'Time cloak' could revolutionize fiber networks

How can you stop a message you can't even see?
Credit: tonechootero/Flickr (CC BY-SA 2.0)

Researchers at Purdue University may have created the first practical way to communicate with absolute secrecy by concealing messages in time using tricks of laser light and fiber optics.

The technique, called "temporal cloaking," conceals not just the content of a message, but even the fact that it was sent, by tucking messages into a gap in the flow of photons sent by one of two lasers down a single strand of fiberoptic cable.

The effect is similar to putting a stick in a stream of water, creating a gap on the downstream side; water flows around the stick, closes the gap and eliminates any evidence it existed. Look closely enough, however, and it's possible to see that the individual molecules of water that hit or detoured around the stick lag slightly behind those that didn't, and now carry a few molecules of stick along with them.

Making it work requires systems what can physically separate some frequencies from the others in a pulse of light, then manipulate the height and frequency of those strands so that they interfere with and cancel themselves out.

That doesn't stop the flow of energy, but does slow the progress of the photons and drops the intensity of the light to zero – thereby making it invisible and slowing the photons in that portion of the beam.

By slowing and releasing the drag on those strands of light, researchers can create a series of gaps into which they can tuck encoded data – data that will arrive at the other end slightly out of sync with the rest of the photons surrounding it. The effect is to create a bubble in time, which is why researchers at McGill called it a "spacetime cloak" in a paper that suggested the first practical way to create one.

Temporal cloak hides data by creating time gaps in light sent over fiber networks Lukens,Weiner et al

Temporal cloak sender/receiver setup. The red channel represents data channel modulated to counteract its intensity and, effectively, make it invisible.

“It looks like no signal is being sent," co-author and graduate student Joseph M. Lukens said in an announcement from Perdue after the publication of an earlier version of the system in 2013. "It's a potentially higher level of security because it doesn't even look like you are communicating. Eavesdroppers won't realize the signal is cloaked because it looks like no signal is being sent."

The Purdue team announced in 2013 that it had found a way to pack 1.5 megabits of data into a temporal cloak using a superfast "femtosecond" laser and off-the-shelf optical networking equipment, proving for the first time that the technique might be practical in the real world as well as in the lab.

Unfortunately, at that time, was no way to get data tucked into a time cloak back out of it again. That meant time cloaking would be more effective to block someone else's communication by irretrievably cloaking messages than it would be to try to use it to hide your own messages from someone else.

The same Purdue team turned that around, however, by finding a way to cloak data, and then transmit the hidden data along another wavelength of light.

The result is a two-way method of communication that can keep observers from ever discovering that messages are being passed, according to new paper by Lukens, senior investigator Andrew Weiner and two co-authors in the Nov. 28 issue of the journal Optica (PDF of full paper).

The technique has more potential than simply as a super-cool time-travel data-encryption process; it could make ordinary data transmission safer and more resistant to tampering by other time-cloak operators and by reducing the chances that other types of interference would corrupt a stream of data.

In last year's paper the Perdue team described a system it used to cloak data at rates far higher than anyone had before, but couldn't decode the cloaked data so it could be received on the other end. However, by "cloaking" 46 percent of the data capacity of the fiber and sending high-energy bursts of modulated nonsense data, they did make it almost impossible for anyone else's data to get through.

It wasn't clear whether the two-way cloaking system could defeat interference from the old version, however. The team took one of the two lasers off the fiber to keep the data stream from being cloaked in the beam from the second laser, and pitted one system against the other.

They found that the one-way system really did destroy any effort to get data through the fiber. When they turned on the modulation that would have hidden the data stream completely in a two-laser setup, however, they found it was the interference from the one-way attack modulator that disappeared. The demodulator received data cleanly and without any sign that the interference bothered it at all.

So the same modulation and encoding process that cloaked data streams completely when all its components were in place, protected itself from interference by systems just as smart, but with fewer outlets for their need to communicate.

The result is a set of three potential uses for a single bit of cool time-manipulation: A cloaking modulator with no receiver at the other end can scramble data sent by someone else, unless it's "cloaked" by a similar device; a cloaking system running on a regular fiberoptic network with no extra laser or power of invisibility can be counted on to work reliably even when attackers use its own tricks against it; and a cloaking system with all the bells and whistles functioning can make it impossible for outsiders to even see a stream of data that needs to stay really secret.

Unfortunately, even though the Purdue systems used mostly off-the-shelf hardware, there is still too much research to be done to expect invisible networks to show up in networking catalogs during the next year or so.

The multiple advances in that one set of experiments do open a lot of new possibilities, however. Not just for hiding data or defending against interference, but also for the rich possibilities inherent in the ability to manipulate the light sent over fiberoptic cables to make data and photons do things few people interested in high-speed networks had really considered before.

Invisible invincibility may turn out not to be the greatest superpower, inside a glass tube filled with data, at least.

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