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Quantum physics puts a speed limit on electro

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image: An ultra-short laser pulse (blue) creates free charge carriers, another pulse (red) accelerates them in opposite directions.
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Credit: TU Wien

How fast can electronics go? As computer chips operate with ever-shorter signals and time intervals, at some point they come up against physical limits. Quantum mechanical processes that allow the generation of electric current in a semiconductor material take some time. This limits the speed of signal generation and transmission.

TU Wien (Vienna), TU Graz and the Max Planck Institute for Quantum Optics in Garching have now been able to explore these limits: the speed certainly cannot be increased beyond one petahertz (one million gigahertz), even if the material is optimally excited with laser pulses. This result has just been published in the scientific journal Nature Communication.

Fields and currents

Electric current and light (i.e. electromagnetic fields) are always linked. This is also the case in microelectronics: in microchips, electricity is controlled using electromagnetic fields. For example, an electric field can be applied to a transistor, and depending on whether the field is on or off, the transistor passes electric current or blocks it. In this way, an electromagnetic field is converted into an electrical signal.

In order to test the limits of this conversion of electromagnetic fields into current, laser pulses – the fastest and most precise electromagnetic fields available – are used, rather than transistors.

“The materials studied do not initially conduct electricity at all,” explains Professor Joachim Burgdörfer from the Institute for Theoretical Physics at TU Wien. “These are struck by an ultra-short laser pulse with a wavelength in the extreme UV range. This laser pulse moves the electrons to a higher energy level, so they can suddenly move freely In this way, the laser pulse transforms the material into an electrical conductor for a short period of time.” As soon as there are free-moving charge carriers in the material, they can be moved in a certain direction by a second, slightly longer laser pulse. This creates an electric current which can then be detected with electrodes on both sides of the material.

These processes occur extremely rapidly, on a timescale of atto- or femtoseconds. “For a long time, such processes were considered instantaneous,” explains Prof. Christoph Lemell (TU Wien). “Today, however, we have the technology to study the time evolution of these ultrafast processes in detail.” The crucial question is: how fast does the material react to the laser? How long does the signal generation take and how long does it take until the material can be exposed to the next signal? The experiments were carried out in Garching and Graz, the theoretical work and the complex computer simulations were carried out at TU Wien.

Time or energy – but not both

The experiment leads to a classic uncertainty dilemma, as often occurs in quantum physics: To increase the speed, extremely short UV laser pulses are needed, so that free charge carriers are created very quickly. However, the use of extremely short pulses means that the amount of energy that is transferred to the electrons is not precisely defined. Electrons can absorb very different energies. “We can tell exactly when free charge carriers are created, but not what energy state they are in,” explains Christoph Lemell. “Solids have different energy bands, and with short laser pulses, many of them are inevitably populated with free charge carriers at the same time.”

Depending on the amount of energy they carry, electrons react very differently to the electric field. If their exact energy is unknown, it is no longer possible to control them precisely and the current signal produced is distorted, especially at high laser intensities.

“It turns out that about one petahertz is an upper limit for controlled optoelectronic processes,” explains Joachim Burgdörfer. Of course, this does not mean that it is possible to produce computer chips with a clock frequency just below one petahertz. Realistic technical upper limits are most likely considerably lower. Although the laws of nature determining the ultimate speed limits of optoelectronics cannot be overcome, they can now be analyzed and understood through sophisticated new methods.


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