Hybrid quantum gates between flying photon and diamond nitrogen-vacancy centers assisted by optical microcavities
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2015-08-14 |
| Journal | Scientific Reports |
| Authors | HaiâRui Wei, GuiâLu Long |
| Institutions | Tsinghua University |
| Citations | 41 |
| Analysis | Full AI Review Included |
Hybrid Quantum Gate Implementation Using MPCVD Diamond NV Centers: Technical Analysis and 6CCVD Solutions
Section titled âHybrid Quantum Gate Implementation Using MPCVD Diamond NV Centers: Technical Analysis and 6CCVD SolutionsâThis technical documentation analyzes the requirements and findings of the research paper âHybrid quantum gates between flying photon and diamond nitrogen-vacancy centers assisted by optical microcavitiesâ to demonstrate how 6CCVDâs specialized MPCVD diamond materials and engineering services can accelerate research in hybrid quantum computing.
Executive Summary
Section titled âExecutive SummaryâThis paper presents a highly compact and efficient scheme for implementing universal hybrid quantum logic gates using a flying photon (control qubit) coupled to solid-state diamond Nitrogen-Vacancy (NV) electron spins (target qubits) via optical microcavities.
- Core Achievement: Deterministic implementation of universal three-qubit gates (CNOT, Toffoli, and Fredkin) using a hybrid photon-NV system without requiring additional auxiliary qubits.
- Mechanism: Exploitation of cavity Quantum Electrodynamics (cQED) in the strong coupling regime ($g/\sqrt{\kappa\gamma} \ge 5$) combined with optical spin selection rules.
- Material Necessity: The scheme relies critically on the ultra-long coherence time (1.8 ms at room temperature) and stable optical transitions of the electron spins associated with NV centers in high-quality diamond.
- Performance Metrics: Theoretical average fidelities ($F$) and efficiencies ($\eta$) approach unity when the strong coupling condition ($g/\kappa\gamma \ge 0.5$) is met.
- Integration Challenge: Success requires precise embedding of NV centers into resonant optical microcavities (e.g., photonic crystal cavities or microtoroidal resonators) to maximize the coupling strength ($g$) and ZPL emission enhancement ($\times 70$).
- Value Proposition: The resulting quantum circuits are significantly simplified and more resource-economic compared to schemes synthesizing three-qubit gates from multiple two-qubit operations.
Technical Specifications
Section titled âTechnical SpecificationsâThe table below summarizes critical hard data points and performance requirements extracted from the analysis, essential for replicating or advancing this hybrid quantum system.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Ground State Spin Splitting ($D$) | $\approx 2.87$ | GHz | Zero external magnetic field |
| Electron Spin Coherence Time ($T_2$) | $1.8$ | ms | Achieved at room temperature |
| Optical Pumping Wavelength | 532 | nm | NV Center preparation (standard green laser) |
| Excited State Spin-Orbit Interaction | $5.5$ | GHz | Key parameter defining NV excited state structure |
| Strong Coupling Ratio (Theoretical Target) | $\ge 5$ | unitless | Required for deterministic interaction ($g / \sqrt{\kappa\gamma}$) |
| High Efficiency Ratio (Theoretical Target) | $\ge 0.5$ | unitless | Required for high gate efficiency ($g / \kappa\gamma$) |
| Reported Coupling Strength ($g/2\pi$) | 55 | MHz | Observed (Park et al. 2006) NV in silica microsphere |
| Required Strain Tuning Range | $\gt 10$ | GHz | Used to tune optical transition frequencies into resonance |
| ZPL Emission Enhancement Factor | $\times 70$ | unitless | Achieved in optimized photonic crystal cavities (Faraon et al. 2012) |
| CNOT Gate Synthesis Cost (Conventional) | 6 | CNOT gates | Complexity avoided by direct implementation |
Key Methodologies
Section titled âKey MethodologiesâThe compact quantum circuits rely on precise control over the input photon, the NV electronic spin, and strong coupling within the microcavity.
- NV Center Preparation: The diamond NV electron spin is initialized using 532 nm optical pumping and coherently controlled via microwave excitation.
- Qubit Encoding:
- Control Qubit: Encoded on the polarization of the flying single photon (Right-circular $|R\rangle$ and Left-circular $|L\rangle$).
- Target Qubit(s): Encoded on the electron spin sublevels of the diamond NV center ground state ($|m_s = \pm 1\rangle$).
- Photon Path Control: Polarizing Beam Splitters (PBS) are utilized to separate the $|R\rangle$ and $|L\rangle$ components of the input photon. Only the $L$-polarized photon component interacts selectively with the NV center.
- Spin-Photon Interaction: The NV center is coupled strongly to an optical microcavity. When the photon frequency matches the NV transition frequency ($\omega = \omega_0 = \omega_p$), the photon reflection coefficient $r(\omega_p)$ changes dramatically, inducing a controlled phase shift only when the NV spin state is $|L\rangle$.
- Hadamard Operation ($H_e$): A Hadamard operation is performed on the NV electron spin before and after the controlled interaction step to translate the controlled phase shift into a CNOT operation (spin flip).
- Gate Completion: A Half-Wave Plate (HWP) is used on the output photon path to complete the unitary transformation ($\sigma_z$), resulting in the desired hybrid CNOT, Toffoli, or Fredkin gate.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâReplicating or extending this hybrid quantum computation research requires ultra-high-quality diamond materials and specialized fabrication expertise, areas where 6CCVD excels as a core supplier of MPCVD diamond.
Applicable Materials
Section titled âApplicable MaterialsâTo achieve the requisite long coherence times (1.8 ms $T_2$) and stable optical transitions essential for strong coupling and high gate fidelity, researchers require Single Crystal Diamond (SCD) with superior quality control.
| 6CCVD Material Recommendation | Specification Rationale |
|---|---|
| Optical Grade SCD | Essential for minimizing spectral diffusion and charge fluctuation, critical hurdles identified in the paper. |
| Low Strain / High Purity SCD | Ensures maximal spin coherence time and facilitates precise tuning of optical transition frequencies required for resonance (up to 10 GHz tuning range). |
| Controlled Doping | SCD wafers must allow for precise creation of high-quality, negatively charged NV centers, either through in-situ growth doping or post-growth ion implantation (which 6CCVD can facilitate preparation for). |
Customization Potential for Cavity Integration
Section titled âCustomization Potential for Cavity IntegrationâThe implementation of these gates relies heavily on confining the NV center within a resonant microcavity (nanopillars, photonic crystals, microtoroids). 6CCVD provides the necessary platform customization:
- Precision Thickness Control: We supply SCD films with thicknesses ranging from 0.1 ”m up to 500 ”m. These thin films are ideal for etching high-aspect-ratio nanopillars or performing lift-off techniques necessary for coupling to high-Q microresonators.
- Custom Dimensions and Etching Preparation: 6CCVD provides custom plate/wafer dimensions up to 125 mm (PCD) and offers high-precision laser cutting/micromachining services, enabling researchers to define samples specifically for nanofabrication workflows (e.g., preparation for Focused Ion Beam milling or Reactive Ion Etching of photonic crystal structures).
- Ultra-low Roughness Polishing: Achieving high cavity Q-factors and maximizing the coupling strength ($g$) demands extremely low surface losses. Our SCD polishing capability achieves Ra < 1 nm, providing an ideal surface finish for fabricating high-performance optical interfaces.
- Integrated Metalization Services: While the current scheme focuses on optical gates, future integration with microwave circuits (as mentioned for spin manipulation) may require metal electrodes. 6CCVD offers in-house deposition of standard metals, including Au, Pt, Pd, Ti, W, and Cu, crucial for microwave components.
Engineering Support
Section titled âEngineering SupportâThe paper identifies key challenges in maintaining NV spectral stability due to charge fluctuation and strain. 6CCVDâs in-house PhD team specializes in the synthesis and characterization of MPCVD diamond and can assist researchers with:
- Material Selection: Determining the optimal SCD substrate orientation and purity level to minimize inherent strain and defect densities that degrade NV performance.
- Process Consultation: Advising on material pre-processing methods to enhance the charge state stability (NV$^-$) required for deterministic hybrid gates.
- Project Scaling: Providing materials in custom dimensions needed for scaling complex quantum circuits involving multiple integrated NV centers (Toffoli and Fredkin gates).
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract Hybrid quantum gates hold great promise for quantum information processing since they preserve the advantages of different quantum systems. Here we present compact quantum circuits to deterministically implement controlled-NOT, Toffoli and Fredkin gates between a flying photon qubit and diamond nitrogen-vacancy (NV) centers assisted by microcavities. The target qubits of these universal quantum gates are encoded on the spins of the electrons associated with the diamond NV centers and they have long coherence time for storing information and the control qubit is encoded on the polarizations of the flying photon and can be easily manipulated. Our quantum circuits are compact, economic and simple. Moreover, they do not require additional qubits. The complexity of our schemes for universal three-qubit gates is much reduced, compared to the synthesis with two-qubit entangling gates. These schemes have high fidelities and efficiencies and they are feasible in experiment.