Enhancement of quantum coherence in solid-state qubits via interface engineering
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-07-01 |
| Journal | Nature Communications |
| Authors | Wing Ki Lo, Yaowen Zhang, H. Chow, Jiahao Wu, Man Yin Leung |
| Institutions | Hong Kong University of Science and Technology, University of Hong Kong |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Coherence Breakthrough: Quantum coherence time (T2) for shallow Nitrogen-Vacancy (NV) centers in diamond was extended to over 1 ms (1063.8 ”s max), approaching the theoretical T1 limit (1.6 ± 0.3 ms).
- Interface Engineering: The enhancement is achieved via a Graphene/Oxygen-terminated (O-terminated) diamond heterojunction, which suppresses surface spin noise.
- Mechanism Verified: Raman spectroscopy and Density Functional Theory (DFT) confirmed that the O-termination facilitates electron transfer from the gapless Graphene, resulting in hole doping (~1012 cm-2). These transferred electrons pair with unpaired surface electrons.
- Spin Noise Reduction: Double Electron-Electron Resonance (DEER) spectroscopy confirmed an order of magnitude reduction in unpaired electron spin concentration (~0.72 Ă 1011 cm-2) after patching.
- Sensitivity Improvement: AC magnetic field sensitivity was enhanced from 50 nT/Hz1/2 to 16 nT/Hz1/2 using the CPMG-64 sequence.
- Nanoscale Sensing Demonstrated: The enhanced T2 enabled the detection of weakly coupled internal 13C nuclear spins and external 11B nuclear spins from a hexagonal boron nitride (h-BN) capping layer, achieving nanoscale NMR.
- Robust Platform: A protective h-BN top layer stabilizes the platform, allowing the device to be repeatedly cleaned via acid boiling and reused without removing the coherence-enhancing Graphene patch.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Coherence Time (T2) | 1063.8 ± 71.2 | ”s | Shallow NV center (NV 17) using CPMG-64 sequence. |
| T1 Limit of NV Centers | 1.6 ± 0.3 | ms | Approaching limit achieved by interface engineering. |
| Hahn-Echo T2 Improvement | 1.2 to 3.3 | fold | Enhancement observed across 20 shallow NV centers. |
| Best AC Magnetic Field Sensitivity | 16 | nT/Hz1/2 | Achieved using CPMG-64 sequence. |
| Initial AC Magnetic Field Sensitivity | 50 | nT/Hz1/2 | Before interface engineering. |
| Unpaired Electron Spin Concentration | ~0.72 Ă 1011 | cm-2 | Measured via DEER after graphene patching. |
| Graphene Hole Doping Concentration | ~1012 | cm-2 | Estimated from Raman G band blue shift. |
| G Band Blue Shift (Graphene/O-term) | ~3.8 | cm-1 | Relative to Graphene/h-BN reference. |
| NV Center Depth | ~5 to 20 | nm | Depth of implanted NV centers below diamond surface. |
| 13C Hyperfine Coupling Strength (Aâ„) | 28 and 17 | kHz | Weakly coupled 13C nuclear spins detected. |
| Graphene Transfer Annealing Temp | 150 | °C | Annealing temperature for 1 h post-transfer. |
| Graphene Transfer Sacrificial Layer Removal | 50 | °C | Acetone bath temperature for 1 h. |
| External Magnetic Field (B) | 286 | G | Standard field used for T2 and DEER measurements. |
Key Methodologies
Section titled âKey Methodologiesâ- Diamond Surface Termination: The diamond sample was cleaned using triacid boiling (concentrated perchloric, nitric, and sulfuric acids) to achieve Oxygen-termination (O-terminated), which stabilizes the NV negative charge state.
- NV Center Implantation: NV centers were created by growing a 6 nm p+ boron layer, followed by 9.8 keV 15N ion implantation (~109 N/cm2 dose), and subsequent annealing at 950 °C for 2 h.
- Graphene Patching: Graphene was transferred onto the O-terminated diamond surface using a standard floating method, immediately following the triacid boiling.
- Post-Transfer Curing: The graphene/diamond assembly was air-dried (30 min), annealed at 150 °C (1 h), and stored under vacuum (> 24 h) to enhance adhesion.
- Sacrificial Layer Removal: The sacrificial layer was removed by immersion in 50 °C acetone (1 h), followed by isopropyl alcohol (1 h), and N2 gas drying.
- Protective Capping Layer: A few micrometers of h-BN were transferred on top of the graphene-patched diamond to create the robust h-BN/Graphene/Diamond hetero-structure for external sensing.
- Quantum Sensing Sequences: Coherence time was measured using Hahn-echo (T2) and extended using Carr-Purcell-Meiboom-Gill (CPMG) dynamical decoupling sequences (up to N=64).
- Charge Transfer Analysis: Raman spectroscopy (532 nm excitation, 500 ”W power) was used to measure G and 2D band shifts, indicating hole doping. DFT calculations modeled the band alignment and charge transfer mechanism for O-terminated versus OH-terminated surfaces.
Commercial Applications
Section titled âCommercial Applicationsâ- Nanoscale NMR and Spectroscopy: High-resolution detection and characterization of molecular structures, proteins, and biological systems at the atomic scale, enabled by the extended T2 coherence.
- Quantum Material Characterization: Sensing weak magnetic signals from exotic two-dimensional (2D) magnetic materials and studying their interactions at the atomic level.
- Reporter Spin Networks: Utilizing the highly coherent shallow NV centers as robust, sensitive reporters for imaging individual proton spins on the diamond surface.
- High-Sensitivity Magnetometry: Development of practical, high-sensitivity AC magnetic field sensors (16 nT/Hz1/2) for use in ambient conditions without requiring complex purification or cryogenic setups.
- Reusable Quantum Devices: The h-BN/Graphene capping layer creates a chemically robust platform that can be repeatedly cleaned using harsh acids and reloaded with new target samples (e.g., bio-samples, high-pressure materials) for continuous use.
- Interface Engineering for Qubits: The demonstrated charge transfer mechanism provides a generalizable strategy for mitigating surface noise in other shallow solid-state qubits (e.g., SiV, GeV centers).