‘Time Reflections’ Are Possible. They Could Speed Up Our Cell Phones and Computers

To reverse signals in time, we’ve always used a digital approach. Now, a new analog method could dramatically improve wireless communications.

• If scientists are able to successfully conduct analog time-reflection of electromagnetic waves, they can greatly speed up some computer and phone applications.

• When a research team created a highly customized circuit earlier this year, they were able to reflect signals in time, which has never been reported before.

• When a signal is reflected in time, it is inverted in time and its frequency is altered.

If you shine a light at a mirror, the beam is reflected inward to a central focal point, bouncing back toward you in a predictable manner—it’s a simple reflection of light, a spatial inversion.

But what if you could reverse an analog signal in time?

It sounds like the stuff of sci-fi, but earlier this year, a paper published in the journal Nature Physics showed the first evidence of a time-reversed electromagnetic signal. “One of the big holy grails of physics is time-reversal symmetry,” Michael Pravica, Ph.D., a professor of physics at the University of Nevada, Las Vegas, tells Popular Mechanics.

When a signal is reflected in time, it travels back toward its source in reverse order and with a new frequency. Scientists have known that this phenomenon of time reflection could happen, theoretically, since 1958. Now, as the new experiment has shown, this effect can occur in a highly customized circuit.

Andrea Alù, Ph.D., founding director of the City University of New York’s Advanced Science Research Center Photonics Initiative and corresponding author of the paper, says that his team accomplished this surprising result by building a circuit that contains a six-meter-long copper trace that runs back and forth across a low-loss dielectric substrate and has around 50 capacitor switches in it. (The dielectric base is an insulating material.)

The switches have to be very dense so that the wave cannot “see” that they are discrete, Alù explains. “We need to make sure that the switches are much more dense than the wavelength of the signal,” he tells Popular Mechanics.

When the capacitor switches turn on, Alù says, the index of refraction of the circuit changes by a factor of two, which results in the frequency of the reflected signal being halved. “The material with switches on has two times the index of refraction of the material without switches on. We moved from a low index to a large index by adding capacitors, and we moved from a large index to a low index by disconnecting capacitors. A capacitor is something that stores electrical energy.”

Designing the circuit involved altering its reflectivity to signals faster than the speed of the signal itself, with a sufficient level of contrast between the material when the switches are on and when they are off, Alù says. “If the contrast is too small, then you don’t see reflection. The ingredients to have strong time reflections are speed, uniformity, and large contrast.”

There is an upper limit to the frequencies that this circuit can time-reflect, Alù says. His team’s goal is now to expand this limit. “The next steps are to push the frequencies higher. Right now, we have a 70-MHz maximum frequency.”

The physicists were surprised by the results of the experiment because they found that the reflection coefficient was the opposite sign from what they expected, Alù says. “This is very fascinating. We had to redevelop the entire theory, and what we found was that our time interface was different from the usual time interface. And that was originally a mystery—and then we understood why.”

It will be a long time before this technology is ready for commercial use, Alù says. But it has the potential to greatly simplify and streamline the process of reversing signals in time, which is currently done digitally. Digital time reflection uses much more energy and memory than this new, analog approach. Time reflection could be used for imaging technology, distortion compensation, or wireless communication. For example, cell phones currently use a digital approach to reflect signals in time when they are communicating from a distance.

“What is impressive is that in very fast time and with very small energy, you can frequency-convert over a very large range of frequencies,” Alù says. “This is very difficult to do.”

“What we’re doing next is to shrink this down to an integrated system,” Alù says. “This will make it much smaller, much more convenient to integrate in a cell phone or in a circuit. It can push the frequencies up by a couple of orders of magnitude.”

Some researchers are looking at materials that can respond to light very fast to create circuits like this, Alù says. “There are some materials that have such a strong interaction with light that you can actually realize sharp changes in the optical response in the range of femtoseconds. Then you can really do these effects for light.”

It’s much more challenging working with signals at optical frequencies because those signals oscillate very rapidly, Alù says. “We had this breakthrough because we came up with these ideas of adding switches. These switches will not work when you want to work at optical frequencies. They stop working at 1 THz.”

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