Frequency Limits of Sequential Readout for Sensing AC Magnetic Fields Using Nitrogen-Vacancy Centers in Diamond

At a Glance

Metadata	Details
Publication Date	2023-08-31
Journal	Sensors
Authors	Santosh Ghimire, Seong-Joo Lee, Sangwon Oh, Jeong Hyun Shim
Institutions	Korea Research Institute of Standards and Science, Korea University of Science and Technology
Citations	4
Analysis	Full AI Review Included

Executive Summary

This study experimentally determined the frequency limits and optimal performance of AC magnetic field sensing using Nitrogen-Vacancy (NV) centers in diamond via the Sequential Readout (SR) method.

Peak Sensitivity: A maximum AC magnetic field sensitivity of 229 pT/√Hz was achieved at a near-optimal sensing frequency of 1 MHz.
Protocol: The sensing utilized the Sequential Readout (SR) scheme combined with the XY4-(4) Dynamical Decoupling (DD) sequence.
High-Frequency Limit: The upper frequency limit for sensing is clearly determined by the Rabi frequency (approx. 6.3 MHz), which is constrained by the finite width of the pi pulses (80 ns).
Low-Frequency Constraint: The lower frequency limit (observed at ~200 kHz) is primarily governed by the duration of the optical repolarization time (T_Laser) relative to the spin polarization time (T_p = 15 µs).
T₂ Limit Overridden: This T_Laser constraint overrides the intrinsic T₂ decoherence limit (calculated at 82 kHz), demonstrating that optical repolarization is the dominant limiting factor for NV ensembles at low frequencies when the dwell time (T_SR = 50 µs) is fixed.
Model Validation: A theoretical model incorporating both finite pi-pulse width and the optical repolarization efficiency successfully describes the observed frequency dependence of the sensitivity.

Technical Specifications

Parameter	Value	Unit	Context
Maximum AC Sensitivity	229	pT/√Hz	Achieved at 1 MHz using SR method.
Optimal Sensing Frequency	1	MHz	Frequency yielding maximum sensitivity.
High-Frequency Limit (Rabi)	6.3	MHz	Determined by finite pi-pulse width.
Observed Low-Frequency Limit	200	kHz	Governed by optical repolarization time (T_Laser).
T₂ Decoherence Limit (Theoretical)	82	kHz	Limit if T_Laser constraint were removed.
DD Sequence Used	XY4-(4)	N/A	Dynamical Decoupling sequence.
Pi-Pulse Width (τ_π)	80	ns	Used in the DD sequence.
Sequential Readout Dwell Time (T_SR)	50	µs	Fixed repetition time for constant bandwidth.
NV Spin Polarization Time (T_p)	15	µs	Estimated from fluorescence fitting.
Applied AC Field Intensity (Calibration)	1.9	µT	Used at 1 MHz for calibration.
Static Magnetic Field (B₀)	5.4	mT	Aligned along one NV orientation.
Laser Wavelength	532	nm	Continuous-wave excitation source.
Laser Power Density	45	W/mm²	Used for optical readout/repolarization.
Diamond Nitrogen Doping ([¹⁴N])	~10	ppm	Thin CVD layer.
Diamond Thickness	40	µm	CVD layer thickness.
Electron Irradiation Dose	1 x 10¹⁹	/cm²	1 MeV energy.
Annealing Temperature 1	800	°C	4 hours in vacuum.
Annealing Temperature 2	1000	°C	2 hours in vacuum.

Key Methodologies

The experiment relied on precise control of NV spin states using microwave pulses and optical readout, integrated into the Sequential Readout (SR) protocol.

Diamond Sensor Fabrication: A 40 µm thick, ¹⁴N-doped CVD diamond layer was created. NV centers were formed by 1 MeV electron irradiation (1 x 10¹⁹ /cm²) followed by sequential vacuum annealing at 800 °C (4 h) and 1000 °C (2 h).
Magnetic Alignment: A static magnetic field (5.4 mT) was applied to align the NV centers along a single crystallographic orientation, enabling spin manipulation.
Microwave Pulse Generation: Dynamical Decoupling (DD) sequences (specifically XY4-(4)) were stored in an Arbitrary Waveform Generator (AWG). The microwave signal was mixed with a signal generator output, amplified (100 W), and delivered to the diamond via a 2 mm loop antenna.
Sequential Readout (SR) Implementation: The SR protocol utilized a fixed dwell time (T_SR = 50 µs). Each block consisted of the DD sequence (interrogation time T_sens) and optical readout/repolarization (T_Laser).
Frequency Tuning: To measure sensitivity across the frequency spectrum, the interval (τ) between DD pulses was adjusted for each AC frequency (f_ac) to maintain the commensurability condition (β = pi/2), ensuring optimal coupling to the AC field.
Calibration and Sensitivity Measurement: The intensity of the applied AC field was calibrated at 1 MHz. Sensitivity was determined by measuring the Signal-to-Noise Ratio (SNR) of the down-converted signal (S_sens(t)) after Fourier transformation, yielding the sensitivity η = δB/√Hz.
Limit Analysis: The high-frequency limit was analyzed by observing the signal decay as the interrogation window closed due to the finite pi-pulse width (Rabi frequency limit). The low-frequency limit was analyzed by modeling the reduction in optical signal intensity (S) as T_Laser decreased with increasing T_sens (lower f_ac).

Commercial Applications

The demonstrated high-sensitivity, broadband AC magnetometry using NV ensembles is crucial for applications requiring high spatial resolution in the kilohertz to megahertz range.

Quantum Sensing and Metrology: Development of next-generation solid-state quantum sensors for precise magnetic field measurements over a wide frequency range.
Microscale Nuclear Magnetic Resonance (NMR): Enabling high-resolution NMR spectroscopy on micron-scale liquid samples, particularly where thermal Boltzmann polarization is the primary signal source.
Magnetic Induction Tomography (MIT): Facilitating high spatial resolution imaging in MIT, potentially improving medical or non-destructive testing diagnostics.
Magnetic Communications: Utilizing the wide bandwidth of NV sensors for high-fidelity reception of rapidly varying or modulated AC magnetic signals.
Microstructure Diagnostics: Applications in imaging local diffusion or analyzing time-varying magnetic phenomena within microstructures and materials.

View Original Abstract

The nitrogen-vacancy (NV) centers in diamond have the ability to sense alternating-current (AC) magnetic fields with high spatial resolution. However, the frequency range of AC sensing protocols based on dynamical decoupling (DD) sequences has not been thoroughly explored experimentally. In this work, we aimed to determine the sensitivity of the ac magnetic field as a function of frequency using the sequential readout method. The upper limit at high frequency is clearly determined by Rabi frequency, in line with the expected effect of finite DD-pulse width. In contrast, the lower frequency limit is primarily governed by the duration of optical repolarization rather than the decoherence time (T2) of NV spins. This becomes particularly crucial when the repetition (dwell) time of the sequential readout is fixed to maintain the acquisition bandwidth. The equation we provide successfully describes the tendency in the frequency dependence. In addition, at the near-optimal frequency of 1 MHz, we reached a maximum sensitivity of 229 pT/Hz by employing the XY4-(4) DD sequence.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s23177566

References

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