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Simulating Quantum Circuits with Light-Induced Magnetism

Researchers from the Graz University of Technology have calculated how suitable molecules can be excited by pulses of infrared light to generate magnetic fields. The research, published in the Journal of the American Chemical Society, will help in the construction of quantum computing circuits.

Simulating Quantum Circuits with Light-Induced Magnetism
Schematic representation of a metal phthalocyanine molecule that is set into two vibrations, creating a rotating electric dipole moment in the plane of the molecule and thus a magnetic field. Image Credit: Wilhelmer/Diez/Krondorfer/Hauser - TU Graz

Molecules exposed to infrared radiation start to vibrate because of the energy source. This well-known phenomenon prompted Andreas Hauser of the Institute of Experimental Physics at Graz University of Technology (TU Graz) to investigate whether these oscillations could also be exploited to produce magnetic fields.

This is due to the positively charged nature of atomic nuclei and the creation of magnetic fields when charged particles move. Andreas Hauser and colleagues have now determined that, when infrared pulses operate on metal phthalocyanines, ring-shaped, planar dye molecules, these molecules generate minuscule magnetic fields in the nm range due to their high symmetry.

The calculations suggest that nuclear magnetic resonance spectroscopy could be used to determine the relatively low but highly accurately localized field strength.

Circular Dance of the Molecules

The team used modern electron structure theory on supercomputers at the Vienna Scientific Cluster and TU Graz to calculate how phthalocyanine molecules behave when irradiated with circularly polarized infrared light.

They also drew on preliminary work from the early days of laser spectroscopy, some of which were decades old. The circularly polarized, or helically twisted, light waves excited two simultaneous molecular vibrations at right angles to one another.

As every rumba dancing couple knows, the right combination of forwards-backwards and left-right creates a small, closed loop. And this circular movement of each affected atomic nucleus actually creates a magnetic field, but only very locally, with dimensions in the range of a few nanometers.

Andreas Hauser, Institute of Experimental Physics, Graz University of Technology

Molecules as Circuits in Quantum Computers

According to Andreas Hauser, it is even possible to regulate the magnetic field's strength and direction by carefully adjusting the infrared light. As a result, the molecules would become high-precision optical switches that could be utilized to construct quantum computer circuits.

Experiments as the Next Step

Andreas Hauser is working with colleagues at the TU Graz Institute of Solid-State Physics and a group at the University of Graz to demonstrate that controlled generation of molecule magnetic fields is possible.

For proof, but also for future applications, the phthalocyanine molecule needs to be placed on a surface. However, this changes the physical conditions, which in turn influences the light-induced excitation and the characteristics of the magnetic field, we therefore want to find a support material that has minimal impact on the desired mechanism.

Andreas Hauser, Institute of Experimental Physics, Graz University of Technology

Before testing the most promising versions in experiments, the physicist and his associates wish to compute the interactions between the deposited phthalocyanines, the support material, and the infrared light in a subsequent stage.

Journal Reference:

Wilhelmer, R., et al. (2024) Molecular Pseudorotation in Phthalocyanines as a Tool for Magnetic Field Control at the Nanoscale. Journal of the American Chemical Society.

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