Atomistic Decoherence of Molecular Spin Qubits

Katy Aruachan Universidad de Santiago de Chile

Magnetic molecules exhibit properties that make them promising fundamental units for quantum processing [1] given their long spin coherence timescales that are comparable with conventional solid-state color centers, and the availability of the chemical synthesis methods to produce crystalline molecular spin qubit arrays. The decoherence and relaxation processes that limit molecular spin coherence have so far been studied with electronic structure methods that are computationally demanding [2]. We developed two models to construct the Redfield tensor that determines the open system dynamics of molecular spin qubits, computing relaxation (T1) and decoherence (T2) timescales over a broad range of temperatures and magnetic fields.

The first model is semi-empirical [3], using a stochastic Haken-Strobl framework with fluctuating molecular gyromagnetic tensors and local magnetic fields, and parametrizing the bath spectral densities using a limited set of T1 relaxation measurements [4]. Taking a vanadium-based spin qubit as a case study, the theoretical predictions agree quantitatively with experiments [4] and represent a solid foundation for the theoretical characterization of other spin qubits. The second model considers spin-lattice and hyperfine spin-spin interaction. It incorporates ab-initio phonon- induced fluctuations of the gyromagnetic tensor from an atomistic model of the copper porphyrin qubit, with hyperfine interaction parameters calibrated from experiments [5]. Low magnetic field relaxation is treated with a phenomenological spin spectral density. The model was successfully tested by predicting T1 and T2 in qualitative agreement with experiments. We discuss possible refinements of the spin bath spectral density to reach quantitative agreement with available T1 measurements in porphyrin qubits. Our modeling approach can be used for the characterization of quantum magnetometers based on molecular spin qubits.

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[2] A. Lunghi, and S. Sanvito. Sci. Adv.5,eaax7163 (2019).

[3] K. Aruachan, Y. Colon, D. Aravena, F. Herrera. New J. Phys. 25 09303.

[4] L. Tesi, A. Lunghi, M. Atzori, E. Lucaccini, L. Sorace, F. Totti, R. Sessoli. Dalton Trans., 2016,45, 16635-16643.

[5] C. Yu, M. D. Krzyaniak, M. S. Fataftah, M. R. Wasielewski and D. E. Freedman. Chem. Sci., 2019,10, 1702-1708.