DC Magnetometry at the $T_2$ Limit

At a Glance

Metadata	Details
Publication Date	2016-11-15
Journal	arXiv (Cornell University)
Authors	Ashok Ajoy, Yixiang Liu, Paola Cappellaro
Citations	3
Analysis	Full AI Review Included

DC Magnetometry at the T₂ Limit: Enhanced Quantum Sensing in CVD Diamond

This document analyzes the research detailing an ancilla-assisted method for sensitive DC magnetometry using Nitrogen Vacancy (NV) centers in diamond, extending interrogation times up to the sensor’s intrinsic coherence time ($T_2$). The findings validate the critical need for high-purity, low-strain Single Crystal Diamond (SCD) material, a specialty of 6CCVD.

Executive Summary

Novel Magnetometry Platform: Introduction and experimental demonstration of a hybridized magnetometer using an NV electron spin sensor and a nuclear spin ancilla ($^{14}\text{N}$).
DC-to-AC Upconversion: The ancillary nuclear spin is used to up-convert static (DC) magnetic fields into an AC signal prior to quantum lock-in detection, effectively moving the detection frequency away from intrinsic $1/f$ noise.
T₂-Limited Interrogation: This technique overcomes the limitation of the conventional Ramsey method, extending the sensor interrogation time from the short dephasing time ($T_2^* \approx 1.16 \text{ µs}$) up to the intrinsic coherence time ($T_2 \sim 60 \text{ µs}$ in pulsed sequences, $> 350 \text{ µs}$ in spin-lock).
Achieved Sensitivity: Demonstrated DC field sensitivity of $\approx 6.02 \text{ µT}/\sqrt{\text{Hz}}$, comparable to Ramsey methods, with projections indicating potential improvement to $\approx 3.01 \text{ µT}/\sqrt{\text{Hz}}$ using Charge State Readout (CSR).
Dual Noise Suppression: The protocol achieves superior noise rejection through effective low-pass filtering of signal noise (cutoff $< 64 \text{ kHz}$) and band-pass filtering of sensor noise, enhancing overall magnetometer robustness.
Material Requirement: Requires high-purity, isotopically engineered Single Crystal Diamond (SCD) wafers (99.99% $^{12}\text{C}$) to minimize environmental noise and maximize $T_2$.
Application: Ushers in a compelling technique for sensitive, full vector DC magnetometry at the nanoscale.

Technical Specifications

Parameter	Value	Unit	Context
Achieved DC Sensitivity (Pulsed)	6.02	µT/√Hz	Ancilla-assisted protocol
Projected DC Sensitivity (with CSR)	3.01	µT/√Hz	Using Charge State Readout (CSR)
Maximum Coherence Time ($T_2$)	350	µs	Achieved using Spin-Lock technique
Sensor Dephasing Time ($T_2^*$)	1.16	µs	Conventional Ramsey limit
Noise Filter Cutoff (Ancilla-Assisted)	63.4	kHz	Low-pass filter (XY8-13 sequence)
Noise Filter Cutoff (Ramsey, Comparison)	869.6	kHz	Conventional filter bandwidth
GSLAC Operating Field ($B_z$)	1025	G	Magnetic field required near Ground State Level Anti-Crossing
Electron Gyromagnetic Ratio ($\gamma_e$)	2.8	MHz/G	NV center property
Diamond Isotopic Purity	99.99	%	Carbon-12 enrichment (required for long $T_2$)
MW Pulse Rate	1.25	GS/s	Arbitrary Waveform Generator (AWG) specification
$\pi$-Pulse Length	50	ns	Required MW pulse fidelity

Key Methodologies

The core of the experiment relies on combining advanced quantum control sequences with precise magnetic field engineering and high-quality CVD diamond.

Material Selection and Preparation:
- Used isotopically pure Single Crystal Diamond (SCD, 99.99% $\text{C}^{12}$) containing implanted NV centers. This isotopic purity is critical for minimizing spin bath noise and achieving long intrinsic $T_2$ coherence times.
Magnetic Field Control (GSLAC Alignment):
- A composite permanent magnet assembly, managed by four motorized actuators, was used for precise 3D translation and rotation.
- The magnetic field ($B_z \approx 1025 \text{ G}$) was aligned to the NV axis to operate near the Ground State Level Anti-Crossing (GSLAC) for maximized sensitivity and controlled level mixing.
Frequency Upconversion:
- The DC field ($B_{\perp}$) is coupled to the native ${}^{14}\text{N}$ nuclear spin ancilla, which acts as a free-running oscillator, up-converting the DC signal to the nuclear spin resonance frequency ($\omega_0$).
Quantum Lock-in Detection:
- The up-converted AC signal is measured using phase-sensitive detection achieved via Dynamical Decoupling (DD) sequences, specifically CPMG/XY8 protocols, where the pulse spacing ($\tau$) is matched to the effective AC field period ($\tau = \pi / \omega_0$).
Microwave (MW) Delivery and Control:
- MW pulses (e.g., 50 ns $\pi$-pulse) were generated using a 1.25 GS/s arbitrary waveform generator (AWG) and delivered via a $25 \text{ µm}$ $\text{Cu}$ wire terminated in a $50 \text{ Ω}$ load, demonstrating the need for nanoscale integration of antennas.
Readout:
- NV spin state was measured optically via fluorescence quenching using a home-built confocal microscope and a single-photon counting module (SPCM).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced diamond materials and fabrication services required to replicate, optimize, and scale this groundbreaking $T_2$-limited magnetometry research.

Applicable Materials: Quantum Grade Diamond for Enhanced $T_2$

This research confirms that maximizing DC magnetometry performance fundamentally relies on diamond material quality, specifically isotopic purity and low strain, to achieve long coherence times.

Material	Description	6CCVD Capability Match	Research Requirement
High-Purity Quantum Grade SCD	Single Crystal Diamond grown with ultra-low nitrogen incorporation ($< 5 \text{ ppb N}$).	Essential for minimizing background defects and achieving long electron $T_2$.	The foundation for high-fidelity NV spin qubits.
Isotopically Engineered SCD	SCD wafers with high enrichment (e.g., $\text{C}^{12} > 99.99%$).	Directly matches the material used in the paper, ensuring minimal spin bath noise from residual $\text{C}^{13}$ nuclei.	Required to push interrogation time from $T_2^*$ to $T_2$.
Heavy Boron-Doped Diamond (BDD)	Polycrystalline or Single Crystal BDD.	Excellent for electrochemical sensing applications or robust, large-area ensemble magnetometers, offering increased charge carrier density.	Offers an alternative path for high-sensitivity, high-dynamic-range sensing, potentially simplifying the required GSLAC alignment apparatus.

Customization Potential: Meeting Fabrication and Integration Needs

The experimental setup required specialized integration of MW delivery structures and highly precise sensor dimensions, aligning perfectly with 6CCVD’s fabrication expertise:

Custom Dimensions: 6CCVD can supply SCD substrates in custom laser-cut dimensions (up to $125 \text{ mm}$ plates) and thicknesses (0.1 µm to 500 µm) necessary for integration into complex, high-precision magnetic field apparatus (e.g., customized chips for GSLAC alignment stages).
Integrated Metalization: The successful delivery of MW pulses relies on precise antenna structures. 6CCVD offers in-house deposition of standard metal stacks, including Ti/Pt/Au, W, or Cu, directly patterned onto the diamond surface for high-frequency MW control and signal routing.
Ultra-Low Roughness Polishing: For integration of solid-immersion lenses or enhanced optical coupling (critical for single NV center readout), 6CCVD guarantees surface roughness of $Ra < 1 \text{ nm}$ on SCD wafers.

Engineering Support

Achieving $T_2$-limited metrology requires controlling diamond growth and processing parameters that dictate the NV environment. Our in-house PhD team specializes in optimizing MPCVD growth recipes to control nitrogen concentration, strain, and crystal orientation. We offer dedicated engineering consultation to researchers seeking to:

Optimize SCD substrate orientation for specific magnetic field alignment protocols.
Determine optimal isotopic purity levels for target $T_2$ coherence times in [Quantum Sensing] projects.
Design and fabricate custom metalization patterns suitable for high-speed dynamic decoupling sequences like the XY8 protocol used in this study.

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

View Original Abstract

Sensing static or slowly varying magnetic fields with high sensitivity and spatial resolution is critical to many applications in fundamental physics, bioimaging and materials science. Several versatile magnetometry platforms have emerged over the past decade, such as electronic spins associated with Nitrogen Vacancy (NV) centers in diamond. However, their high sensitivity to external fields also makes them poor sensors of DC fields. Indeed, the usual method of Ramsey magnetometry leaves them prone to environmental noise, limiting the allowable interrogation time to the short dephasing time T2*. Here we introduce a hybridized magnetometery platform, consisting of a sensor and ancilla, that allows sensing static magnetic fields with interrogation times up to the much longer T2 coherence time, allowing significant potential gains in field sensitivity. While more generally applicable, we demonstrate the method for an electronic NV sensor and a nuclear ancilla. It relies on frequency upconversion of transverse DC fields through the ancilla, allowing quantum lock-in detection with low-frequency noise rejection. In our experiments, we demonstrate sensitivities better than 6uT/vHz, comparable to the Ramsey method, and narrow-band signal noise filtering better than 64kHz. With technical optimization, we expect more than an one order of magnitude improvement in each of these parameters. Since our method measures transverse fields, in combination with the Ramsey detection of longitudinal fields, it ushers in a compelling technique for sensitive vector DC magnetometry at the nanoscale.

Tech Support

Original Source

DOI: https://doi.org/10.48550/arxiv.1611.04691