Moti Fridman, a postdoc in applied and engineering physics, and his colleagues, demonstrated for the first time the invisibility of an event in time, a phenomenon known as temporal cloaking. Although different than the invisibility cloak of Harry Potter fame, which illustrates spatial cloaking, Fridman’s research prevents light from interacting with a whole activity in time, rendering it, like Harry under his magical cloak, undetectable.
According to their study, which was published in the January version of Nature, the researchers masked an entire event for fifty picoseconds, or fifty-trillionths of a second, a fraction of time so minuscule, the measurement can only be detected with a computer.
“Seeing something is always about the interaction of light with an object,” Fridman said. “To make an object invisible, you prevent light from interacting with what you’re trying to hide. It’s not disappearing, the object is still there, but the light is not detecting it.”
Fridman is a member of the lab headed by Prof. Alex Gaeta, applied and engineering physics. The lab specializes in manipulating laser beams in a time domain, according to Gaeta.
In November 2010, Prof. Martin McCall, physics, at Imperial College in London published a theoretical proposal for space-time cloaking, Gaeta said. This paper prompted him to investigate if temporal cloaking could be successfully demonstrated in the lab.
“McCall’s paper was an interesting idea, it proposed a technique that was very challenging,” Gaeta said.
Opening the Gap
The first step in cloaking an event in time is opening a gap in which events may occur but cannot be detected by a beam of light, according to McCall’s paper.
The proposal suggested that accelerating and decelerating parts of light could open a gap. This is done by changing the composition of the fiber at a rate fast enough to change the velocity of the light travelling through the fiber, which is not an easy task Fridman said.
“Instead,” Fridman explained, “we thought rather than changing the property of the material, we could change the property of the light—this is much easier, and has the same effect as changing the composition.”
Fridman changed the property of light travelling through the fiber by using time lenses. These special lenses, allowed for the alteration of the frequency of light, which work to accelerate and decelerate light through a ‘four-wave mixing’ technique, the lenses bump the beams of light up and down to open the gap. The beams then continued to pass through the optical fiber until both wavelengths are slowed and reconvened, closing the gap.
Optical fibers were the medium used in the experiment. The speed of light in the fiber depended on the color and frequency of the light. By changing the color of the light, parts of the light were accelerated and decelerated, which created the gap needed to cloak an event.
The alternating higher and lower frequencies created a time gap that goes unrecognized by the beam of light passing through.
“We knew that it was only a matter of time before we could actually cloak something once we knew how to open the gap. Most of the experiment and setup was making the gap,” Fridman said.
The Race to Success
The allure of creating a cloaking device is not unique to researchers at Cornell. Many other universities have done similar work in the field of invisibility.
For example, at Purdue University, Prof. Vladimir Shalaev, electrical and computer engineering, employed similar fiber optic techniques. An article describing the cloaking methods was published in 2009, but he himself did not display temporal cloaking in the lab.
Additionally, at the CUNY Grove School of Engineering, Prof. David Crouse, a professor electrical engineering and Cornell alum, has worked with the engineering of meta-materials, an integral component of spatial cloaking. “Cloaking is a very exciting field. It has really taken off in the past ten-fifteen years,” Crouse said.
Once Fridman was confident that the designed procedure would merit success, he did not waste time carrying out the experiment. He borrowed equipment from other labs and worked long hours on the weekend to complete the project. “We were very worried that someone would do it before us. Once we had the idea we knew it would work, we just had to prove it,” Fridman said.
Optimistic Next Steps
Fridman and Gaeta plan to continue working on the project, perhaps widening the gap to cloak an event for a nanosecond or longer.
In terms of the applications of temporal cloaking, there are many possibilities Fridman said. “You don’t need to have too much of an imagination to know how this science could help,” he explained. “If you have the ability to change a beam of light, to move something from one part to another, you can alter information in that space—without anyone knowing.” Fridman’s research is sponsored in part by the Pentagon’s Defense Advanced Research Project Agency or DARPA, which develops futuristic technology for the military.
According to Gaeta, temporal cloaking may be useful in the field of telecommunications, allowing scientists to inject data into a data stream and later remove it without interruption.
The wavelengths in which Fridman and Gaeta demonstrated temporal cloaking fell within the infrared spectrum. But as Crouse stated, “people are making progress, working to cloak at wavelengths closer and closer to the visible spectrum.”
In fact, Fridman said he plans to get more people involved in doing science. “Every time I have the opportunity to do something that other people, not just in the science community, will be interested in, I will take it,” he said.
One of the unique aspects of temporal cloaking and the concept of invisibility is its seemingly magical appeal while abiding by the laws of physics. “Maybe more kids will be drawn to science because of this experiment... if this is the case, then I have done my job,” Fridman concluded.
“It’s nice to achieve something once in a while that appeals to the non-science community as well. Every scientist should experience the feeling at least once in their lifetime,” Gaeta said.