Spectroscopic analysis of vibronic relaxation pathways in molecular spin qubit $[$Ho(W$_5$O$_{18}$)$_2]^{9-}$: sparse spectra are key

TL;DR

This study combines magneto-infrared spectroscopy and ab initio calculations to analyze vibronic relaxation pathways in a molecular spin qubit, revealing how sparse spectra and specific vibrational modes influence decoherence.

Contribution

It provides the first detailed experimental and theoretical analysis of vibronic coupling in a Ho-based molecular spin qubit, highlighting the role of spectral sparsity in coherence.

Findings

Vibrational decoherence pathways identified via spectroscopy and calculations.

Field-induced spectral changes linked to crystal field excitations.

Sparse vibrational spectra limit vibronic coupling, enhancing coherence.

Abstract

Molecular vibrations play a key role in magnetic relaxation processes of molecular spin qubits as they couple to spin states, leading to the loss of quantum information. Direct experimental determination of vibronic coupling is crucial to understand and control the spin dynamics of these nano-objects, which represent the limit of miniaturization for quantum devices. Herein, we measure the vibrational properties of the molecular spin qubit $[$Ho(W$_5$O$_{18}$)$_2]^{9-}$ by means of magneto-infrared spectroscopy. Our results allow us to unravel the vibrational decoherence pathways in combination with $ab$ $initio$ calculations including vibronic coupling. We observe field-induced spectral changes near 63 and 370 cm$^{-1}$ that are modeled in terms of $f$-manifold crystal field excitations activated by odd-symmetry vibrations. The overall extent of vibronic coupling in this system is limited by a transparency window in the phonon density of states that acts to keep the intramolecular vibrations and $M_J$ levels apart. These findings advance the understanding of vibronic coupling in molecular magnets, place significant constraints on the pattern of crystal field levels in these systems, and provide a strategy for designing molecular spin qubits with improved coherence lifetimes.