Laser threshold magnetometry

At a Glance

Metadata	Details
Publication Date	2016-01-08
Journal	New Journal of Physics
Authors	Jan Jeske, Jared H. Cole, Andrew D Greentree, Jan Jeske, Jared H. Cole
Institutions	RMIT University, Quantum (Australia)
Citations	51
Analysis	Full AI Review Included

Technical Documentation & Analysis: Laser Threshold Magnetometry (LTM)

This document analyzes the requirements for implementing Laser Threshold Magnetometry (LTM) using Nitrogen-Vacancy (NV^-) centers in diamond, focusing on how 6CCVD’s advanced MPCVD diamond materials and processing capabilities meet the stringent demands of this high-sensitivity quantum sensing application.

Executive Summary

The research proposes Laser Threshold Magnetometry (LTM), a novel room-temperature sensing technique utilizing NV^- ensembles in diamond as a laser gain medium.

Breakthrough Sensitivity: LTM is predicted to achieve shot-noise limited d.c. sensitivity of 1.86 fT/√Hz and a.c. sensitivity of 3.97 fT/√Hz.
Performance Parity: This sensitivity is 2-3 orders of magnitude better than current NV^- demonstrations and comparable to state-of-the-art, cryogenically operated SQUID magnetometers.
Room-Temperature Operation: The device operates at room temperature, offering a significant technological advantage for industrial and biomedical applications (e.g., MEG).
Material Requirement: Implementation requires high-quality, high-purity Single Crystal Diamond (SCD) with a high, controlled NV^- concentration (up to 16 ppm) and long inhomogeneous coherence time (T₂*).
Device Structure: The sensor requires a 1 mm³ diamond volume integrated into an external cavity, pumped by a CW green laser and driven by an RF field.
6CCVD Solution: 6CCVD provides the necessary Quantum Grade SCD substrates with custom doping, isotopic purification (low ¹³C), and ultra-low roughness polishing (Ra < 1 nm) essential for high-Q cavity integration.

Technical Specifications

The following parameters are critical for achieving the predicted ultra-high sensitivity LTM device:

Parameter	Value	Unit	Context
Predicted DC Sensitivity (Optimal)	1.86	fT/√Hz	Shot-noise limited, 1-second measurement
Predicted AC Sensitivity (Optimal)	3.97	fT/√Hz	Low frequency, small signal amplitudes
NV^- Concentration (Optimal)	16	ppm	Required for optimal sensitivity
Coherence Time (T₂*)	0.181	µs	Required for optimal sensitivity
Laser Medium Volume (V_NV)	1	mm³	Required volume for LTM gain medium
External Cavity Volume (V_c)	2	mm³	Assumed cavity volume
Lasing Wavelength (λ)	709	nm	Red three-phonon sideband emission
Crystal Field Splitting	2.88	GHz	Zero magnetic field spin ±1 states
Rabi Frequency (Ω) (Example)	3.67	MHz	Used in Figure 2 calculations
Cavity Loss Rate (κ) (Optimal)	63.1	GHz	Used for optimal sensitivity (Q ≈ 8.9 x 10⁸)
Response Time (t_r)	0.5	µs	Limited by non-spin conserving transition rate

Key Methodologies

The LTM concept relies on precise control over the NV^- spin state and cavity dynamics:

Gain Medium Preparation: Utilize high-purity Single Crystal Diamond (SCD) doped with Nitrogen (N) and subsequently converted to a high density of negatively-charged Nitrogen-Vacancy (NV^-) centers (up to 16 ppm).
Optical Pumping: Employ a Continuous Wave (CW) green pump laser to drive population into the phonon-added excited states, initiating spin polarization into the spin 0 ground state.
RF Spin Drive: Apply a radio-frequency (RF) Rabi-drive field (Ω) to induce coherent oscillations between the spin 0 and spin ±1 ground states.
Threshold Operation: Set the laser pumping rate (Λ) such that the system is just below the lasing threshold when the RF drive is resonant (magnetic field B = 0).
Magnetic Field Readout: An external magnetic field (B) causes detuning (Δ), shifting the spin populations and pushing the system above the lasing threshold, resulting in a coherently amplified output signal (P_out) proportional to B.
Lasing Transition: Lasing occurs on the red three-phonon sideband transition (709 nm).
Cavity Integration: The diamond medium must be integrated into a high-Q optical cavity to enhance stimulated emission and achieve milliwatt output power.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials required to realize and scale Laser Threshold Magnetometry (LTM) devices.

Applicable Materials

To replicate or extend this research, high-quality diamond with precise defect engineering is mandatory.

Quantum Grade Single Crystal Diamond (SCD): Required for its superior crystalline quality, low defect density, and ability to host high concentrations of NV^- centers.
Controlled Nitrogen Doping: The optimal sensitivity requires a high NV^- concentration of 16 ppm. 6CCVD specializes in custom MPCVD growth recipes to control N incorporation and subsequent processing to maximize NV^- conversion efficiency.
Isotopic Purification: To achieve the required T₂* coherence time (0.181 µs), the diamond must have low concentrations of paramagnetic impurities, particularly low natural abundance ¹³C. 6CCVD offers isotopically purified SCD to minimize decoherence sources.

Customization Potential

The LTM device requires specific geometries and surface preparation for optimal cavity coupling.

Requirement from Paper	6CCVD Capability	Technical Advantage
Volume (1 mm³)	Custom Dimensions & Laser Cutting: We provide SCD substrates up to 500 µm thick and up to 10 mm thick for substrates. We offer precise laser cutting services to achieve the required 1 mm³ geometry.	Ensures precise volume control for ensemble size (N_at).
High-Q Cavity Integration	Ultra-Low Roughness Polishing: We guarantee Ra < 1 nm surface roughness on SCD plates.	Essential for minimizing scattering losses and achieving the high cavity Q-factor (Q ≈ 8.9 x 10⁸) required for LTM.
RF Drive Integration	Custom Metalization: We offer internal metalization services (Au, Pt, Pd, Ti, W, Cu).	Allows for integration of on-chip RF strip lines or electrodes directly onto the diamond surface for efficient Rabi driving (Ω).
Scaling Potential	Large Area PCD/SCD: We offer plates/wafers up to 125 mm (PCD) and large-area SCD, enabling wafer-level processing and scaling of LTM arrays.	Facilitates industrial applications and high-throughput manufacturing.

Engineering Support

LTM is a complex quantum sensing scheme dependent on material quality and defect control.

Recipe Optimization: 6CCVD’s in-house PhD team provides expert consultation on material selection and growth recipe optimization. We assist researchers in tuning N concentration and post-growth annealing protocols to maximize the yield and stability of the NV^- charge state.
Application Focus: Our team can assist with material selection for similar quantum sensing projects, including Magnetoencephalography (MEG), where the predicted fT/√Hz sensitivity is critical for measuring weak brain activity fields.
Global Logistics: We offer reliable global shipping (DDU default, DDP available) to ensure timely delivery of sensitive quantum materials worldwide.

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

View Original Abstract

We propose a new type of sensor, which uses diamond containing the optically\nactive nitrogen-vacancy (NV$^-$) centres as a laser medium. The magnetometer\ncan be operated at room-temperature and generates light that can be readily\nfibre coupled, thereby permitting use in industrial applications and remote\nsensing. By combining laser pumping with a radio-frequency Rabi-drive field, an\nexternal magnetic field changes the fluorescence of the NV$^-$ centres. We use\nthis change in fluorescence level to push the laser above threshold, turning it\non with an intensity controlled by the external magnetic field, which provides\na coherent amplification of the readout signal with very high contrast. This\nmechanism is qualitatively different from conventional NV$^-$—based\nmagnetometers which use fluorescence measurements, based on incoherent photon\nemission. We term our approach laser threshold magnetometry (LTM). We predict\nthat an NV$^-$—based laser threshold magnetometer with a volume of 1mm$^3$ can\nachieve shot-noise limited d.c.~sensitivity of 1.86 fT$/\sqrt{\rm{Hz}}$ and\na.c.~sensitivity of 3.97fT$/\sqrt{\rm{Hz}}$.\n

Tech Support

Original Source

DOI: https://doi.org/10.1088/1367-2630/18/1/013015