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Understanding decoherence in molecular spin qudits

Leonardo Ratini1,2,*, Giacomo Sansone1,2,*, Elena Garlatti1,2,3, Francesco Petiziol4, Stefano Carretta1,2,3,†, and Paolo Santini1,2,3,‡

  • *These authors contributed equally to this work.
  • †Contact author: stefano.carretta@unipr.it
  • ‡Contact author: paolo.santini@unipr.it
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Phys. Rev. Research 7, 043125 – Published 3 November, 2025

DOI: https://doi.org/10.1103/16rd-tv1q

Abstract

Molecular nanomagnets are quantum spin systems potentially serving as qudits for future quantum technologies thanks to their many accessible low-energy states. At low temperatures, the primary source of error in these systems is pure dephasing, caused by their interactions with the bath of surrounding nuclear spins degrees of freedom. Most importantly, as the system’s dimensionality grows going from qubits to qudits, the control and mitigation of decoherence become more challenging. Here, we analyze the characteristics of pure dephasing in molecular qudits under spin-echo sequences. We use a realistic description of their interaction with the bath, whose non-Markovian dynamics is accurately computed by the cluster correlation expansion technique. First, we show that the differences in the expectation values of the local spin operators on the eigenstates of the qudit are the source of decoherence in these systems. Indeed, we demonstrate that this is a necessary and sufficient condition to prevent the decay of coherence with time, also introducing a parameter to quantify the deviation from such ideal condition. We illustrate this with two paradigmatic systems: a single giant spin and a composite antiferromagnetic spin system. We then advance a proposal for optimized nanomagnets, identifying key ingredients for engineering robust qudits for quantum technologies.

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Phys. Rev. Research 7, 043125– Published 3 November, 2025
  • Received 11 April 2025
  • Accepted 3 September 2025
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