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Research Team observes Squeezing of a Dark Nuclear Spin State for the first time

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Schematic explanation of nuclear spin squeezing © Kirstein, E., Smirnov, D.S., Zhukov, E.A. et al. 2023
Distribution function of the nuclear spins, left before illumination with the laser, right after illumination and formation of the dark state. The "squeezing" after illumination is clearly visible. The structure of the formamidinium lead tribromide crystal is shown schematically below, with the red arrows indicating the changed orientation of the nuclear spins of the lead atoms.
Quantum mechanical states consisting of many particles are much more robust against disturbances that threaten information stored in this state than corresponding single-particle states. More than 20 years ago, researchers theoretically predicted a particularly robust many-particle state of nuclear spins - the angular momentum of atomic nuclei: this "dark nuclear spin state" is created by irradiation with laser light, but becomes immune to illumination and thus dark after its formation. An international team including researchers from the Department of Physics at TU Dortmund University has now succeeded in demonstrating this state experimentally. These findings were recently published in the renowned journal "Nature Communications".

The researchers first chemically synthesised a suitable crystal from the group of perovskites, namely formamidinium lead tribromide. Polarised light pulses then orientate 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. This interaction ultimately creates a collective nuclear spin state. The nuclear spins involved - at least 35 of them - therefore no longer act independently of each other, as a detailed analysis shows, but are coupled with each other. In quantum mechanics, the dependency between the nuclear spins is called "entanglement".

State potentially usable for quantum technologies

The interaction specifically changed the orientation of the nuclear spins of the lead atoms in the experiment. While they were disordered before illumination with the laser light - characterised by the quantum mechanical uncertainty principle - the nuclear spins orientated themselves preferentially along the direction of the optical illumination by the laser after sufficient illumination. In addition, the nuclear spins fluctuated significantly less in their orientation - both in this longitudinal direction and transversely perpendicular to the direction of illumination. Such a reduction in the fluctuations of a quantum mechanical state is called squeezing. The research team was thus able to observe the squeezing of such a collective nuclear spin state for the first time.

The squeezing achieved the predicted dark nuclear spin state, which is insensitive to further optical excitation. Due to the resulting robustness, it could be used to store quantum mechanical information - an important prerequisite for many quantum technologies such as a quantum computer.

Link to the original paper

 

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