Nitrogen-Vacancy Center Coupled to an Ultrasmall-Mode-Volume Cavity - A High-Efficiency Source of Indistinguishable Photons at 200 K
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2021-03-10 |
| Journal | Physical Review Applied |
| Authors | Joe A. Smith, Chloe Clear, Krishna C. Balram, Dara P. S. McCutcheon, John Rarity |
| Institutions | University of Bristol |
| Citations | 13 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive SummaryāThis research proposes a highly efficient, scalable source of indistinguishable single photons utilizing a Nitrogen Vacancy (NV) center in diamond coupled to a novel photonic crystal cavity (PhCC).
- Elevated Temperature Operation: The system is designed to operate effectively at 200 K (Peltier cooling regime), significantly reducing the complexity and cost associated with liquid helium cooling (4 K).
- High Indistinguishability: The core achievement is an indistinguishability (I) of 0.54 (unfiltered) at 200 K, which is nine orders of magnitude better than an uncoupled NV center at room temperature. This can be boosted to I = 0.73 using external spectral filtering.
- High Efficiency: The design achieves a calculated photon extraction efficiency of 99%, enabling on-demand emission under pulsed excitation.
- Cavity Design: A planar Silicon Nitride (Si3N4) PhCC is used, optimized for a moderate Quality factor (Q ~ 1000) and an ultra-small mode volume (Vm = 0.075 (Ī»/n)3) to maximize the Purcell enhancement rate (R).
- Scalability: The planar geometry is compatible with standard silicon fabrication processes, pointing toward scalable, dense integration of multiple quantum sources on a chip.
- Dephasing Mitigation: The strong Purcell enhancement ensures that emission leaves the atom-like environment faster than the thermal dephasing rate (Ī³* = 1 THz at 200 K), overcoming the primary limitation of solid-state emitters at elevated temperatures.
Technical Specifications
Section titled āTechnical Specificationsā| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Temperature | 200 | K | Target for non-cryogenic (Peltier) cooling |
| Maximum Indistinguishability (I) | 0.73 | Dimensionless | Achieved with external filtering (Kf = 0.3 THz) |
| Extraction Efficiency (Ī²) | 29 | % | Efficiency corresponding to I = 0.73 |
| Unfiltered Indistinguishability (I) | 0.54 | Dimensionless | Optimal planar cavity design at 200 K |
| ZPL Linewidth (Ī³*) | 1 | THz | Pure dephasing rate at 200 K |
| Cavity Quality Factor (Q) | 1000 | Dimensionless | Optimal Q for planar design |
| Cavity Mode Volume (Vm) | 0.075 (Ī»/n)3 | Volume | Loaded planar PhCC design |
| Waveguide Material | Silicon Nitride (Si3N4) | n = 2.0 | Refractive Index |
| Emitter Material | Nanodiamond | n = 2.4 | Modeled as a 20 nm cube |
| ZPL Wavelength | 637 | nm | Corresponding to 470 THz |
| Cavity Length | < 10 | Āµm | Total length including mirror and taper pairs |
| Room Temp. ZPL Linewidth (Ī³*) | 15 | THz | Broadened linewidth at 300 K |
Key Methodologies
Section titled āKey Methodologiesā- Cavity Design and Simulation: Initial design utilized a ābow tieā Photonic Crystal Cavity (PhCC) in a 200 nm thick Silicon Nitride waveguide to achieve ultra-small mode volume (Vm ~ 0.0052 (Ī»/n)3). This was later simplified to a planar PhCC structure for fabrication ease.
- Gaussian Mode Generation: The planar cavity was realized by quadratically tapering the air hole diameter (from 140 nm to 100 nm) across 10 mirror pairs and 5 taper pairs to form a Gaussian mode profile.
- Emitter Modeling: The NV center was modeled as a dipole embedded within a 20 nm cube of nanodiamond (n = 2.4) placed at the cavity center, and the resulting loaded cavity parameters (Vm, Q) were extracted using FDTD simulations and harmonic inversion.
- Quantum Dynamics Modeling: The NV center-cavity interaction was analyzed using a Lindblad master equation, valid at and beyond strong coupling, incorporating both the ZPL broadening (Ī³*) and the broad phonon sideband (SB).
- Indistinguishability Calculation: The overall indistinguishability (I) was calculated by treating the phonon sideband as incoherent and assuming the cavity acts as a spectral filter (width Īŗc) to suppress off-resonant sideband emission.
- Robustness Analysis (Three-Level Model): The system was tested against practical variations, including the random orientation of the NV center dipole using a three-level system model (incorporating the dynamic Jahn-Teller effect and polarization relaxation at 200 K).
- Optimization for Dephasing: The cavity parameters (Q, Vm) were optimized to ensure the Purcell-enhanced spontaneous emission rate (R) was maximized relative to the thermal dephasing rate (Ī³* = 1 THz), preventing emission splitting due to strong coupling while maximizing sideband filtration.
Commercial Applications
Section titled āCommercial Applicationsā- Linear Optic Quantum Computing (LOQC): Provides the necessary high-efficiency, high-indistinguishability single-photon sources required for scalable quantum computation protocols.
- Quantum Communication and Networks: Enables the creation of robust, non-cryogenic quantum network nodes and repeaters, crucial for generating entanglement over long distances.
- Integrated Quantum Photonics: The use of Silicon Nitride and planar fabrication methods allows for dense, chip-scale integration of multiple NV sources, compatible with existing silicon processing infrastructure.
- High-Temperature Quantum Sensing: The ability to maintain strong spin-photon coupling at 200 K could facilitate single-shot electron-spin readout at elevated temperatures, enhancing the utility of NV centers in quantum sensing applications.
- Solid-State Emitter Technology: The methodology for overcoming broad phonon sidebands via cavity filtration is directly transferable to other dephased solid-state emitters (e.g., SiV, GeV centers) to improve their quantum performance above cryogenic limits.
View Original Abstract
Solid state atom-like systems have great promise for linear optic quantum\ncomputing and quantum communication but are burdened by phonon sidebands and\nbroadening due to surface charges. Nevertheless, coupling to a small mode\nvolume cavity would allow high rates of extraction from even highly dephased\nemitters. We consider the nitrogen vacancy centre in diamond, a system\nunderstood to have a poor quantum optics interface with highly distinguishable\nphotons, and design a silicon nitride cavity that allows 99 % efficient\nextraction of photons at 200 K with an indistinguishability of > 50%,\nimprovable by external filtering. We analyse our design using FDTD simulations,\nand treat optical emission using a cavity QED master equation valid at and\nbeyond strong coupling and which includes both ZPL broadening and sideband\nemission. The simulated design is compact (< 10 um), and owing to its planar\ngeometry, can be fabricated using standard silicon processes. Our work\ntherefore points towards scalable fabrication of non-cryogenic atom-like\nefficient sources of indistinguishable photons.\n