Multipoint Lock-in Detection for Diamond Nitrogen-Vacancy Magnetometry Using DDS-Based Frequency-Shift Keying

At a Glance

Metadata	Details
Publication Date	2023-12-21
Journal	Micromachines
Authors	Qidi Hu, Luheng Cheng, Yushan Liu, Xinyi Zhu, Yu Tian
Institutions	Hefei University of Technology, Zhejiang Lab
Citations	1
Analysis	Full AI Review Included

Technical Documentation & Analysis: Multipoint Lock-in Detection for NV Magnetometry

This document analyzes the research paper “Multipoint Lock-in Detection for Diamond Nitrogen-Vacancy Magnetometry Using DDS-Based Frequency-Shift Keying” and outlines how 6CCVD’s advanced MPCVD diamond materials and processing capabilities directly support and enhance this quantum sensing technology.

Executive Summary

This research presents a significant advance in miniaturized quantum sensing by developing an integrated digital system for multipoint lock-in detection using Nitrogen-Vacancy (NV) centers in diamond.

Miniaturization & Integration: The system integrates two FPGAs and a Direct Digital Synthesizer (DDS) onto a small 60 mm x 50 mm PCB, enabling highly mobile and portable quantum sensor applications.
Single-Source Multi-Resonance: Overcomes the complexity of traditional methods by using a single self-built microwave (MW) source and Frequency-Shift Keying (FSK) to track multiple NV resonances simultaneously.
High-Speed Frequency Hopping: Achieves rapid frequency shifting at the sub-µs level across a maximum 1.4 GHz bandwidth, ensuring high data timeliness and efficient multipoint measurement.
Vector Sensing Capability: The ability to detect magnetic resonance shifts from different NV orientations allows for potential applications in magnetic vector detection and joint temperature-magnetic sensing.
Validation: Performance is verified through continuous-wave Optically Detected Magnetic Resonance (cw-ODMR) and simultaneous tracking of four distinct NV resonances (e.g., 2824.4 MHz, 2843.1 MHz, 2893.6 MHz, 2911.2 MHz).
Material Requirement: The success of this system relies fundamentally on high-quality, low-strain Single Crystal Diamond (SCD) substrates for optimal NV center coherence and optical readout fidelity.

Technical Specifications

The following hard data points were extracted from the experimental implementation of the DDS-based FSK system:

Parameter	Value	Unit	Context
Maximum MW Bandwidth	1.4	GHz	DDS theoretical capability for frequency hopping
Practical MW Output Control	> 500	MHz	Used in practical application for cleaner signal
Minimum Frequency Shift Time	Sub-µs	Level	Time required for DDS frequency shifting
ODMR Frequency Sweep Range	2.8 to 2.95	GHz	Experimental validation range
Frequency Step Size (ODMR)	400	kHz	Interval between Frequency Turning Words (FTWs)
MW Duration per Frequency	30	ms	Residence time during cw-ODMR sweep
MW Output Power Level	~17	dBm	Power used during cw-ODMR experiment
Laser Excitation Wavelength	532	nm	Used for NV center excitation
Fluorescence Detection Range	637 to 800	nm	NV center emission spectrum
ADC Resolution	14	bit	Data acquisition module specification (LTC2145-14)
Maximum Sampling Rate	125	MSPS	Master FPGA sampling module rate
System PCB Footprint	60 x 50	mm	Integrated system size for portability

Key Methodologies

The core innovation lies in the synchronization and modulation techniques applied to the microwave source and data acquisition module:

Integrated Digital Architecture: The system utilizes a Master FPGA (Xilinx Zynq) controlling a Slave FPGA (Xilinx Artix-7) and a DDS chip (AD9914) to generate the microwave signal and manage the DAQ/DSP processes.
DDS-Based FSK Modulation: Frequency-Shift Keying (FSK) is implemented by storing Frequency Turning Words (FTWs) and Frequency Offset Words (FOWs) in the Slave FPGA RAM. This allows the microwave frequency to rapidly hop between $f_{out} + f_{offset}$ and $f_{out} - f_{offset}$ within a single cycle.
Synchronization via Trigger Signal: A single trigger signal synchronizes the MW source frequency hopping with the DAQ sampling module, ensuring that the collected fluorescence data corresponds precisely to the current microwave frequency.
Differential Sampling: The DAQ process is divided into two parts (Section A and Section B) corresponding to the two shifted frequencies. A short “Delay Window” is implemented after the trigger edge to allow the DDS frequency to settle, preventing errors.
Data Encoding and Processing: The Digital Signal Processor (DSP) encodes the accumulated 14-bit data based on the trigger level (A or B marker) and packages it into 128-bit UDP packets for differential analysis by the PC terminal.
Multipoint Detection: By cycling through multiple FTWs, the system can sequentially measure the resonance peaks of different NV orientations (four groups detected) in an extremely short period, enabling vector magnetic field sensing.

6CCVD Solutions & Capabilities

The successful replication and commercialization of this DDS-based NV magnetometry system depend critically on high-quality diamond material. 6CCVD is uniquely positioned to supply the necessary substrates and customization services.

Applicable Materials

To achieve the high coherence times and optical fidelity required for quantum sensing, the research necessitates the use of high-purity diamond.

Optical Grade Single Crystal Diamond (SCD): This is the ideal material for replicating this research. 6CCVD supplies high-purity SCD plates with low strain, essential for maximizing the $T_{2}$ coherence time of the NV centers and ensuring sharp, detectable ODMR resonances.
Controlled Doping: 6CCVD can provide SCD substrates with controlled nitrogen concentrations, allowing researchers to optimize the density of NV centers for ensemble sensing applications, balancing signal strength against coherence time.

Customization Potential

The integrated nature of the reported system (60 mm x 50 mm PCB) demands highly precise, custom-sized diamond substrates.

Research Requirement	6CCVD Capability	Sales Advantage
Precision Substrate Size	Custom Dimensions: Plates and wafers available up to 125 mm (PCD) and custom sizes for SCD.	We provide the exact, small-footprint SCD wafers required for miniaturized, integrated systems, ensuring seamless fit onto the 60 mm x 50 mm PCB.
Optimal Optical Readout	Advanced Polishing: SCD surfaces polished to Ra < 1 nm.	Ultra-low surface roughness minimizes laser scattering (532 nm) and maximizes fluorescence collection (637-800 nm), directly improving the signal-to-noise ratio for the 14-bit ADC.
Integrated MW Delivery	Custom Metalization: In-house deposition of Au, Pt, Pd, Ti, W, Cu.	For future iterations requiring on-chip microwave antennas (e.g., Q-type antennas mentioned in the paper) or striplines, 6CCVD can deposit high-quality metal layers directly onto the diamond surface.
Thick Substrates/Heat Sinks	Substrate Thickness: Up to 10 mm available.	While the sensing layer is thin, thicker diamond substrates can be used as high-performance heat spreaders, managing thermal load from the integrated FPGAs and power amplifiers, which is critical for maintaining temperature stability in highly mobile devices.

Engineering Support

6CCVD’s in-house team of PhD material scientists specializes in optimizing MPCVD diamond for quantum applications. We offer comprehensive engineering support to researchers and developers working on similar quantum magnetic sensing projects. This includes consultation on:

Selecting the optimal diamond grade (SCD vs. PCD) based on required coherence time and cost constraints.
Designing custom metalization schemes for efficient MW coupling.
Specifying surface termination and polishing requirements for maximum optical fidelity.

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

View Original Abstract

In recent years, the nitrogen-vacancy (NV) center in diamonds has been demonstrated to be a high-performance multiphysics sensor, where a lock-in amplifier (LIA) is often adopted to monitor photoluminescence changes around the resonance. It is rather complex when multiple resonant points are utilized to realize a vector or temperature-magnetic joint sensing. In this article, we present a novel scheme to realize multipoint lock-in detection with only a single-channel device. This method is based on a direct digital synthesizer (DDS) and frequency-shift keying (FSK) technique, which is capable of freely hopping frequencies with a maximum of 1.4 GHz bandwidth and encoding an unlimited number of resonant points during the sensing process. We demonstrate this method in experiments and show it would be generally useful in quantum multi-frequency excitation applications, especially in the portable and highly mobile cases.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi15010014

References

2020 - Sensitivity optimization for nv-diamond magnetometry [Crossref]
2020 - Efficient implementation of a quantum algorithm in a single nitrogen-vacancy center of diamond [Crossref]
2008 - Multipartite entanglement among single spins in diamond [Crossref]
2013 - Quantum logic readout and cooling of a single dark electron spin [Crossref]
2016 - Direct measurement of topological numbers with spins in diamond [Crossref]
2015 - Quantum simulation of helium hydride cation in a solid-state spin register [Crossref]
2008 - Nanoscale magnetic sensing with an individual electronic spin in diamond [Crossref]
2012 - High spatial and temporal resolution wide-field imaging of neuron activity using quantum nv-diamond [Crossref]
2017 - Measuring broadband magnetic fields on the nanoscale using a hybrid quantum register [Crossref]
2022 - Multiplexed sensing of magnetic field and temperature in real time using a nitrogen-vacancy ensemble in diamond [Crossref]