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Electron spin superpositions represent a critical component of emergent quantum technologies in computation, sensing, encryption, and communication. However, spin relaxation (T1) and decoherence (Tm) represent major obstacles to the implementation of molecular quantum bits (qubits). Synthetic strategies have made substantial progress in enhancing spin coherence times by minimizing contributions from surrounding electron and nuclear spins. For room-temperature operation, however, the lifetime of spin coherence becomes limited by coupling with vibrational modes of the lattice. Using pulse electron paramagnetic resonance (EPR) spectroscopy, we measure the spin-lattice relaxation of a vanadyl tetrapyrazinoporphyrazine complex appended with eight peripheral 2,6-diisopropylphenol groups (VOPyzPz-DIPP) and compare it to the relaxation of the archetypical vanadyl phthalocyanine molecular qubit (VOPc). The added peripheral groups lead to distinctly different spin relaxation behavior. While similar relaxation times are observed at low temperatures and ambient conditions, significant changes are observed for the orientation dependence of T1at 100 K, as well as the temperature dependence of T1over the intermediate temperature range spanning [Formula: see text]10–150 K. These results can be tentatively interpreted as arising from loosened spin-phonon coupling selection rules and a greater number of accessible acoustic and optical modes contributing to the spin relaxation behavior of VOPyzPz-DIPP relative to VOPc.
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This content will become publicly available on December 14, 2026
Tensorial spin-phonon relaxation reveals mode-selective relaxation pathways in a single-molecule qubit
Understanding and controlling spin relaxation in molecular qubits is essential for developing chemically tunable quantum information platforms. We present a first-principles-parametrized analytical framework for evaluating spin relaxation dynamics in vanadyl phthalocyanine (VOPc) and its oxygenated derivative, VOPc(OH)8. By expanding the spin Hamiltonian in vibrational normal modes and computing both linear and quadratic spin–phonon coupling tensors via finite differences of the g-tensor, we construct a relaxation tensor that enters a Lindblad-type master equation, capturing both direct (one-phonon) and Raman (two-phonon) processes. A mode-resolved analysis reveals that relaxation is funneled through only a handful of low-frequency vibrations: in VOPc, three out-of-plane distortions of the phthalocyanine ring and V–O unit dominate, whereas in VOPc(OH)8, the additional oxygens shift these modes downward and suppress two of them, leaving a single strongly coupled mode as the main decoherence pathway. Both longitudinal (T1) and transverse (T2) relaxation are governed by this same set of vibrational modes, indicating that coherence loss is controlled by a common microscopic mechanism. This mode-selective picture offers a design strategy for engineering longer-lived molecular qubits.
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- Award ID(s):
- 2404788
- PAR ID:
- 10659638
- Publisher / Repository:
- AIP
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 163
- Issue:
- 22
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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