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Publication in „Nature Communications“

International Research Team Observes Squeezing of a Dark Nuclear Spin State for the First Time

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  • Particle Physics
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A graphic on nuclear spin squeezing. © Kirstein, E., Smirnov, D.S., Zhukov, E.A. et al. 2023
Distribution function of the nuclear spins before illumination with the laser (left) and after illumination and establishment of the dark state (right). The „squeezing“ after illumination is clearly visible. The structure of the formamidinium lead tribromide crystal is shown below schematically, with the red arrows indicating the altered orientation of the nuclear spins of the lead atoms.
Quantum-mechanical states consisting of many particles are significantly more robust with respect to disturbances that threaten information stored in this state than corresponding single-particle states. More than 20 years ago, researchers theoretically predicted an especially robust many-body state of nuclear spins – the angular momenta of atomic nuclei: This „dark nuclear spin state“ can be created through illumination with laser light but is immune to illumination, and therefore dark, once it has been established. An international team, including participants from the Department of Physics of TU Dortmund University, has now managed to demonstrate this state experimentally. These findings were recently published in the renowned journal Nature Communications.

First the researchers chemically synthesized a suitable crystal from the perovskite group, namely formamidinium lead tribromide. Then polarized light pulses were used to orient the spins of positively charged charge carriers in this crystal. When the positive charge carriers come into contact with lead nuclei in the crystal, they transfer their spin to the nuclear spins of the lead atoms. A collective nuclear spin state arises through this interaction. After that the nuclear spins involved – at least 35 of them – no longer act independently of one another, as a detailed analysis shows, but rather are linked together. The mutual dependence of the nuclear spins is referred to in quantum mechanics as entanglement.

State may be usable for quantum technologies

In the experiment, the interaction altered the orientation of the nuclear spins of the lead atoms in a very specific way. While before illumination with laser light their orientation was disordered – as characterized by the quantum-mechanical uncertainty principle – after sufficient illumination the nuclear spins oriented themselves preferentially along the direction of the optical illumination by the laser. In addition, the orientation of the nuclear spins fluctuated significantly less – not only in this longitudinal direction, but also transversely, perpendicular to the direction of illumination. Such a reduction of fluctuations in a quantum-mechanical state is called squeezing. The research team was able to observe the squeezing of such a collective nuclear spin state for the first time.

Through squeezing, the predicted dark nuclear spin state was achieved, which is insensitive to further optical excitation. Because of the resulting robustness, such a state could be used to store information quantum-mechanically – an important prerequisite for many quantum technologies, such as a quantum computer.

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