The forces between particles, atoms, molecules, or even macroscopic objects
like magnets are determined by the interactions of nature. For example, two
closely lying bar magnets realign themselves under the influence of magnetic
forces. A team led by Prof. Dr Matthias Weidemüller and Dr Gerhard Zürn at the
Center for Quantum Dynamics of Heidelberg University has now succeeded in its
aim to change not only the strength but also the nature of the interaction
between microscopic quantum magnets, known as spins. Instead of falling into a
state of complete disorder, the especially prepared magnets can maintain their
original orientation for a long period. With these findings, the Heidelberg
physicists have successfully demonstrated a programmable control of spin
interactions in isolated quantum systems.
Magnetic systems can exhibit surprising behaviour when they are prepared in
an unstable configuration. For example, constraining a collection of
spatially disordered magnetic dipoles, such as bar magnets, to be aligned in
the same direction, will lead to a subsequent reorientation of the magnets.
This ultimately results in an equilibrium in which all magnets are randomly
oriented. While the majority of investigations used to be limited to
classical magnetic dipoles, it has recently become possible to expand the
approaches to quantum magnets using what are called quantum simulators.
Synthetic atomic systems mimic the fundamental physics of magnetic phenomena
in an extremely well-controlled environment where all relevant parameters
can be adjusted almost at will.
In their quantum simulation experiments, the researchers used a gas of atoms
that was cooled down to a temperature near absolute zero. Using laser light,
the atoms were excited to extremely high electronic states, separating the
electron by almost macroscopic distances from the atomic nucleus. These
“atomic giants”, also known as Rydberg atoms, interact with each other over
distances of almost a hair’s breadth. “An ensemble of Rydberg atoms exhibits
exactly the same characteristics as interacting disordered quantum magnets,
making it an ideal platform to simulate and explore quantum magnetism,”
states Dr Nithiwadee Thaicharoen, who was a postdoc on Prof. Weidemüller’s
team at the Institute for Physics and now continues her research as a
professor in Thailand.
The essential trick of the Heidelberg physicists was to steer the dynamics
of the quantum magnets by adopting methods from the field of nuclear
magnetic resonance. In their experiments, the researchers apply especially
designed periodic microwave pulses to modify the atomic spin. A major
challenge was to precisely control the interaction between the atomic spins
using this technique, known as Floquet engineering. “The microwave pulses
had to be applied to the Rydberg atoms at timescales of a billionth of a
second, with these atoms being super-sensitive at the same time to any
external perturbation, however tiny, like minute electric fields,” says Dr
Clément Hainaut, a postdoc on the team who recently moved to the University
of Lille (France).
“We nonetheless succeeded in stalling the spin’s seemingly inevitable
reorientation and maintaining a macroscopic magnetisation through our
control protocol,” explains doctoral student Sebastian Geier. “Using our
Floquet engineering approach, it should now be possible to reverse the
timeline such that the spin system inverts its evolution after having gone
through a very complex dynamic. It would be like a broken glass magically
reassembling itself after it has crashed onto the floor.”
The studies are an important step towards a better understanding of basic
processes in complex quantum systems. “After the first and second quantum
revolution, which led to the understanding of the systems and the precise
control of single objects, we are confident that our technique of
dynamically adjusting interactions in a programmable fashion opens a path to
Quantum Technologies 3.0,” concludes Matthias Weidemüller, professor at the
Institute for Physics and Director of Heidelberg University’s Center for
Quantum Dynamics.
The experiments were conducted in the framework of the STRUCTURES Cluster of
Excellence and the “Isolated quantum systems and universality under extreme
conditions” Collaborative Research Centre (ISOQUANT) of Heidelberg
University. The activities are also part of PASQuans, the “Programmable
Atomic Large-Scale Quantum Simulation” collaboration, within the European
Quantum Technologies Flagship.
The research results were published in the journal “Science”.
Reference:
S. Geier, N. Thaicharoen, C. Hainaut, T. Franz, A. Salzinger, A. Tebben, D.
Grimshandl, G. Zürn, M. Weidemüller: Floquet Hamiltonian Engineering of an
Isolated Many-Body Spin System, Science, Vol 374, Issue 6571, pp. 1149-1152
(25 November 2021)
DOI: 10.1126/science.abd9547
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