Quantum physics sets a speed limit to electronics

Date:
March 25, 2022
Source:
Vienna University of Technology
Summary:
Semiconductor electronics is getting faster and faster - but at
some point, physics no longer permits any increase. The speed
can definitely not be increased beyond one petahertz (one million
gigahertz), even if the material is excited in an optimal way with
laser pulses.



FULL STORY ========================================================================== Semiconductor electronics is getting faster and faster -- but at some
point, physics no longer permits any increase. The speed can definitely
not be increased beyond one petahertz (one million gigahertz), even if
the material is excited in an optimal way with laser pulses.


==========================================================================
How fast can electronics be? When computer chips work with ever shorter
signals and time intervals, at some point they come up against physical
limits. The quantum-mechanical processes that enable the generation of
electric current in a semiconductor material take a certain amount of
time. This puts a limit to the speed of signal generation and signal transmission.

TU Wien (Vienna), TU Graz and the Max Planck Institute of Quantum
Optics in Garching have now been able to explore these limits: The
speed can definitely not be increased beyond one petahertz (one million gigahertz), even if the material is excited in an optimal way with laser pulses. This result has now been published in the scientific journal
Nature Communications.

Fields and currents Electric current and light (i.e. electromagnetic
fields) are always interlinked. This is also the case in microelectronics:
In microchips, electricity is controlled with the help of electromagnetic fields. For example, an electric field can be applied to a transistor,
and depending on whether the field is switched on or off, the transistor
either allows electrical current to flow 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
to current, laser pulses -- the fastest, most precise electromagnetic
fields available -- are used, rather than transistors.

"Materials are studied that initially do not conduct electricity at all," explains Prof. Joachim Burgdo"rfer from the Institute for Theoretical
Physics at TU Wien. "These are hit by an ultra-short laser pulse with a wavelength in the extreme UV range. This laser pulse shifts the electrons
into a higher energy level, so that they can suddenly move freely. That
way, the laser pulse turns the material into an electrical conductor
for a short period of time." As soon as there are freely 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
that can then be detected with electrodes on both sides of the material.

These processes happen extremely fast, on a time scale of atto-
or femtoseconds. "For a long time, such processes were considered instantaneous," says Prof. Christoph Lemell (TU Wien). "Today, however,
we have the necessary 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 one have to wait until the material can be exposed
to the next signal? The experiments were carried out in Garching and
Graz, the theoretical work and complex computer simulations were done
at TU Wien.

Time or energy -- but not both The experiment leads to a classic
uncertainty dilemma, as it often occurs in quantum physics: in order to increase the speed, extremely short UV laser pulses are needed, so that
free charge carriers are created very quickly.

However, using extremely short pulses implies that the amount of energy
which is transferred to the electrons is not precisely defined. The
electrons can absorb very different energies. "We can tell exactly
at which point in time the free charge carriers are created, but
not in which energy state they are," says Christoph Lemell. "Solids
have different energy bands, and with short laser pulses many of them
are inevitably populated by free charge carriers at the same time."
Depending on how much energy they carry, the electrons react quite
differently to the electric field. If their exact energy is unknown, it
is no longer possible to control them precisely, and the current signal
that is produced is distorted -- especially at high laser intensities.

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


========================================================================== Story Source: Materials provided by Vienna_University_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. M. Ossiander, K. Golyari, K. Scharl, L. Lehnert, F. Siegrist, J. P.

Bu"rger, D. Zimin, J. A. Gessner, M. Weidman, I. Floss, V. Smejkal,
S.

Donsa, C. Lemell, F. Libisch, N. Karpowicz, J. Burgdo"rfer,
F. Krausz, M.

Schultze. The speed limit of optoelectronics. Nature Communications,
2022; 13 (1) DOI: 10.1038/s41467-022-29252-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220325093932.htm

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