Entanglement of dark electron-nuclear spin defects in diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-06-09 |
| Journal | Nature Communications |
| Authors | Maarten Degen, S. J. H. Loenen, H. P. Bartling, C. E. Bradley, Aletta Meinsma |
| Institutions | Element Six (United Kingdom), QuTech |
| Citations | 43 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research establishes the dark P1 center in diamond as a viable qubit platform by demonstrating individual control and entanglement within a dense spin bath.
- Core Achievement: Demonstrated heralded initialization, coherent control, single-shot readout, and entanglement of individual P1 electron-nuclear spin defects (dark spins) surrounding a Nitrogen-Vacancy (NV) center.
- Qubit Platform: P1 centers are utilized as qubits, leveraging their multiple degrees of freedom (Jahn-Teller axis, 14N nuclear spin, and charge state) for selective access and control.
- Initialization Innovation: Developed a method using projective DEER measurements to herald the preparation of specific configurations of the P1 centerâs auxiliary degrees of freedom, enabling selective access to individual spins (S1, S2, etc.).
- High Fidelity Control: Achieved high combined initialization and single-shot readout fidelity for the P1 electron spin (F|â)/|â) = 0.95 ± 0.01) and demonstrated coherent control of the 14N nuclear spin.
- Entanglement Demonstration: Realized an entangled state between two P1 electron spins (S1 and S2) via their direct magnetic-dipole coupling, achieving a state fidelity of F = 0.81 ± 0.05.
- Coherence Metrics: Measured long coherence times for the individual P1 electron spin (T2e = 1.00 ms) and longitudinal relaxation times (T1e = 21 s).
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Temperature | 3.3 | K | Cryogenic environment (Montana Cryostation) |
| Diamond Growth Method | Homoepitaxial | CVD | Sample provided by Element Six Innovation |
| 13C Concentration | ~0.01 | % | Isotopic purification level of the diamond |
| P1 Concentration | ~75 | ppb | Estimated density of P1 defects |
| NV Electron T1 | > 30 | s | Longitudinal relaxation time of the NV center |
| P1 Electron T1e | 21(7) | s | Longitudinal relaxation time of individual P1 electron spin |
| P1 Electron T2e (Spin-Echo) | 1.00(4) | ms | Coherence time of individual P1 electron spin |
| P1 Nuclear T2N (Spin-Echo) | 4.2(2) | ms | Coherence time of 14N nuclear spin |
| P1 Electron T2e (Ramsey) | 50(3) | ”s | Dephasing time of the electron spin |
| P1-P1 Dipolar Coupling (J) | -2Ï Â· 17.8(5) | kHz | Interaction strength between S1 and S2 |
| Two-Qubit Entanglement Fidelity | 0.81(5) | - | Fidelity of the S1-S2 entangled state |
| P1 Electron Readout Fidelity | 0.95(1) | - | Combined initialization and single-shot readout fidelity |
| NV Rabi Frequency (Peak) | ~26 | MHz | Microwave control speed |
Key Methodologies
Section titled âKey MethodologiesâThe experiment relies on high-quality diamond material and advanced quantum control techniques performed at cryogenic temperatures:
- Sample Engineering: Used a homoepitaxially CVD-grown diamond sample (Element Six) with low 13C concentration (~0.01%) to minimize environmental noise. Photon collection was enhanced using a solid-immersion lens and an aluminum-oxide anti-reflection coating.
- Cryogenic and Magnetic Stabilization: Experiments were conducted at 3.3 K. The magnetic field (B) was actively stabilized via a feedback loop based on interleaved NV transition frequency measurements, achieving stability of less than 3 mG along the z-axis.
- P1 Spin Bath Spectroscopy: Double Electron-Electron Resonance (DEER) spectroscopy was used, applying a simultaneous Ï-pulse to selectively recouple resonant P1 centers to the NV electron spin. A slightly tilted magnetic field was used to lift the degeneracy of the four Jahn-Teller axes.
- Heralded Initialization (JT/14N/Charge State): Repeated DEER measurements were used as projective measurements. By analyzing correlations in the measurement outcomes (time traces), discrete jumps corresponding to individual P1 centers were identified. Thresholds were set on the signal strength N(k) to herald the initialization of a specific P1 center (e.g., S1) in a desired state (e.g., |+1, D>).
- Fast Logic and Reset: To speed up the low-probability heralded initialization, fast logic (ADwin-Pro II) was implemented to identify unsuccessful attempts in real-time and apply a 5 ”s 515 nm laser pulse to photoexcite and scramble the P1 center states, resetting the system.
- Electron Spin Readout (DEER(y)): A modified DEER sequence (DEER(y)) was developed with a rotated readout axis (final Ï/2 pulse along -y) to projectively measure the P1 electron spin state, achieving single-shot readout.
- Nuclear Spin Control: Coherent control of the 14N nuclear spin was achieved using resonant radio-frequency (RF) pulses, implementing an electron-controlled CNOT gate conditional on the P1 electron spin state.
- Two-Qubit Entanglement: Entanglement between two P1 electron spins (S1 and S2) was generated using a double echo sequence for an interaction time 2Ï = Ï/J, implementing a CPHASE gate based on their magnetic-dipole coupling.
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to individually control and entangle dark spins in diamond opens pathways for scalable quantum technologies:
- Scalable Quantum Registers: P1 centers provide additional, controllable qubits in the environment of optically addressable defects (like the NV center), enabling the construction of larger solid-state quantum registers beyond the nuclear spins intrinsic to the NV center.
- Quantum Computation Architectures: The P1 centers can form electron spin chains for quantum computation or be used to control nearby 13C nuclear spins, forming robust, isolated quantum memories.
- Enhanced Quantum Sensing: Utilizing entanglement among P1 centers (which form the dominant spin bath) can enable environment-assisted quantum sensing protocols, potentially enhancing sensitivity for magnetometry and other sensing applications.
- Quantum Networks: P1 nuclear spins, with their long coherence times (T2N = 4.2 ms), serve as robust quantum memories for quantum network nodes, connected indirectly to the optical interface via the P1 electron spin and the NV center.
- High-Purity Diamond Supply: This technology relies critically on high-quality, isotopically purified (low 13C) CVD diamond substrates, supporting the market for advanced diamond materials (e.g., those supplied by Element Six or 6ccvd.com) required for solid-state quantum devices.