A logic gate developed by researchers from the University of Rochester and the Friedrich-Alexander-Universität (FAU) Erlangen-Nürnberg operates at femtosecond timescales, potentially enabling information processing at the petahertz limit. Logic gates, which are the basic building blocks necessary for computation, control how incoming information taking the form of a 1 or 0 is processed. Logic gates require two input signals and yield a logic output. In recent years, lasers have been developed that are able to generate pulses lasting a few femtoseconds to generate ultrafast bursts of electrical currents. This is done by illuminating tiny graphene-based wires connecting two gold metals. The ultrashort laser pulse sets in motion, or excites, the electrons in graphene and sends them in a particular direction, therefore generating a net electrical current. Laser pulses are able to generate electricity far faster than any traditional method and can do so in the absence of applied voltage. The direction and magnitude of the current can be controlled by varying the shape of the laser pulse, or changing its phase, in other words. Synchronized laser pulses (red and blue) generate a burst of real and virtual charge carriers in graphene that are absorbed by gold metal to produce a net current. Courtesy of Michael Osadciw/University of Rochester. In trying to reconcile the experimental measurements at Erlangen with computational simulations at Rochester, the team found that it could generate two types of particles carrying the charges — real and virtual — in gold-graphene-gold junctions. Real charge carriers are electrons excited by light that remain in directional motion even after the laser pulse is turned off. Virtual charge carriers are only set in motion while the laser pulse is on. As such, they are elusive and only exist transiently during illumination. Because the graphene is connected to gold, both real and virtual charge carriers are absorbed by the metal to produce a net current. The team found that by changing the laser pulse’s shape, it could generate currents where only the real or virtual charge carriers play a role. This finding, the ability to independently control two types of currents, drastically augments the design elements of lightwave electronics. Using this augmented control landscape, the team experimentally demonstrated, for the first time, logic gates that operated on the femtosecond timescale. In the experiment, the input signals are the shape or phase of two synchronized laser pulses, each one chosen to only generate a burst of real or virtual charge carriers. Depending on the laser phases used, these two contributions to the currents can either add up or cancel. The net electrical signal can be assigned logical information 0 or 1, yielding an ultrafast logic gate. “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,” said Tobias Boolakee, who led the experimental efforts as a Ph.D. student at FAU. The study represents the culmination of more than 15 years of research by Ignacio Franco, an associate professor of chemistry and physics at Rochester. The work was published in Nature (www.doi.org/10.1038/s41586-022-04565-9).