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6CCVD Research

Vibronic Relaxation Pathways in Molecular Spin Qubit Na9[Ho(W5O18)2]·35H2O under Pressure

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2023-02-09
JournalMagnetochemistry
AuthorsJ. L. Musfeldt, Zhenxian Liu, Diego López‐Alcalá, Yan Duan, Alejandro Gaita‐Ariño
InstitutionsUniversity of Illinois Chicago, University of Tennessee at Knoxville
Citations1
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This research investigates the control of vibronic decoherence pathways in the molecular spin qubit Na9[Ho(W5O18)2]·35H2O using high external pressure.

  • Qubit System: The study focuses on a holmium (Ho3+) polyoxometalate molecular spin qubit, known for its long coherence time (8.4 µs in diluted systems) and protection via atomic clock transitions (ACTs).
  • Decoherence Mechanism: Vibronic coupling (interaction between spin states and molecular vibrations/phonons) is a primary cause of quantum information loss (decoherence).
  • Pressure Effect (Experimental): Applied compression (up to 5.2 GPa) systematically hardens the molecular phonons (rate of ~0.9 cm-1/GPa) and increases the crystal field splitting of the Ho3+ MJ levels.
  • Transparency Window Closure: Crucially, compression causes the transparency window (a “hole” in the phonon density of states that limits vibronic overlap) to systematically close and fill in.
  • Conclusion on Compression: The closure of the transparency window increases the spectral overlap between phonons and MJ levels, suggesting that positive pressure (compression) is detrimental to qubit performance in this system.
  • Proposed Strategy: The findings strongly suggest that negative pressure (tensile strain, elongational strain, or chemical pressure) should be explored to further open the transparency window and enhance the performance of this rare earth-containing molecular spin qubit.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Molecular Spin QubitNa9[Ho(W5O18)2]·35H2ON/AModel system featuring Atomic Clock Transitions (ACTs).
Coherence Time (T2)8.4 µsN/AMeasured in diluted systems.
Operating Frequency (ACT)~9GHzX-band frequency, sensitive to small distortions.
Experimental Pressure Range0 to 5.2GPaApplied via Diamond Anvil Cell (DAC) at room temperature.
Phonon Hardening Rate~0.9cm-1/GPaRate at which vibrational modes shift under compression.
Key Vibronic Coupling Modes63 and 370cm-1Frequencies coupling to Ho3+ MJ levels (±7, ±5, ±2).
Transparency Window Range220-310cm-1Frequency range where the phonon density of states is sparse.
Spectroscopy Range60-680cm-1Synchrotron-based Far Infrared (FIR) measurement range.
DFT Compressive Strain0.5 to 2%Triaxial strain applied in theoretical simulations.
Vibronic Coupling Constant~0.25cm-1Modest coupling constant limiting overall vibronic interaction.

Key Methodologies

Section titled “Key Methodologies”

The study combined high-pressure experimental techniques with advanced computational modeling to analyze the vibronic response of the molecular spin qubit.

  1. High-Pressure Setup (DAC):

    • High-quality single crystals were loaded into a symmetric diamond anvil cell (DAC) with 500 µm culets.
    • Hydrocarbon grease (petroleum jelly) and an annealed ruby ball were used as the pressure medium to maintain quasi-hydrostatic conditions.
    • Pressure was monitored and calibrated using the ruby R1 fluorescence line shift (up to 5.2 GPa).

  2. Synchrotron Far Infrared (FIR) Spectroscopy:

    • FIR spectroscopy (60-680 cm-1, 4 cm-1 resolution, transmittance geometry) was performed at the National Synchrotron Light Source II.
    • Two optical density trials were conducted: high density (55-370 cm-1) for weaker features and low density (300-625 cm-1) for stronger features.

  3. Density Functional Theory (DFT) Calculations:

    • Molecular geometries were optimized using the Gaussian09 package with the PBE0 hybrid-exchange functional and Grimme D3BJ dispersion corrections.
    • Relativistic basis sets (Stuttgart RSC ANO for Ho3+, CRENBL for W) were employed, along with corresponding effective core potentials (ECP).
    • Triaxial compressive strain (0.5% to 2%) was simulated by decreasing Cartesian coordinates, followed by constrained optimization to maintain the pressure effect.

  4. Semi-Empirical Crystal-Field Calculations:

    • The Semi-Empirical Radial Effective Charge (REC) model, implemented in the SIMPRE package, was used to calculate the evolution of the Ho3+ MJ energy levels under applied strain.
    • Calculations confirmed a linear increase in crystal field splitting under compression, consistent with shorter metal-ligand distances.

Commercial Applications

Section titled “Commercial Applications”

The findings provide critical design rules for engineering molecular spin qubits, particularly concerning their integration into solid-state devices where strain is inherent or controllable.

  • Quantum Information Processing (QIP): The primary application is the development of robust molecular spin qubits (MSQs) for quantum computing and memory, leveraging the protection offered by ACTs.
  • Strain Engineering for Qubit Control: Utilizing device surfaces and interfaces to apply tensile strain (negative pressure) to MSQ films or crystals. This technique can actively manage decoherence by expanding the phonon transparency window, leading to longer T1 and T2 times.
  • High-Temperature Quantum Devices: Strategies that effectively decouple spin states from thermal vibrations (phonons) are essential for enabling MSQs to operate at temperatures higher than cryogenic, reducing cooling costs and complexity.
  • Integrated Quantum Sensors: The system’s known magnetoelectric coupling suggests potential for strain-tuned electric field control, enabling the development of highly sensitive, integrated quantum sensors.
  • Crystal Engineering and Host Matrix Control: The results inform the chemical design of new MSQs or host matrices, guiding the selection of counterions or ligands that naturally induce lattice expansion (chemical pressure) to maximize spectral sparsity.
View Original Abstract

In order to explore how spectral sparsity and vibronic decoherence pathways can be controlled in a model qubit system with atomic clock transitions, we combined diamond anvil cell techniques with synchrotron-based far infrared spectroscopy and first-principles calculations to reveal the vibrational response of Na9[Ho(W5O18)2]·35H2O under compression. Because the hole in the phonon density of states acts to reduce the overlap between the phonons and f manifold excitations in this system, we postulated that pressure might move the HoO4 rocking, bending, and asymmetric stretching modes that couple with the MJ = ±5, ±2, and ±7 levels out of resonance, reducing their interactions and minimizing decoherence processes, while a potentially beneficial strategy for some molecular qubits, pressure slightly hardens the phonons in Na9[Ho(W5O18)2]·35H2O and systematically fills in the transparency window in the phonon response. The net result is that the vibrational spectrum becomes less sparse and the overlap with the various MJ levels of the Ho3+ ion actually increases. These findings suggest that negative pressure, achieved using chemical means or elongational strain, could further open the transparency window in this rare earth-containing spin qubit system, thus paving the way for the use of device surfaces and interface elongational/compressive strains to better manage decoherence pathways.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2013 - Lanthanide single-molecule magnets [Crossref]
  2. 2017 - Molecular single-ion magnets based on lanthanides and actinides: Design considerations and new advances in the context of quantum technologies [Crossref]
  3. 2019 - The second quantum revolution: Role and challenges of molecular chemistry [Crossref]
  4. 2020 - Design of high-temperature f-block molecular nanomagnets through the control of vibration-induced spin relaxation [Crossref]
  5. 2020 - Molecular magnetism: From chemical design to spin control in molecules, materials and devices [Crossref]
  6. 2014 - Construction of a general library for the rational design of nanomagnets and spin qubits based on mononuclear f-block complexes: The polyoxometalate case [Crossref]
  7. 2021 - Effect of spin-orbit coupling on phonon-mediated magnetic relaxation in a series of zero-valent vanadium, niobium, and tantalum isocyanide complexes [Crossref]
  8. 2022 - Ultrahard magnetism from mixed-valence dilanthanide complexes with metal-metal bonding [Crossref]
  9. 2022 - Data-driven design of molecular nanomagnets [Crossref]
  10. 2014 - Room temperature quantum coherence in a potential molecular qubit [Crossref]

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