wave, and when the pump is turned back on, the machine reconverts the spin wave back to an image.
In the future, Howell and his team would like to enable image processing while the light is being stored. They also want to perform actual interference measurement and compare one image to another image at a different time. In addition, they hope to produce time-varying images in the future. Howell says that they would like to look at a biological system and see it on a very fast time scale. “That would be our goal,” he explains.
Though Howell has been working with the speed of light for about three years, he says he has no idea exactly where his research will go. Describing himself as spontaneous, Howell says the research his team undertakes can change. When he began this line of work, he was unaware of the possibilities he would discover. “It’s been fun pursuing it just because so many things have opened up that we didn’t anticipate,” he says. Howell adds that he was skeptical initially about anything interesting arising from the research.
An area that does interest him now is demonstrating that his team can preserve quantum images, and the group is currently building a device to do just that. Howell explains that many people are unaware that a whole image can be placed on a single photon. The misconception, he says, is that photons are small when really there are no limits to their size. He wants to show that his team can put an image on a photon and preserve it in a slow-light medium. The team impresses images on the photons and then can take those images and with a single device be able to determine the image. According to Howell, people measure photons in the wrong basis, so they need many photons to do this type of work with images. Howell’s team measures in terms of two-dimensional arrays or pixels. Essentially, the team is enabling a method to obtain very low-light level target recognition.
It turns out “light speed” has a very different meaning to researchers and the military than it does to pilots in science fiction.
Web Resources
DARPA Slow Light: www.darpa.mil/dso/thrusts/physci/funphys/slowing/index.htm
U.S. Army Research Laboratory’s Army Research Office: www.aro.army.mil
