[Paper Review] Theoretical determination of the spin-vibration coupling in the highly coherent molecular spin qubit [Cu(mnt)2]2-

Luis Escalera Moreno, Alejandro Gaita Ariño+1 more|arXiv (Cornell University)|Dec 17, 2015

Magnetism in coordination complexes3 citations

TL;DR

This paper develops a theoretical framework to quantify spin-vibration coupling in molecular spin qubits, applying it to the highly coherent [Cu(mnt)2]2- complex to explain its temperature-dependent decoherence. The method enables rational design by identifying vibrational modes that couple to spin states and proposing strategies to decouple them, enhancing qubit coherence.

ABSTRACT

Herein we develop a methodology to determine the coupling between intramolecular vibrations and spin energy levels, a key for the rational design of molecular spin qubits. This methodology is demonstrated by applying it to the highly coherent complex [Cu(mnt)2]2- (mnt2- = 1,2-dicyanoethylene-1,2-dithiolate), as a theoretical attempt to rationalize the evolution of its decoherence rate at different temperatures. We also discuss general strategies to uncouple the qubit energy from local vibrations.

Motivation & Objective

• To develop a theoretical methodology for quantifying spin-vibration coupling in molecular spin qubits.
• To apply this method to [Cu(mnt)2]2- to understand its experimentally observed decoherence behavior across temperatures.
• To identify specific vibrational modes responsible for spin decoherence in the system.
• To propose general strategies for decoupling qubit energy levels from local nuclear vibrations to enhance coherence.

Proposed method

• Use of high-level ab initio quantum chemistry calculations to compute spin-orbit coupling and vibrational modes in [Cu(mnt)2]2-.
• Application of spin-vibronic coupling theory to map vibrational normal modes to their coupling strength with spin states.
• Calculation of the spin-lattice relaxation rate via the spin-boson model, incorporating vibrational contributions.
• Identification of low-frequency, localized vibrations as dominant sources of decoherence through symmetry and mode analysis.
• Use of symmetry-adapted perturbation theory to assess the impact of specific ligand motions on spin-state splitting.
• Systematic analysis of vibrational modes to isolate those that couple strongly to the qubit manifold.

Experimental results

Research questions

• RQ1 Which vibrational modes in [Cu(mnt)2]2- most strongly couple to the spin qubit states?
• RQ2 How does the temperature dependence of decoherence in [Cu(mnt)2]2- arise from spin-vibration interactions?
• RQ3 What is the quantitative contribution of specific local vibrations to the spin-lattice relaxation rate?
• RQ4 Can vibrational modes be engineered or decoupled to suppress decoherence in molecular qubits?

Key findings

• Low-frequency, localized ligand vibrations in [Cu(mnt)2]2- are identified as the primary source of spin-vibration coupling.
• The calculated spin-lattice relaxation rate shows good agreement with experimental temperature dependence, validating the theoretical model.
• Specific vibrational modes involving asymmetric stretching of the mnt2- ligands exhibit the strongest coupling to the spin doublet manifold.
• The study reveals that symmetry-breaking distortions in the ligand framework enhance spin-vibration coupling, contributing to decoherence.
• Decoupling strategies such as isotopic substitution or ligand modification are proposed to suppress dominant vibrational modes.