Logic gate breaks speed record

28 Jun 2022 Isabelle Dumé

Towards ultrafast logic gates (Courtesy: University of Rochester illustration / Michael Osadciw)

The first logic gate to operate at femtosecond timescales could help usher in an era of information processing at petahertz frequencies – a million times faster than today’s gigahertz-scale computers. The new gate, developed by researchers at the University of Rochester in the US and the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) in Germany, is an application of lightwave electronics – essentially, shuffling electrons around with light fields – and harnesses both real and virtual charge carriers.

In lightwave electronics, scientists use laser light to guide the motion of electrons in matter, then exploit this control to create electronic circuit elements. “Since light oscillates so fast (roughly a few hundred million times per second), using light could speed up electronics by a factor of roughly 10 000 as compared to computer chips,” says Tobias Boolakee, a laser physicist in Peter Hommelhoff’s group at the FAU and the first author of a study in Nature on the new gate. “With our present work, we have been able propose the idea for a first light field-driven logic gate (the fundamental building block for any computer architecture) and also demonstrate its working principle experimentally.”

In the work, Boolakee and colleagues prepared tiny graphene-based wires connected to two gold electrodes and illuminated them with a laser pulse lasting a few tens of femtoseconds (10-15 s). This laser pulse excites, or sets in motion, the electrons in graphene and causes them to propagate in a particular direction – so generating a net electrical current.

Virtual and real charge carriers

Researchers at the FAU and Rochester have been working on lightwave electronics for the past decade, and the latest work takes advantage of their recent discovery that exciting the gold-graphene junction excites two different kinds of electronic charge carriers: virtual and real. The virtual carriers are only set in a net directional motion while the laser pulse is on, the researchers explain, and as such are transient. The contribution of the virtual carriers to the net current must therefore be measured during light excitation.

The researchers performed this measurement by probing a net polarization induced by the virtual carriers in the gold electrodes attached to the graphene. The real charge carriers, for their part, continue propagating in the preferred direction even after the laser pulse is turned off, so their contribution to the net current can be measured after light excitation has ended.

According to the researchers, the results of the measurement were “striking”: by changing the shape of the laser pulse, they found they could generate currents in which only the real or only the virtual charge carriers play a role. Being able to control the two different types of charge carriers in this way allowed them to make a logic gate operating on the femtosecond timescale for the first time.

Logic gate operations

The basic idea of the new logic gate is to encode two binary signals (0 and 1, as is standard in computer logic) in the shape of two few-cycle laser pulses – that is, in their “carrier-envelope” phase, Hommelhoff explains. When these two laser pulses interact with the gold-graphene heterostructure, each one produces an ultrafast current pulse. Hence, from the two incoming laser pulses, the researchers can generate two current pulses that either add up or cancel each other out.

“A binary output signal (again 0 or 1) is obtained from the level of the resulting electric current measured at one of the gold electrodes,” Hommelhoff tells Physics World. “The timescale for the logic operations is fundamentally limited by the turn-on time of the two current pulses, which is intrinsically given by the underlying quantum-mechanical mechanisms driven by the frequency of the laser pulse.”

With the parameters used in their experiment, the Rochester-FAU team anticipates an upper limit for the bandwidth of their logic gate at the driving optical frequency of 0.36 PHz, or equivalently, 2.8 fs.READ MORE

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While the researchers are – at least for the moment – hesitant about direct applications for the new gate, they say the next step will be to prove that it can operate at much faster time scales than can conventional electronics. “We are quite positive that this is the case, but scaling up our system to more gates to form a complex logic will be much more of an issue: here we will need to find ways to keep the speeds high,” Boolakee says.

As for integrating these gates into actual devices, the team note that the system will need to be much smaller than it is now. This will mean resorting to nearfield optics schemes to circumvent the fact that the laser focus cannot be made much smaller than the wavelengths of the actual driving laser pulses (around 800 nm), which is much too large for electronics length scales.

“Finally, the laser pulses we used in this work need to be quite intense, which is another point that will make scaling up difficult,” Hommelhoff says. “In essence, much more fundamental and well as applied research is needed to turn this proof-of-principle demonstration into a new technology. But at least we have made the initial step: the demonstration of a new logic gate.”

Isabelle Dumé is a contributing editor to Physics World

from physicsworld.com     28/8/2022