Memory-assisted quantum key distribution with a single nitrogen-vacancy center

At a Glance

Metadata	Details
Publication Date	2017-11-13
Journal	Physical review. A/Physical review, A
Authors	Nicolò Lo Piparo, Mohsen Razavi, William J. Munro
Institutions	National Institute of Informatics, University of Leeds
Citations	33
Analysis	Full AI Review Included

Technical Documentation and Analysis: Memory-Assisted QKD using Single NV Centers

This document analyzes the technical requirements of the research paper “Memory-Assisted Quantum Key Distribution with a Single Nitrogen Vacancy Center” and correlates them with the specialized capabilities of 6CCVD’s diamond material fabrication and processing services.

Executive Summary

The analyzed research proposes an efficient Memory-Assisted Measurement-Device-Independent Quantum Key Distribution (MA-MDI-QKD) scheme relying on a single Nitrogen Vacancy (NV) center embedded in a microcavity. This work leverages the unique properties of diamond quantum memories, providing a compelling pathway toward scalable quantum network implementation.

Key findings and material requirements include:

Performance Enhancement: The scheme is projected to significantly outperform conventional repeaterless QKD systems (PLOB bound) in terms of secure distance and key rate, especially when leveraging the long coherence time of the nuclear spin ($T_{n}$).
Material Core: The success hinges entirely on ultra-high purity Single Crystal Diamond (SCD) material capable of hosting high-quality, long-lived NV centers embedded in an optical microcavity structure.
Storage Mechanism: The electron spin is used for fast photon interaction ($T_{int}$ = 10 ns), while the nuclear spin is used for long-term quantum storage, requiring coherence times ($T_{n}$) up to 10 seconds.
Efficiency Metrics: High cooperativity ($C=50$) is required for effective double-encoding and entanglement generation, necessitating precision fabrication of the diamond microcavity structure.
Scalability: The single-memory setup is scalable to a three-leg quantum repeater architecture, theoretically extending the secure distance to approximately 1400 km.

Technical Specifications

The following table extracts key parameters critical to the performance of the proposed MA-MDI-QKD scheme, derived primarily from Table I and numerical results in the paper.

Parameter	Value	Unit	Context
Material Requirement	Ultra-High Purity SCD	N/A	Essential for long nuclear spin coherence time ($T_{n}$)
Cooperativity ($C$)	50	N/A	Required for strong coupling in the NV-cavity system
Electron Spin Interaction Time ($T_{int}$)	10	ns	Time required for photon entanglement generation
Initialization Time ($T_{init}$)	11.5	ns	Time required for NV center state preparation
Electron-to-Nuclear Spin Swap Time ($T_{swap}$)	1.1	µs	Time required for transferring the qubit to memory
Logic Gate Error Probability (CZ Gate, $p_{cz}$)	2 x 10^-4	N/A	Target error rate for deterministic BSM
Nuclear Spin Coherence Time ($T_{n}$)	10	s	Maximum coherence time assumed for long-distance QKD
Attenuation Length ($L_{att}$)	25	km	Standard telecom fiber channel loss parameter
Maximum QKD Distance (Single-Memory)	~400	km	Distance where the proposed scheme crosses the 1 GHz PLOB bound
Maximum Repeater Distance (Three-Leg)	~1400	km	Theoretical limit imposed by dark count rate

Key Methodologies

The following ordered list outlines the critical steps and requirements necessary to implement the single-memory MA-MDI-QKD protocol relying on the NV center in a cavity (referenced against Figure 3 and Appendix B).

NV Center Initialization and Cavity Integration:
- A single NV center is embedded in a one-sided microcavity structure built on a diamond substrate, optimized for a high Cooperativity ($C=50$).
- The NV center is initialized into the desired state using the double-encoding module, taking $T_{init}$ = 11.5 ns.
Alice State Loading (Step 1):
- Alice’s photon (BB84 encoded) is directed toward the cavity-NV system.
- A side-BSM is performed using the double-encoding technique to teleport Alice’s state onto the electron spin ($e_{A}$).
Spin Transfer to Nuclear Memory (Step 2):
- Upon successful loading, the state from the electron spin ($e_{A}$) is transferred to the highly coherent nuclear spin ($n$) via a controlled evolution based on the hyperfine interaction ($t = \pi/A_{net} \approx 165$ ns).
- The total swap time ($T_{swap}$) for the transfer operation is 1.1 µs.
Bob State Loading (Step 3):
- Optical switches redirect Bob’s photon (BB84 encoded) to the BSM module.
- Bob’s state is teleported onto the electron spin ($e_{B}$), using spin echo ($T_{dis}$) during the loading process to protect the nuclear spin memory.
Final Deterministic BSM (Step 4):
- A Controlled-Z (CZ) gate is performed on the entangled nuclear ($n$) and electron ($e_{B}$) spins.
- An X-basis measurement is performed to create the final correlated key bit between Alice and Bob.

6CCVD Solutions & Capabilities

6CCVD is positioned as the ideal partner for sourcing the foundation material required for this breakthrough quantum memory research. The realization of long-coherence NV centers and complex microcavity structures depends on high-quality, customized MPCVD diamond.

Applicable Materials

The proposed research demands exceptional material quality to support the necessary spin coherence times ($T_{n}$ up to 10 s) and the structural integrity required for microcavity fabrication.

Material	Description & Requirements Match	6CCVD Offering
High Purity Single Crystal Diamond (SCD)	Required substrate for intrinsic NV centers, guaranteeing ultra-low nitrogen content (ppm/ppb levels) to minimize decoherence sources and reduce strain/impurities.	Quantum Grade SCD: Optimized for NV center applications with extremely high crystalline purity and low birefringence. SCD wafers from 0.1 µm to 500 µm thickness available.
Structured Diamond Substrates	The successful operation relies on embedding the NV center in a small-volume optical microcavity (p. 7). This requires precision etching and structural control.	Custom SCD Processing: We supply highly polished SCD substrates (Ra < 1nm) suitable for subsequent epitaxial growth or micro-nanofabrication (e.g., reactive ion etching) for cavity creation.
Polycrystalline Diamond (PCD) / Boron-Doped (BDD)	While not the primary material for the quantum memory itself, these materials are often used for associated components (e.g., stable heat sinks, electrodes, or supporting structures).	Engineering Grade Materials: Available in plates up to 125mm in diameter and thicknesses up to 10mm for system integration and thermal management.

Customization Potential

The complexity of integrating the NV center into a functional device (microcavity and BSM unit) requires high-precision starting materials and post-processing steps.

Research Requirement Implication	6CCVD Customization Service	Value Proposition
Microcavity Fabrication Base	Precision polishing (Ra < 1nm for SCD) and custom laser cutting/shaping services.	Provides mechanically stable and optically smooth substrates necessary for high Cooperativity ($C=50$) cavity structures.
Advanced Quantum Device Integration	Internal metalization capabilities (Au, Pt, Pd, Ti, W, Cu).	Enables rapid prototyping and integration of contact electrodes or optical reflective coatings necessary for controlling spin states or integrating detectors.
System Scale-Up	Custom dimensions for plates and wafers (up to 125mm for PCD, inch-size SCD).	Supports researchers moving from single-device proof-of-concept toward array-based or multi-node quantum repeater architectures.

Engineering Support

The demanding specifications of this MA-MDI-QKD project—especially achieving the necessary $T_{n}$ coherence time of 10 seconds—require close collaboration between material suppliers and quantum engineers.

Materials Consultation: 6CCVD’s in-house PhD material science team specializes in customizing MPCVD diamond growth parameters to optimize specific properties (e.g., ultra-low nitrogen content, specific crystallographic orientation, and strain reduction) critical for long-lived NV center Quantum Memory (QM) projects.
Recipe Refinement: We assist in selecting the optimal substrate thickness (0.1 µm - 500 µm) and geometry for micro-cavity and membrane fabrication, ensuring compatibility with the required 10 ns interaction time.
Global Logistics: We provide reliable global shipping options (DDU default, DDP available) to ensure timely delivery of high-value quantum materials worldwide.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Memory-assisted measurement-device-independent quantum key distribution\n(MA-MDI-QKD) is a promising scheme that aims to improve the\nrate-versus-distance behavior of a QKD system by using the state-of-the-art\ndevices. It can be seen as a bridge between current QKD links to quantum\nrepeater based networks. While, similar to quantum repeaters, MA-MDI-QKD relies\non quantum memory (QM) units, the requirements for such QMs are less demanding\nthan that of probabilistic quantum repeaters. Here, we present a variant of\nMA-MDI-QKD structure that relies on only a single physical QM: a\nnitrogen-vacancy center embedded into a cavity where its electronic spin\ninteracts with photons and its nuclear spin is used for storage. This enables\nus to propose a simple but efficient MA-MDI-QKD scheme resilient to memory\nerrors and capable of beating, in terms of rate and reach, existing QKD\ndemonstrations. We also show how we can extend this setup to a quantum repeater\nsystem, reaching, thus, larger distances.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physreva.96.052313