Ultrafast Computers Are Coming: Laser Bursts Drive Fastest-Ever Logic Gates

Synchronized laser pulses (red and blue) cause a burst of real and virtual charge carriers in graphene to be absorbed by gold metal, resulting in a net current.

“We clarified the role of virtual and real charge carriers in laser-induced currents, and that opened the way to the creation of ultrafast logic gates,”explains Ignacio Franco, associate professor of chemistry and physics at the University of Rochester.

Researchers have taken a decisive step toward creating ultrafast computers.
Science and technology have long sought to develop electronics and information processing that function on the shortest timescales permitted by the laws of nature.

Lightwave electronics is a promising way to achieving this aim that entails using laser light to regulate the migration of electrons in matter and then using this control to construct electronic circuit elements.

Surprisingly, lasers now allow us to generate femtosecond bursts of electricity—that is, in a millionth of a billionth of a second. Despite this, our ability to handle data at such extreme speeds has remained elusive. Researchers have taken a significant step forward in the development of ultrafast computers.

Researchers from the University of Rochester and Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have demonstrated a femtosecond logic gate, a fundamental building component of computation and information processing. The achievement was done by harnessing and independently regulating the actual and virtual charge carriers that make up these ultrafast bursts of electricity for the first time, as published in the journal Nature on May 11.

The breakthroughs made by the researchers have paved the way for information processing at the petahertz limit, which allows one quadrillion computational processes to be executed every second. That's over a million times quicker than today's computers, which use gigahertz clock speeds (1 petahertz equals one million gigahertz).

“This is a great example of how fundamental science can lead to new technologies,” says Ignacio Franco, an associate professor of chemistry and physics at Rochester who worked on the theoretical calculations that led to the finding with doctoral student Antonio José Garzón-Ramrez '21 (PhD).

Lasers generate ultrafast bursts of electricity

Scientists have just discovered how to manufacture ultrafast bursts of electrical currents using laser pulses lasting a few femtoseconds. This can be accomplished by illuminating microscopic graphene-based wires that join two gold metals, for example. The ultrashort laser pulse sets the electrons in graphene in motion and, more significantly, directs them in a certain direction, resulting in a net electrical current.

Laser pulses may generate electricity significantly more quickly than any other method, even when no voltage is provided. Furthermore, by modifying the shape of the laser pulse, the direction and amount of the current may be regulated (that is, by changing its phase). Science and technology have long sought to develop electronics and information processing that function on the shortest timescales permitted by the laws of nature.

The breakthrough: Harnessing real and virtual charge carriers

For several years, Franco's and Peter Hommelhoff's research groups have been attempting to convert light waves into ultrafast current pulses.

The scientists realized that in gold-graphene-gold junctions, two flavors of the particles transporting the charges that make up these bursts of electricity can be generated—"real" and "virtual."

"Real" charge carriers are electrons that have been stimulated by light and continue to move in a directed direction after the laser pulse has stopped.

Electrons that are only set in net directional motion while the laser pulse is on are called "virtual" charge carriers. As a result, they are elusive creatures that only exist during periods of illumination.

Because graphene is coupled to gold, the metal absorbs both actual and virtual charge carriers, resulting in a net current.

Surprisingly, the researchers realized that by modifying the structure of the laser pulse, they could construct currents in which only real or virtual charge carriers are involved. In other words, they not only learned to generate two types of currents, but also how to manage them independently, a discovery that greatly enhances the design components in lightwave electronics.

Logic gates through lasers

The team was able to experimentally demonstrate logic gates that work on a femtosecond timescale using this augmented control landscape for the first time.

The building blocks of computation are logic gates. They regulate the processing of incoming data in the form of 0 or 1 (bits). Logic gates have two input signals and output logic.

The shape or phase of two synchronized laser pulses, each designed to only generate a burst of actual or virtual charge carriers, are used as input signals in the researchers' experiment. These two contributions to the currents can either add up or cancel out depending on the laser phases used. An ultrafast logic gate can be created by assigning logical information 0 or 1 to the net electrical signal.

“It will probably be a very long time before this technique can be used in a computer chip, but at least we now know that lightwave electronics is practically possible,” says Tobias Boolakee, a PhD student at FAU who led the experimental work.

“Our results pave the way toward ultrafast electronics and information processing,” says Garzón-Ramrez, a postdoctoral researcher at McGill.

“What is amazing about this logic gate,” Franco explained, “is that the operations are performed not in gigahertz, like in regular computers, but in petahertz, which are one million times faster. This is because of the really short laser pulses used that occur in a millionth of a billionth of a second.”

From fundamentals to applications

Fundamental studies of how charge can be driven in nanoscale systems using lasers gave rise to this new, potentially disruptive technology.

“Through fundamental theory and its connection with the experiments, we clarified the role of virtual and real charge carriers in laser-induced currents, and that opened the way to the creation of ultrafast logic gates,” Franco continues.

Franco has spent over 15 years researching this study. He developed a way to generate ultrafast electrical currents in molecular wires subjected to femtosecond laser pulses while a PhD student at the University of Toronto in 2007. This first concept was later tested in 2013 and the Franco group discussed the precise methodology behind the trials in a 2018 publication. Since then, there has been "explosive" experimental and theoretical growth in this field, according to Franco.

“This is an area where theory and experiments challenge each other and, in doing so, unveil new fundamental discoveries and promising technologies,” he explains.

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