Paul van Gerven
31 March

Researchers from TU Wien, TU Graz and the Max Planck Institute of Quantum Optics have determined the upper limit for the speed of electronic signal generation and transmission to be about 1 petahertz (1015 hertz). Considering the Guinness World Record for the highest CPU clock rate is 8.4 gigahertz (set by an overclocked AMD chip that was cooled with liquid helium and nitrogen), there’s still plenty of room for improvement.

To be more precise, the researchers tested the speed at which electromagnetic fields can be converted to current. Instead of transistors, they used lasers, the most precise electromagnetic fields available. Firing ultrashort laser pulses onto materials that are normally insulating, electrons are excited and free to move around for a short time. This can be measured by encouraging movement with a second laser pulse and detecting electrical current with electrodes attached to the material.

speed limit
An ultrashort laser pulse (blue) creates free charge carriers, another pulse (red) accelerates them in opposite directions. Credit: TU Wien

Shooting shorter and shorter laser pulses at the material, the researchers eventually hit a limit in the current they observed. This limit stems from a famous cornerstone of quantum mechanics: the uncertainty principle. The shorter the laser pulses, the quicker free charge carriers are created, but, at the same time, the less well-defined the energy that’s transferred to the electrons. Since the energy state of an electron determines its reaction to an electric field, the current signal becomes more and more distorted as the laser pulses get shorter. At about 1 petahertz pulses, the signal vanishes.

The team doesn’t expect electronics to ever come close to the limit it has determined. “Realistic technical upper limits are most likely considerably lower. Even though the laws of nature determining the ultimate speed limits of optoelectronics can’t be outsmarted, they can now be analyzed and understood with sophisticated new methods,” reads a press release.