Scalable spin–photon entanglement by time-to-polarization conversion
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-01-28 |
| Journal | npj Quantum Information |
| Authors | Rui Vasconcelos, Sarah Reisenbauer, C. L. Salter, Georg Wachter, Daniel Wirtitsch |
| Institutions | University of Vienna, Vienna Center for Quantum Science and Technology |
| Citations | 40 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Scalable Spin-Photon Entanglement
Section titled “Technical Documentation & Analysis: Scalable Spin-Photon Entanglement”Executive Summary
Section titled “Executive Summary”This research validates a scalable Time-to-Polarization Conversion (TPC) protocol for generating spin-photon entanglement using Nitrogen-Vacancy (NV) centers in diamond. The findings directly inform the material requirements for next-generation quantum communication and computing architectures.
- Protocol Validation: Successful experimental demonstration of the TPC protocol using a single optical transition in an NV center, significantly relaxing the stringent requirements of previous cluster-state machine gun (CSMG) schemes.
- High Fidelity Achieved: The system demonstrated high qubit initialization fidelity (97.9 ± 1.6%) and a lower bound entanglement fidelity of F ≥ 64.7 ± 1.3%.
- Material Requirement: The experiment relied on high-quality, artificial Single-Crystal Diamond (SCD) with a specific {1, 1, 1} surface orientation for optimal NV axis alignment.
- Fabrication Necessity: Focused-Ion-Beam (FIB) milling was used to create solid-immersion lenses (SILs) on the diamond surface, highlighting the need for ultra-smooth, high-purity substrates suitable for advanced micro-fabrication.
- Scaling Limitation: The current system is limited by low Zero-Phonon Line (ZPL) photon collection efficiency (~2 x 10-5 pulse to click) and excited-state spin mixing, issues directly addressed by 6CCVD’s ultra-low strain, high-coherence SCD materials and custom integration services.
- 6CCVD Value Proposition: We provide the necessary high-purity, low-strain SCD substrates, custom orientations, and integrated metalization required to replicate this work and scale the system using high-cooperativity optical resonators.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the experimental realization of the TPC protocol:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Material Substrate | Single-Crystal Diamond (SCD) | N/A | Artificial, natural isotopic abundance |
| Surface Orientation | {1, 1, 1} | N/A | Used for NV center alignment |
| Operating Temperature | ~4.5 | K | Closed-cycle cryostat cooling |
| Resonant Laser Wavelength | 637.2 | nm | Used for optical $\pi$-pulses |
| Working Transition Detuning | 0.87 | GHz | Ensures negligible cross-excitation |
| Qubit Initialization Fidelity | 97.9 ± 1.6 | % | Electron spin subspace |
| Nuclear Polarization | 83.8 ± 1.9 | % | Within the mI = -1 manifold |
| Entanglement Fidelity (Lower Bound) | F ≥ 64.7 ± 1.3 | % | Relative to ideal Bell-state |
| ZPL Efficiency (Pulse to Click) | ~2 x 10-5 | N/A | Limited by decay into Phonon-Side Band (PSB) |
| Interferometer Propagation Delay | 262 | ns | Time difference between fiber arms |
| Surface Coating | 110 | nm | SiO2 layer to reduce Fresnel reflection |
Key Methodologies
Section titled “Key Methodologies”The experimental demonstration required precise material engineering and controlled quantum manipulation steps:
- Material Selection: Utilization of artificial, single-crystal diamond (SCD) with natural isotopic abundance, selected for its NV center properties and {1, 1, 1} surface orientation.
- Micro-Fabrication: Focused-Ion-Beam (FIB) milling was employed to machine solid-immersion lenses (SILs) over pre-allocated NV centers to improve photon collection efficiency.
- Surface Preparation: The diamond surface was coated with 110 nm of SiO2 to minimize Fresnel reflection losses and laser backscatter at the high-index interface.
- Spin Initialization: Iterative resonant optical pumping and nuclear spin selective microwave pulses were used to initialize the electron and nuclear spins into the high-fidelity state |ms = -1, mI = 0>.
- Entanglement Sequence: The core TPC protocol involved a sequence of microwave Ry($\pi$/2) rotation, a resonant optical $\pi$-pulse (a1), an intermediate microwave $\pi$-pulse, and a second optical $\pi$-pulse (a2).
- Time-to-Polarization Conversion: ZPL photons were routed into a polarization-maintaining Mach-Zehnder interferometer, where the 262 ns propagation delay matched the time between the two optical $\pi$-pulses, erasing path information and converting time-bin entanglement to polarization entanglement.
- Tomography: Partial spin-photon tomography was performed by measuring correlations in the $\sigma$z $\otimes$ $\sigma$z and $\sigma$x $\otimes$ $\sigma$x bases to quantify entanglement fidelity.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”6CCVD is uniquely positioned to supply the high-quality MPCVD diamond required to replicate, optimize, and scale the TPC entanglement protocol for commercial and academic quantum applications.
Applicable Materials
Section titled “Applicable Materials”To achieve the high coherence and low strain necessary for robust NV center operation and high fidelity, the following 6CCVD materials are recommended:
| 6CCVD Material | Specification | Application Relevance |
|---|---|---|
| Optical Grade SCD | Ultra-low strain, high purity (N < 1 ppb) | Essential for minimizing spin mixing in the excited state and maximizing coherence time, directly addressing a key limitation cited in the paper. |
| Custom SCD Substrates | {1, 1, 1} Orientation, Thickness 0.1 µm - 500 µm | Required for aligning the NV axis perpendicular to the surface, crucial for efficient spin-photon coupling. |
| Substrates up to 10 mm | High mechanical stability | Provides robust mounting platforms for cryostat integration and subsequent FIB milling or optical resonator attachment. |
Customization Potential
Section titled “Customization Potential”The research highlights the need for precise material geometry and integrated components (e.g., bond wires for microwave delivery). 6CCVD offers comprehensive customization services to meet these demands:
- Custom Dimensions and Orientation: We provide SCD plates and wafers up to 125 mm (PCD) and offer precise control over crystal orientation, including the required {1, 1, 1} surface.
- Advanced Polishing: Our SCD substrates are polished to an industry-leading surface roughness of Ra < 1nm, which is critical for high-quality FIB milling of SILs and for integration with high-Q optical resonators (a known solution for improving ZPL efficiency).
- Integrated Metalization: We offer in-house deposition of thin-film metals (Au, Pt, Pd, Ti, W, Cu) for creating integrated microwave delivery structures directly on the diamond surface, simplifying the experimental setup and improving microwave transfer fidelity.
Engineering Support
Section titled “Engineering Support”The path to scalable quantum systems requires overcoming complex material-science challenges, such as minimizing strain and maximizing photon collection.
- Strain Management: 6CCVD’s in-house PhD team specializes in material selection and growth recipes designed to minimize internal strain, a critical factor limiting the fidelity of NV center entanglement schemes.
- Optical Enhancement Consultation: We assist researchers in selecting optimal diamond thickness and surface preparation for integrating external structures, such as high-cooperativity optical resonators, which are cited as a solution to drastically improve ZPL photon collection efficiency (currently 2 x 10-5).
- Global Logistics: We ensure reliable global shipping (DDU default, DDP available) to deliver sensitive quantum materials directly to cryostat environments worldwide.
Call to Action: For custom specifications or material consultation regarding scalable spin-photon entanglement projects, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract The realization of quantum networks and quantum computers relies on the scalable generation of entanglement, for which spin-photon interfaces are strong candidates. Current proposals to produce entangled-photon states with such platforms place stringent requirements on the physical properties of the photon emitters, limiting the range and performance of suitable physical systems. We propose a scalable protocol, which significantly reduces the constraints on the emitter. We use only a single optical transition and an asymmetric polarizing interferometer. This device converts the entanglement from the experimentally robust time basis via a path degree of freedom into a polarization basis, where quantum logic operations can be performed. The fundamental unit of the proposed protocol is realized experimentally in this work, using a nitrogen-vacancy center in diamond. This classically assisted protocol greatly widens the set of physical systems suited for scalable entangled-photon generation and enables performance enhancement of existing platforms.