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Condensed Matter Theory Groups
Department of Physics
Students in summer on north campus, red gears in background. © Roland Baege​/​TU Dortmund

NIC Project of Excellence

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Portraits of Andreas Fischer, Prof. Frithjof Anders, Dr. Natalie Jäschke, and Iris Kleinjohannn © Fabian Eickhoff​/​TU Dortmund
Andreas Fischer, Prof. Frithjof Anders, Dr. Natalie Jäschke, and Iris Kleinjohannn
From projects ranging from drug and climate research to work in theoretical chemistry, various physics disciplines, mathematics, computer science, and large-scale flow simulations from mechanical engineering, a project on the coherent control of electron spins by the group of Prof. Frithjof Anders from the John von Neumann Institute for Computing at FZ Jülich has been selected for a NIC Excellence Project 2019. Dr. Nina Fröhling, Dr. Natalie Jäschke, Iris Kleinjohann and Andreas Fischer contributed to a total of three high-level publications through their simulations on the Jülich supercomputers JUWELS and JURECA Booster, which they carried out as part of subprojects A4 and A7 of the German-Russian Transregio TRR 160 Dortmund/St. Petersburg, which was recognized by the jury.

Physicists of the Anders group awarded a 2019 NIC Excellence Project

Coherently controlled electron spins in an ensemble of semiconductor quantum dots are discussed as a building block for quantum information processing due to their integrability in existing semiconductor devices. The simulations performed in the Anders group were able to make important contributions to spin relaxation times and to the imposition of non-equilibrium distributions of the nuclear spin alignment by periodic excitation with circularly polarized light. The latter leads to a partial suppression of dephasing. In addition, the research group proposes the measurement of higher order spin noise functions. The calculations show that these quantities are sensitive to very weak interactions.

Optical excitation of electron spins in a quantum dot ensemble
Optical excitation of electron spins in a quantum dot ensemble

In quantum mechanical calculations [1] we investigated the influence of periodic laser pulses on a quantum dot as a function of an external magnetic field. Up to 20 million pulses were considered. We were able to show that a non-equilibrium distribution of the nuclear spin alignment arises via the hyperfine interaction. The periodic maxima of the distribution can be predicted by two analytical stationarity conditions. Synchronization of the spin dynamics leads to a coherent signal before the arrival of the next laser pulse: dephasing is partially suppressed. The calculations also show the influence of the nuclear Zeeman effect on the magnetic field dependence of the amplitude of the signal. Not only the excitation frequency but also the excitation duration of the laser pulse plays a key role.

Small clusters of quantum dots were simulated to represent the ensemble using classical equations of motion [2]. Due to the growth process, not all quantum dots are identical: the excitation energies follow a Gaussian distribution. By selecting different colored lasers or by varying the laser linewidth, a subset of quantum dots of identical excitation energies can be targeted. The experimental findings in such systems can be well understood under the assumption of a weak Heisenberg interaction between the quantum dots. The elucidation of this interaction is of great interest in particular because of the possible emergence of entanglement.

Interestingly, the long-time dynamics of spin echoes in quantum dots is affected by other very weak interactions, which can only be detected and elucidated in spin-noise experiments to a very limited extent. We have therefore performed both quantum mechanical and classical simulations for fourth-order spin noise [3], in which spin noise components of different frequencies are correlated. Both simulations show a clear characteristic signal change when the quadrupolar interactions of the nuclear spins are taken into account. We therefore propose such measurements to provide new information on the nature of the interactions present.

The calculations were also funded by the DFG within the framework of the German-Russian Transregio TRR 160 Dortmund/St. Petersburg.

For more information on the NIC Excellence Project please visit the web page "Coherent control of electronic and nuclear spins in quantum dot ensembles John von Neumann Excellence Project 2019".

Literature references

  1. I. Kleinjohann, E. Evers, P. Schering, A. Greilich, G.S. Uhrig, M. Bayer, F.B. Anders, Phys. Rev. B 98, 155318 (2018).
  2. A. Fischer, E. Evers, S. Varwig, A. Greilich, M. Bayer, F.B. Anders, Phys. Rev. B 98, 205308 (2018).
  3. N. Fröhling, N. Jäschke, F.B. Anders, Phys. Rev. B 99, 155305 (2019).

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