Shaped waves promise speed

By Kimberly Patch, Technology Research News

Scientists recently refined the law of physics that says nothing can travel faster than the speed of light in a vacuum. Though the results aren't likely to make physics textbooks obsolete, they could yield techniques for significantly speeding up computer chips.

The superluminal, or faster-than-light, effects reported last summer use wave shaping techniques to effectively boost the speed of a signal traveling on an electromagnetic wave. Researchers from the University of California at Berkeley have outlined two ways that superluminal effects can speed computer circuits.

The effects do not actually allow information to travel faster than the electromagnetic waves that carry the information signals. According to the laws of physics, that will remain impossible. They could, however, be used to speed information winding through computer circuits because those signals travel considerably slower than the speed of light due to delays introduced by electronics and the structure of the circuits.

The effects could potentially boost computer speeds by as much as two to ten times, said Daniel Solli, one of the researchers and a graduate student at the University of California at Berkeley.

To picture how the faster-than-light effects work, consider a train speeding along from Washington to New York. There's no way any person traveling on that train can get to New York faster than the arrival time of the train. By the same token, information cannot travel any faster than the speed of the wave the information is traveling on.

However, the people exiting the front cars of the train, once it arrives in New York, have arrived a little further along the track than those exiting from the cars in the back. And if a person entered the train's last car in Washington just as it was leaving, walked the length of the train during the trip, then exited from the first car, she actually traveled as far as the train traveled, plus a little more in the time the train took to get from Washington to New York.

The superluminal effects are similar. No information is actually traveling faster than the speed of the wave carrying it, but within a smooth, continuous waveform, if the crest of the wave is pushed forward as the signal travels, it arrives earlier than it would ordinarily.

The researchers put together a circuit that pushed the crest of a wave forward as it was traveling within the circuit, an experiment that opens the possibility that superluminal effects could be used to speed computer signals.

The researchers have found two distinct ways this could be done.

The first way is simply to increase the speed of individual computer signals. This is possible because, although the wave-shaping distorts the wave, the binary electric signals of computer circuits are simple enough to allow a certain amount of distortion. In a computer signal, if the crest of a wave passes a certain height, that bit of information is a one; if it does not the bit is a zero.

Digital signals can be depicted as the presence or absence of a step on a simple graph: a signal, or step happens when the voltage of the circuit reaches a certain value. While the voltage remains less than that value, there's no signal, and the graph remains flat. In reality, however the steps are not sharp, but curve up and down, because it takes time for the voltage to reach the level it needs to trip the signal. "If microprocessors had signals that [really] look like steps, then there would be nothing we could do to improve it," said Solli. But in reality the circuits are waiting for some voltage thresholds to appear, he said.

Superluminal effects can be used to push the front of the wave forward so the voltage reaches its threshold value sooner, both on the way up and on the way down, according to Solli. "You can use this scheme to sharpen up the signal so it looks more like the actual step," he said.

The use of this effect also depends on the sensitivity of the electronics, he added. "We need to find out exactly how much is possible -- we don't really know that yet," he said.

The second way the effects could be used has to do with clock skew, or mismatches in the time it takes signals to travel.

Electrons traveling around computer circuits are generally plagued by various delays, said Solli. "The... processing speed of microprocessors is limited by transistor latency and interconnect delays [caused by] finite resistances and capacitances which can never be fully eliminated from any real electronic components. In high-speed electronics these become important limiting factors," he said.

Computers are constantly comparing two signals, and often they are waiting for the second signal to arrive because the different paths around the circuit board have different delay times, said Solli. This difference in time is the clock skew. The computer "just sits there and waits for the other signal to get there... that time is just totally wasted," he said.

Instead of eliminating the positive delays introduced by the transistors and interconnects, the researchers are proposing to compensate for them using superluminal effects, effectively making all paths the same length for the electron signals that traverse them.

"What our device would do is create a path independence. Basically it [wouldn't] matter what path you take to get from one point to another on a chip, all paths would have almost the same delay," he said.

According to the researchers, using the faster-than-light effects in these ways could eventually speed computers significantly. "It's complicated... we don't know exactly how much the electronics will be able to speed up individual signals [or] how important the path independence idea will be. But... these effects should result in speed up of several factors, not just a small percentage. We think that speed up on the order of anywhere from two to tenfold might be possible," said Solli.

It's an open question whether or not the kinds of superluminal effects which have been found over the past few years could be useful, said Aephraim Steinberg, an assistant professor of physics at the University of Toronto. Although transmitting information faster than light remains impossible, "in real physical systems, we almost always transmit information significantly slower than light, and there are good reasons to believe that these new superluminal effects might be useful for compensating such delays, and bringing our overall transmission rate closer to the ultimate speed limit," he said.

More research is needed to be able to tell whether this will actually prove practical, however, Steinberg added.

The researchers' next steps are "looking at what the best way of implementing these things are in real computer circuits [and] actually building some prototype computer circuits," said Solli. "We hope that our ideas will be ready for practical application in less than 10 years," he said.

Solli's research colleagues were Raymond Y. Chiao of the University of California at Berkeley and Jandir M. Hickmann from the University of California at Berkeley, on leave from the Federal University of Alagoas in Brazil. The research was funded by the Office of Naval Research (ONR) and NASA.

Timeline:   < 10 years
Funding:   Government
TRN Categories: Integrated Circuits; Physics
Story Type:   News
Related Elements:  Technical paper, "Faster-Than-Light Effects and Negative Group Delays in Optics Electronics, and their Applications," posted on CoRR: Technical paper, "Gain Assisted Superluminal Light Propagation," Nature, July 20, 2000.


April 25, 2001

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