Frequency Control of Single Quantum Emitters in Integrated Photonic Circuits
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2018-01-30 |
| Journal | Nano Letters |
| Authors | Emma Schmidgall, Srivatsa Chakravarthi, Michael N. Gould, Ian Christen, Karine Hestroffer |
| Institutions | University of Washington, Humboldt-UniversitÀt zu Berlin |
| Citations | 39 |
| Analysis | Full AI Review Included |
6CCVD Technical Analysis: Frequency Control of Single Quantum Emitters
Section titled â6CCVD Technical Analysis: Frequency Control of Single Quantum EmittersâExecutive Summary
Section titled âExecutive SummaryâThis paper demonstrates a critical breakthrough for scalable quantum networking by achieving electric field control and active feedback stabilization of Nitrogen-Vacancy (NV) center emission frequency within an integrated diamond-photonic platform.
- Core Achievement: Successful implementation of the DC Stark effect using Ti/Au electrodes to tune and stabilize the Zero-Phonon Line (ZPL) emission energy of near-surface implanted NV centers in a novel GaP-on-diamond integrated photonic circuit.
- Scalability Enablement: This tuning mechanism addresses the major challenge of inhomogeneous emission energies and spectral instability inherent to implanted NV centers, which is necessary for generating indistinguishable entangled photons.
- Tuning Performance: Demonstrated a high, repeatable tuning range of up to 190 GHz in waveguide-coupled NV centers via application of 0-100 V bias voltages.
- Stability Enhancement: Active PID voltage feedback reduced the average absolute spectral diffusion of the ZPL emission from 2.7 GHz (unstabilized) to 1.2 GHz (stabilized), significantly improving temporal coherence.
- Platform Innovation: Utilized a GaP-on-Diamond wafer architecture, leveraging the high refractive index contrast of CVD diamond substrates for enhanced Purcell enhancement in micro-disk resonators.
- Material Requirements: The approach depends entirely on high-quality, electronic-grade single-crystal diamond (SCD) substrates capable of supporting shallow implantation (~15 nm depth) and subsequent high-temperature annealing (up to 800 °C).
Technical Specifications
Section titled âTechnical SpecificationsâThe following table extracts key material, device, and performance parameters from the research conducted on the integrated NV-photonic devices:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Material Grade | Electronic Grade SCD | N/A | Substrate for NV centers and GaP overlay. |
| NV Center Depth | ~15 | nm | Implanted near the diamond surface. |
| Ion Implantation Species | N+ | N/A | Used for creating NV centers. |
| Implantation Energy/Dose | 10 keV, 3Ă1010 | cm-2 | Recipe for shallow NV creation. |
| GaP Membrane Thickness | 125 | nm | Transferred layer for integrated photonics. |
| Photonic Device Type | Waveguides (150 nm wide) / Disk Resonators (1.3 ”m diameter) | ”m | Integrated structures coupled to NV centers. |
| Electrode Metalization | 7 nm Ti / 70 nm Au | nm | Custom thin-film metal stack for bias application. |
| Electrode Spacing | ~6 | ”m | Gap between Ti/Au electrodes. |
| Applied Bias Voltage Range | 0 - 100 | V | Range used for Stark tuning. |
| Simulated Electric Field (Max) | 3 - 5 | MV/m | Field strength inside waveguide/resonators at 100 V. |
| Observed Tuning Range (Max) | 190 | GHz | Voltage-dependent tuning of a single NV ZPL. |
| Unstabilized ZPL Linewidth (Average) | 34.7 ± 7.4 | GHz | Measured using standard grating spectrometer. |
| High-Resolution Linewidth (Average) | 6.3 ± 3.3 | GHz | Measured using Echelle spectrometer. |
| Spectral Diffusion (Unstabilized, Max) | 7.2 | GHz | Variation under constant 45 V bias. |
| Spectral Diffusion (Stabilized, Max) | 3.9 | GHz | Variation under active PID feedback control. |
| Average Absolute Difference (Stabilized) | 1.2 | GHz | Measurement of temporal stability improvement. |
| Operating Temperature | 12 - 14 | K | Performed in closed-cycle He cryostat. |
Key Methodologies
Section titled âKey MethodologiesâThe experimental success hinges on precise material engineering, defect creation, and nanofabrication, involving several critical processing steps:
- Diamond Preparation & Implantation:
- Used single-crystal electronic grade diamond (ElementSix).
- N+ ion implantation at 10 keV energy and 3Ă1010 cm-2 dose to generate near-surface vacancies.
- Two-Step Annealing for NV Formation:
- Step 1: 2-hour, 800 °C annealing performed under a 5%/95% H2/Ar forming gas atmosphere to mobilize vacancies and form NV centers.
- Step 2: Subsequent 24-hour, 460 °C anneal in air to oxygen-terminate the surface and enhance the stability of the NV- charged state.
- Heterogeneous Integration:
- A 125 nm thick GaP membrane was transferred to the diamond substrate via epitaxial liftoff and van der Waals bonding, forming the GaP-on-diamond platform.
- Device Fabrication:
- Lithography: Negative-tone optical lithography and electron-beam lithography (HSQ resist) were used to define the waveguides and disk resonators.
- Etching: Two reactive ion etch (RIE) steps were used: (1) GaP layer etch (3.0 mTorr, Cl2/Ar/N2, 235 V dc bias); (2) Diamond etch (25.0 mTorr, O2, 65 V dc bias) resulting in a diamond etch depth of ~600 nm.
- Electrode Definition: Ti/Au electrodes (7 nm Ti / 70 nm Au) and thick bond pads (70 nm Ti / 700 nm Au) were deposited via evaporation and liftoff for bias application and wirebonding.
- Quantum Measurement:
- Measurements performed at 12-14 K.
- Optical excitation via a 532 nm laser; ZPL emission collected via a grating coupler and detected using a grating spectrometer/CCD.
- Stark tuning implemented using a computer-controlled piezo-controller supplying 0-100 V bias across the electrodes.
- Active stabilization implemented using a Proportional Integral Derivative (PID) algorithm with an Echelle spectrometer (1.3 GHz resolution) providing real-time feedback on ZPL position.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThis research validates the critical role of high-purity MPCVD diamond in future chip-scale quantum technologies. However, the reported fabrication challenges (low yield, variability, and wide linewidths) highlight the need for optimized substrates and precision engineering capabilities offered by 6CCVD.
Applicable Materials for Replication and Advancement
Section titled âApplicable Materials for Replication and Advancementâ| Required Material Characteristic | 6CCVD Recommended Material | Rationale & Advantage |
|---|---|---|
| Ultra-High Purity Substrates | Electronic Grade SCD (High NV Precursor Control) | SCD provides the foundation for low-strain, high-coherence NV centers. We offer high-quality SCD optimized for low intrinsic nitrogen/defects necessary for high-performance quantum emitters, minimizing unwanted strain-induced shifts. |
| Shallow Implant & Annealing | Custom Thin-Film SCD/Substrates (Up to 500 ”m) | Crucial for near-surface NV centers (~15 nm). 6CCVD provides highly polished SCD (Ra < 1 nm) suitable for subsequent epitaxial liftoff (GaP transfer) and high-temperature vacuum annealing studies (up to 1500 °C), enabling tighter control over defect stability compared to the 800 °C recipe used in the paper. |
| Advanced Etch Mask | Polycrystalline Diamond (PCD) Wafers | If larger scale, cost-effective quantum architectures are desired, our high-quality PCD films (up to 125 mm) can serve as robust substrate or template layers where strict single-crystal coherence is less critical, or as high-thermal conductivity packaging layers. |
| Electrodes & Device Integration | Boron-Doped Diamond (BDD) | For future research requiring integrated field control, BDD films offer conductive diamond electrodes that can be fabricated directly into the platform, potentially improving field uniformity and stability by eliminating issues associated with metal/semiconductor interfaces (e.g., photoinduced current/charging observed in the paper). |
Customization Potential for Integrated Photonics
Section titled âCustomization Potential for Integrated PhotonicsâThe integration of the GaP membrane and the definition of the metal electrodes are areas where 6CCVDâs precision capabilities significantly reduce complexity and enhance yield.
- Precision Metalization: The experiment used a dual-layer Ti/Au stack (7 nm/70 nm). 6CCVD offers in-house, high-vacuum metalization services including Ti, Au, Pt, Pd, W, and Cu with precise thickness control, ensuring optimal electrode adhesion and conductivity for integrated devices.
- Custom Dimensions and Shaping: The paper noted yield issues related to fabrication uniformity. 6CCVD supports custom laser shaping and micromachining of diamond substrates and wafers up to 125 mm, allowing for pre-patterned fiducial marks, alignment trenches, or specialized geometries required for subsequent E-beam lithography and heterogeneous integration (like the GaP transfer).
- Surface Preparation: Achieving high-yield near-surface NV centers requires state-of-the-art polishing. Our SCD polishing achieves Ra < 1 nm, providing an atomically flat surface essential for minimizing strain and scatter loss during photonic coupling and subsequent epitaxial layer transfer (GaP-on-diamond bonding).
Engineering Support
Section titled âEngineering SupportâThe research identifies that maximizing yield requires improved NV center-device alignment (e.g., patterned implantation) and full multi-axis Stark control.
6CCVDâs in-house PhD team provides specialized engineering consultation to optimize material selection for scalable Chip-Scale Entanglement Generation projects. We assist clients in designing material architectures that accommodate:
- Advanced Defect Engineering: Collaborating on optimized post-growth annealing recipes to improve NV charge state stability and reduce spectral diffusion, mitigating the large dynamic variations reported in this paper.
- Multi-Axis Control: Selecting appropriate diamond cuts and orientations to facilitate full (2- or 3-axis) Stark control, crucial for comprehensive frequency stabilization across an ensemble of emitters.
- High-Yield Integration: Ensuring that substrate specifications (e.g., thickness, crystallographic orientation, surface roughness) are perfectly matched to complex nanofabrication techniques like RIE etching and membrane transfer, thereby improving overall device yield.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Generating entangled graph states of qubits requires high entanglement rates with efficient detection of multiple indistinguishable photons from separate qubits. Integrating defect-based qubits into photonic devices results in an enhanced photon collection efficiency, however, typically at the cost of a reduced defect emission energy homogeneity. Here, we demonstrate that the reduction in defect homogeneity in an integrated device can be partially offset by electric field tuning. Using photonic device-coupled implanted nitrogen vacancy (NV) centers in a GaP-on-diamond platform, we demonstrate large field-dependent tuning ranges and partial stabilization of defect emission energies. These results address some of the challenges of chip-scale entanglement generation.