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Decoherence in magnetic molecules as qubits

Final Report Summary - DECMMQUBIT (Decoherence in magnetic molecules as qubits)

Objectives
A quantum computing scheme using lanthanoid cations, either chemically encapsulated by polyoxometalates (Ln@POM) or by phthalocyanines (Ln@Pc2). DFT calculations on lanthanoid cations on surfaces, polyoxometalates on surfaces and lanthanoids in polyoxometalates. Synthesis and characterization of Ln@POM. The project placed emphasis in interdisciplinary training.

Work carried out
Two polyoxometalate molecules were studied theoretically, by means of an exploration of the parameter space of the ligand field hamiltonian for a D4d coordination symmetry. They were synthesized and characterized. To check the influence of hyperfine and dipolar interactions, isotopically pure and/or magnetically diluted samples were prepared.

Previous work on the single-molecule magnet Fe8 had shown that at low temperature the main contributions to decoherence are the hyperfine and dipolar interactions. A parallel work was started on the single-molecule-magnet series Ln@Pc2.

Tunneling two-level-systems (TLS) are believed to be the source of phenomena such as the universal low temperature properties in disordered and amorphous solids, and 1/f noise. While on the UBC, M. Schechter and P. Stamp suggested that the low temperature TLSs could be attributed to inversion pairs. The fellow performed ab initio / DFT calculations to determine the coupling constants between phonons and impurities in KBr:CN, in support of this theory.

Results
We published two papers which gave guidelines and ideas for the design of chemical systems that could be useful in a quantum information context. , In particular, the paper by Stamp et al. focused on chemical strategies to minimize the problem of decoherence. In the returning stage, we wrote a third paper with a novel scheme to coherently control the quantum state of an SMM by means of magnetic field pulses.

The first two families of polyoxometalate-based SMMs were magnetically characterized with static and dynamic measurements. , Slow relaxation of the magnetization, typically associated with SMM-like behavior, was observed for several such Ln@POMs. The [Er(W5O18)2]9- derivative exhibited such a behavior above 2 K with an energy barrier for the reversal of the magnetization of 55 K. For a deep understanding of the appearance of slow relaxation of the magnetization in these types of mononuclear complexes, the ligand field parameters and the splitting of the J ground-state multiplet of the lanthanoid ions were estimated.

The interaction of different types of two-level systems in the crystalline lattice KBr(1-x)(CN)x were calculated. CN- flips were positively identified as the relevant TLS excitations dictating the low temperature characteristics, and the estimations compared well with the experimentally measured value for the relevant two-level systems at low energies.

Potential and implications
As this project is basic research, no immediate societal impact is to be expected. In the long term, the project contributes to the understanding of decoherence as one of the ground concepts in quantum mechanics, of quantum aspects of magnetism, and of magnetic molecules as quantum objects.