Whether for use in cybersecurity, gaming or scientific simulation, the world
needs true random numbers, but generating them is harder than one might
think. But a group of Brown University physicists has developed a technique
that can potentially generate millions of random digits per second by
harnessing the behavior of skyrmions—tiny magnetic anomalies that arise in
certain two-dimensional materials.
Their research, published in Nature Communications, reveals previously
unexplored dynamics of single skyrmions, the researchers say. Discovered
around a half-decade ago, skyrmions have sparked interest in physics as a
path toward next-generation computing devices that take advantage of the
magnetic properties of particles—a field known as spintronics.
"There has been a lot of research into the global dynamics of skyrmions,
using their movements as a basis for performing computations," said Gang
Xiao, chair of the Department of Physics at Brown and senior author of the
research. "But in this work, we show that purely random fluctuations in the
size of skyrmions can be useful as well. In this case, we show that we can
use those fluctuations to generate random numbers, potentially as many as 10
million digits per second."
Most random numbers produced by computers aren't random in the strictest
sense. Computers use an algorithm to generate random numbers based on an
initial starting place, a seed number. But because the algorithm used to
generate the number is deterministic, the numbers aren't truly random. With
enough information about the algorithm or its output, it could be possible
for someone to find patterns in the numbers that the algorithm produces.
While pseudorandom numbers are sufficient in many settings, applications
like data security—which uses numbers that can't be guessed by an outside
party—require true random numbers.
Methods of producing true random numbers often draw on the natural world.
Random fluctuations in electrical current flowing through a resistor, for
example, can be used to generate random numbers. Other techniques harness
the inherent randomness in quantum mechanics—the behavior of particles at
the tiniest scale.
This new study adds skyrmions to the list of true random number generators.
Skyrmions arise from the "spin" of electrons in ultra-thin materials. Spin
can be thought of as the tiny magnetic moment of each electron, which points
up, down or somewhere in between. Some two-dimensional materials, in their
lowest energy states, have a property called perpendicular magnetic
anisotropy—meaning the spins of electrons all point in a direction
perpendicular to the film. When these materials are excited with electricity
or a magnetic field, some of the electron spins flip as the energy of the
system rises. When that happens, the spins of surrounding electrons are
perturbed to some extent, forming a magnetic whirlpool surrounding the
flipped electron—a skyrmion.
Skyrmions, which are generally about 1 micrometer (a millionth of a meter)
or smaller in diameter, behave a bit like a kind of particle, zipping across
the material from side to side. And once they're formed, they're very
difficult to get rid of. Because they're so robust, researchers are
interested in using their movement to perform computations and to store
data.
This new study shows that in addition to the global movement of skyrmions
across a material, the local behavior of individual skyrmions can also be
useful. For the study, which was led by Brown postdoctoral fellow Kang Wang,
the researchers fabricated magnetic thin films using a technique that
produced subtle defects in the material's atomic lattice. When skyrmions
form in the material, these defects, which the researchers call pinning
centers, hold the skyrmions firmly in place rather than allowing them to
move as they normally would.
The researchers found that when a skyrmion is held in place, they fluctuate
randomly in size. With one section of the skyrmion held tightly to one
pinning center, the rest of the skyrmion jumps back and forth, wrapping
around two nearby pinning centers, one closer and one farther away.
"Each skyrmion jumps back and forth between a large diameter and a small
diameter," Wang said. "We can measure that fluctuation, which occurs
randomly, and use it to generate random numbers."
The change in skyrmion size is measured through what's known as the
anomalous Hall effect, which is a voltage that propagates across the
material. This voltage is sensitive to the perpendicular component of
electron spins. When the skyrmion size changes, the voltage changes to an
extent that is easily measured. Those random voltage changes can be used to
produce a string of random digits.
The researchers estimate that by optimizing the defect-spacing in their
device, they can produce as many as 10 million random digits per second,
providing a new and highly efficient method of producing true random
numbers.
"This gives us a new way of generating true random numbers, which could be
useful for many applications," Xiao said. "This work also gives us a new way
of harnessing the power of skyrmions, by looking at their local dynamics as
well as their global movements."
Reference:
Kang Wang et al, Single skyrmion true random number generator using local
dynamics and interaction between skyrmions, Nature Communications (2022).
DOI: 10.1038/s41467-022-28334-4
Tags:
Physics