Discovery of atomic clock-like spin defects in simple oxides from first principles
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2024-06-06 |
| Journal | Nature Communications |
| Authors | Joel Davidsson, Mykyta Onizhuk, Christian Vorwerk, Giulia Galli |
| Institutions | University of Chicago, Linköping University |
| Citations | 12 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research predicts a new class of solid-state spin defects in Calcium Oxide (CaO) that exhibit properties highly analogous to the Nitrogen-Vacancy (NV) center in diamond, offering a path toward robust quantum technologies.
- Core Value Proposition: Discovery of NV-like spin defects (XCaVO, where X=Sb, Bi, I) in simple oxides (CaO), a host material that is virtually noiseless due to the scarcity of spinful nuclei.
- Coherence Time Breakthrough: The 209Bi defect (BiCaVO-) exhibits âatomic clock-like transitionsâ (CTs) that significantly protect the electron spin from environmental noise.
- Achieved T2 Coherence: Hahn-echo coherence time (T2) is predicted to increase to 4.7 seconds near a CT, representing an increase of two orders of magnitude over the intrinsic CaO limit (34 ms).
- Quantum Communication Readiness: The BiCaVO- complex emits a Zero Phonon Line (ZPL) at 1624 nm, placing it directly in the telecommunication L-band.
- Optical Cycle Viability: Quantum Defect Embedding Theory (QDET) calculations confirm a triplet ground state (S=1) and a viable optical cycle for qubit initialization, similar to the NV center.
- Host Advantages: CaO has a lower refractive index (1.84) than diamond (2.42), improving photon coupling efficiency into optical fibers.
- Methodology: High-throughput screening (ADAQ) combined with advanced quantum calculations (HSE, QDET, CCE) was used for prediction and validation.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Host Material | Calcium Oxide (CaO) | N/A | Simple oxide, rocksalt structure |
| Host Refractive Index | 1.84 | N/A | Lower than diamond (2.42), beneficial for fiber integration |
| Intrinsic T2 Coherence Limit | 34 | ms | Nuclear spin-limited T2 in natural CaO |
| Bi Defect T2 (Near CT) | 4.7 | s | Hahn-echo T2 at clock transition (22.18 mT) |
| Dopant Nucleus (Bi) | 209Bi (Spin 9/2) | N/A | Provides strong electron-nuclear coupling (1.27 GHz) |
| Defect Class | XCaVO | N/A | Charged complex (X=Sb, Bi, I) between Ca and O vacancies |
| Defect Symmetry | C4v | N/A | Lowered symmetry from host Oh |
| BiCaVO- ZPL Energy | 0.76 | eV | Excitation energy (a1 to e states) |
| BiCaVO- ZPL Wavelength | 1624 | nm | Telecommunication L-band emission |
| BiCaVO- Zero-Field Splitting (ZFS) | 2.46 | GHz | Ground state (3A2) splitting |
| BiCaVO- Radiative Lifetime | 63 | ns | Time for spontaneous emission |
| ICaVO+ ZFS | 3.42 | GHz | Highest ZFS among the predicted defects |
| CaO Band Gap (HSE) | 5.32 | eV | Calculated value (Experimental: 7.09 eV) |
| DFT Exchange Mixing (HSE) | 62.5 | % | Optimized Hartree-Fock mixing parameter |
| Clock Transition Field (Bi) | 22.18 | mT | Magnetic field for optimal T2 coherence |
Key Methodologies
Section titled âKey MethodologiesâThe discovery and characterization relied on a multi-step computational approach combining high-throughput screening with high-accuracy quantum methods:
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High-Throughput Screening (ADAQ):
- The Automatic Defect Analysis and Qualification (ADAQ) software was used to generate and screen 9077 potential defects (vacancies, substitutionals, interstitials, and clusters) in CaO.
- Defects were filtered based on stability (on the defect hull), triplet ground state (S=1), and presence of localized occupied/unoccupied states (indicating a detectable ZPL).
- Initial screening used Density Functional Theory (DFT) at the Generalized Gradient Approximation (GGA/PBE) level.
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Electronic Structure and ZPL Calculation (DFT/HSE):
- Calculations were performed using the Vienna Ab initio Simulation Package (VASP) with the Projector Augmented Wave (PAW) method.
- Hybrid functionals (HSE06 and K-PBEO) were used to accurately predict the band gap and defect properties, with the HSE mixing parameter optimized to 62.5% to match the experimental CaO band gap.
- Zero Phonon Lines (ZPLs) and Transition Dipole Moments (TDMs) were calculated using Constrained DFT (Î-SCF).
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Many-Body State Characterization (QDET):
- The Quantum Defect Embedding Theory (QDET), implemented in the WEST code, was used to accurately determine the singlet-triplet (S-T) splitting and the full many-body level diagram for the BiCaVO- complex.
- QDET confirmed the 3A2 triplet ground state and the presence of two singlet excited states (1B1, 1B2) between the ground state and the first triplet excited state (3E), confirming a viable optical cycle.
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Spin Dynamics and Coherence Time (CCE):
- The Cluster-Correlation Expansion (CCE) method, implemented in the PyCCE code, was used to compute the nuclear spin-limited Hahn-echo coherence time (T2).
- CCE was used to identify and characterize the âclock transitionsâ (CTs)âavoided crossings in the spin energy spectrumâwhere T2 is maximized due to robustness against external perturbations.
Commercial Applications
Section titled âCommercial ApplicationsâThe predicted NV-like defects in CaO are highly attractive for next-generation quantum technologies, particularly those requiring long-distance communication and robust sensing.
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Quantum Communication:
- The BiCaVO- defectâs ZPL emission (1624 nm) falls within the telecommunication L-band, enabling direct integration with existing optical fiber networks for quantum repeaters and long-distance entanglement distribution.
- The low refractive index of CaO (1.84) compared to diamond (2.42) facilitates better photon coupling into fibers.
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Quantum Computing and Memory:
- The long T2 coherence time (4.7 s) makes these defects excellent candidates for solid-state quantum memory and robust qubit operation.
- The strong coupling between the electron spin (qubit) and the high-spin 209Bi nucleus (I=9/2) provides 30 controllable quantum levels, offering a large Hilbert space for complex quantum operations.
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Quantum Sensing:
- The high robustness of the clock transitions against environmental noise makes these defects ideal for high-precision quantum sensing applications (e.g., magnetic field, temperature, or strain sensing) where environmental stability is critical.
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Hybrid Optoelectronic Systems:
- The ability to interface oxides (like CaO) with semiconductors allows for the engineering of hybrid quantum optoelectronic systems, combining the robust spin properties of the oxide defect with the mature processing capabilities of semiconductors.