One million gigahertz is the physical speed limit for computers

March 25, 2022

(News from Nanowerk) The maximum signal transmission speed in microchips is about one petaherz (one million gigahertz), which is about 100,000 times faster than current transistors. Physicists from the Ludwig Maximilian University of Munich, the Max Planck Institute for Quantum Optics, and the Universities of Technology in Vienna and Graz recently published this discovery in the scientific journal Nature Communication (“The speed limit of optoelectronics”).

Whether computer chips of this maximum speed can ever be produced is, however, debatable.

Microelectronics pursues two approaches to making computers faster. On the one hand, work is underway to make the components ever smaller so that the data transmission (signal path from A to B) literally “doesn’t take so long”. The physical limit of this miniaturization is the size of an atom. A circuit cannot be physically smaller.

The second possibility for faster data transmission is to speed up the switching signals of the transistors themselves. These are the components of the microchips that block or allow current to flow. And that’s where the research of the German-Austrian physics group comes in.

High frequency light as a speed booster

In this case, fast means “high frequency”, as Martin Schultze, lead author and director of the Institute for Experimental Physics at the Graz University of Technology (TU Graz) explains: “The faster you want to go, the more the high frequency the electromagnetic signal has to be – and at some point we enter the range of the frequency of light, which can also be considered or used as an electromagnetic signal.

This happens, for example, in optoelectronics, where light is used to excite electrons in the semiconductor from the valence band (the area where electrons normally reside) to the conduction band, so that it jumps from the isolated state to the conductive state. The excitation energy is determined by the semiconductor material itself. It lies in the frequency range of infrared light, which ultimately also corresponds to the maximum achievable speed that can be achieved with such materials.

Dielectric material: top candidate for speed records

Dielectric materials (such as glass or ceramics) could overcome these limitations, as they require much more energy to be excited than semiconductors. More energy in turn enables the use of high frequency light and therefore faster data transmission.

Unfortunately, however, dielectric materials cannot conduct electricity without breaking, as noted by Marcus Ossiander, the study’s first author and currently a post-doctoral researcher at Harvard University: “For example, if you apply an electromagnetic field to glass to make it conduct electricity, it usually causes the glass to break and a hole to form.

The solution the research group chose for their research was to keep the applied voltage pulse or switching frequency so short that the material had no time to break at all.

The right pulse gives the right answers

Specifically, the physicists used an ultra-short laser pulse with a frequency in the extreme UV range for their investigations. They bombarded a sample of lithium fluoride with this laser pulse. Lithium fluoride is dielectric and has the largest band gap of any known material. It is the distance between the valence band and the conduction band.

The ultra-short laser pulse brought the lithium fluoride electrons into a more energetic state so they could move freely. In this way, the material briefly became an electrical conductor. A second, slightly longer laser pulse directed the excited electrons in a desired direction, creating an electrical current that could then be detected with electrodes on both sides of the material. The measurements provided answers to questions of how quickly the material reacted to the ultra-short laser pulse, how long it took to generate the signal, and how long to wait until the material could be exposed to the following signal.

“It follows that at about one petahertz there is an upper limit for controlled optoelectronic processes,” explains Joachim Burgförder from the Institute for Theoretical Physics at TU Wien.

This, of course, does not mean that computer chips can be produced with a clock frequency of just under one petahertz. But one thing is certain: for the time being, optoelectronics will not become faster than what has been shown in experiments. How close future technologies will approach this limit is written in the stars.