Low-Frequency Quantum Sensing

At a Glance

Metadata	Details
Publication Date	2022-09-22
Journal	Physical Review Applied
Authors	Ernst David Herbschleb, Izuru Ohki, Kohki Morita, Y. Yoshii, Hiromitsu Kato
Institutions	Kanazawa University, Kyoto University Institute for Chemical Research
Citations	11
Analysis	Full AI Review Included

Technical Documentation & Analysis: Low-Frequency Quantum Sensing via MPCVD Diamond

Executive Summary

This research demonstrates a significant breakthrough in coherent quantum sensing, utilizing single Nitrogen-Vacancy (N-V) centers in high-purity Single Crystal Diamond (SCD) to measure low-frequency AC magnetic fields with frequency-independent sensitivity.

Breakthrough Method: A novel fitting-based algorithm, termed “QScope,” applied to Free-Induced Decay (FID) sequences, enables coherent sensing below the typical 1 kHz limit imposed by N-V center coherence times (T₂).
Achieved Sensitivity: A frequency-independent sensitivity of 9.4 nT Hz^-0.5 was achieved for AC fields ranging from 1 Hz up to 0.6 kHz.
Material Requirement: The success relies critically on high-quality, epitaxially grown N-type SCD with ultra-high ¹²C enrichment (99.998%) to maximize the spin coherence time (T₂ ≈ 1.05 ms).
Low-Field Capability: The technique was successfully demonstrated at ultralow background fields (as low as 30 nT) and is theoretically extendable to true zero-field operation.
Application Validation: The method was validated by measuring synchronized low-frequency Nuclear Magnetic Resonance (NMR) signals from deionized water and ethanol, achieving high spectral resolution (Hz-order line widths).
Commercial Relevance: This technique is highly promising for nanoscale, highly sensitive low-frequency applications, including low-field NMR, magnetic resonance imaging, and integrated circuit diagnostics.

Technical Specifications

Parameter	Value	Unit	Context
AC Field Sensitivity (Low-Frequency)	9.4 ± 0.1	nT Hz^-0.5	Frequency-independent regime (1 Hz to 0.6 kHz)
Threshold Frequency (f_threshold)	0.36	kHz	Frequency limit for constant sensitivity
Lowest Measured Frequency	1	Hz	Demonstrated AC field sensing
Background Field (Ultralow Test)	30	nT	Field offset for low-field measurement
Apparent Coherence Time (T₂)	1.05 ± 0.05	ms	Measured single N-V center property
Optimal Time Delay (τ)	0.4	ms	Fixed delay between π/2 pulses
Diamond Growth Method	MPCVD	N/A	Microwave Plasma-Assisted Chemical Vapor Deposition
Diamond Orientation	(111)	N/A	Substrate orientation used
¹²C Isotopic Enrichment	99.998	%	Used to achieve long T₂
Phosphorus (N-type) Concentration	5 x 10¹⁶	atoms cm^-3	Doping level for N-V creation
Water NMR Line Width	1.6	Hz	Measured via synchronized sensing

Key Methodologies

The experiment relies on precise material engineering via MPCVD and advanced pulsed quantum control sequences:

Material Synthesis: N-type diamond was epitaxially grown onto Ib-type (111)-oriented diamond substrates using Microwave Plasma-Assisted Chemical Vapor Deposition (MPCVD).
Isotopic Engineering: High isotopic purity diamond (99.998% ¹²C) was used to suppress decoherence caused by nuclear spins, enabling the long T₂ times necessary for low-frequency detection.
Quantum Sequence (FID): The core measurement utilized the Free-Induced Decay (FID) sequence (π/2 - τ - π/2 pulses) to measure the phase accumulation of the N-V electron spin.
QScope Operation: The fixed-delay FID subsequence was repeated many times within the period of the low-frequency AC signal, effectively acting as a quantum oscilloscope for signal averaging.
Time-Domain Fitting: The AC field amplitude (B_ac) was retrieved by fitting the integrated readout signal directly in the time domain, which accurately accounts for the field shape during the subsequence delay.
Low-Field Cancellation: External magnetic fields were actively canceled in the z-direction to below 1 nT to investigate sensing performance near zero field.
Synchronized Sensing: For NMR applications, the N-V center readout was synchronized with the transient free nuclear precession signal following an RF excitation pulse on the sample (water/ethanol).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the specialized MPCVD diamond materials and customization services required to replicate, optimize, and scale this groundbreaking low-frequency quantum sensing research.

Applicable Materials

To achieve the demonstrated sensitivity and long coherence times, researchers require high-quality, isotopically pure Single Crystal Diamond (SCD).

Optical Grade SCD (High Purity): Essential for replicating the long T₂ times (1.05 ms) achieved in the study. 6CCVD provides SCD with ultra-high ¹²C enrichment (> 99.998%) to minimize nuclear spin noise and maximize coherence.
Custom Doped SCD (N-type): We offer precise control over N-type doping (Phosphorus or Nitrogen) during MPCVD growth, allowing researchers to target the specific 5 x 10¹⁶ atoms cm^-3 concentration used for optimal N-V center density and spin properties.
Specific Orientation: 6CCVD routinely supplies (111)-oriented SCD substrates up to 10mm thick, which is critical for maximizing the contrast and alignment of the N-V centers relative to the applied magnetic fields.

Customization Potential

6CCVD’s advanced fabrication capabilities directly address the integration challenges inherent in quantum sensing experiments.

Capability	Relevance to Low-Frequency Sensing	6CCVD Specification
Custom Dimensions	Scaling up ensemble measurements or integrating into microfluidic/NMR systems.	Plates/wafers up to 125mm (PCD); SCD substrates up to 10mm thickness.
Surface Polishing	Minimizing surface defects and noise for near-surface N-V centers (relevant for nanoscale NMR).	SCD polishing to Ra < 1 nm.
Custom Metalization	Integrating microwave delivery structures (e.g., the thin copper wire used in the experiment) directly onto the diamond surface.	In-house deposition of Au, Pt, Pd, Ti, W, Cu for custom antenna designs.
Laser Cutting/Shaping	Creating specific geometries for flux concentrators or integrated coils (as mentioned in related literature).	Precision laser cutting and shaping services available.

Engineering Support

6CCVD’s in-house PhD team provides authoritative professional support for advanced quantum projects:

Material Optimization: We assist researchers in optimizing diamond growth recipes (doping, isotopic purity, orientation) specifically for low-field NMR and high-sensitivity magnetometry applications.
Decoherence Mitigation: Consultation on material selection to mitigate environmental effects that lead to the decay of apparent T₂ during long measurement times.
Global Logistics: We ensure reliable, secure global shipping (DDU default, DDP available) of sensitive, high-value quantum materials.

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

View Original Abstract

Exquisite sensitivities are a prominent advantage of quantum sensors. Ramsey sequences allow precise measurement of direct current fields, while Hahn-echo-like sequences measure alternating current fields. However, the latter are restrained for use with high-frequency fields (above approximately $1$ kHz) due to finite coherence times, leaving less-sensitive noncoherent methods for the low-frequency range. In this paper, we propose to bridge the gap with a fitting-based algorithm with a frequency-independent sensitivity to coherently measure low-frequency fields. As the algorithm benefits from coherence-based measurements, its demonstration with a single nitrogen-vacancy center gives a sensitivity of $9.4$ nT Hz$^{-0.5}$ for frequencies below about $0.6$ kHz down to near-constant fields. To inspect the potential in various scenarios, we apply the algorithm at a background field of tens of nTs, and we measure low-frequency signals via synchronization.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.18.034058