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arXiv:1805.10725 · Review of Scientific Instruments

Extending coherence time of macro-scale diamond magnetometer by dynamical decoupling with coplanar waveguide resonator

Y. MASUYAMA, Kosuke Mizuno, Hayato Ozawa, Hitoshi Ishiwata, Yuji Hatano, Takeshi Ohshima, Takayuki Iwasaki, Mutsuko Hatano

Published Dec 1, 2018• 34 citations
arXivHTMLPDF

Abstract

Ultimate sensitivity for quantum magnetometry using nitrogen-vacancy (NV) centers in a diamond is limited by a number of NV centers and coherence time. Microwave irradiation with a high and homogeneous power density for a large detection volume is necessary to achieve a highly sensitive magnetometer. Here, we demonstrate a microwave resonator to enhance the power density of the microwave field and an optical system with a detection volume of 1.4 × 10−3 mm3. The strong microwave field enables us to achieve 48 ns Rabi oscillation which is sufficiently faster than the phase relaxation time of NV centers. This system combined with a decoupling pulse sequence, XY16, extends the spin coherence time (T2) up to 27 times longer than that with a spin echo method. Consequently, we obtained an AC magnetic field sensitivity of 10.8 pt/Hz using the dynamical decoupling pulse sequence.

DOI
10.1063/1.5047078

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quant-pharXiv:2610.13379•Oct 12, 2026
Ultimate sensitivity for quantum magnetometry using nitrogen-vacancy (NV) centers in a diamond is limited by a number of NV centers and coherence time. Microwave irradiation with a high and homogeneous power density for a large detection volume is necessary to achieve a highly sensitive magnetometer. Here, we demonstrate a microwave resonator to enhance the power density of the microwave field and an optical system with a detection volume of 1.4 × 10−3 mm3. The strong microwave field enables us to achieve 48 ns Rabi oscillation which is sufficiently faster than the phase relaxation time of NV centers. This system combined with a decoupling pulse sequence, XY16, extends the spin coherence time (T2) up to 27 times longer than that with a spin echo method. Consequently, we obtained an AC magnetic field sensitivity of 10.8 pt/Hz using the dynamical decoupling pulse sequence.
Diamond-based quantum magnetometers are more sensitive to oscillating (ac) magnetic fields than static (dc) fields because the crystal impurity-induced ensemble dephasing time T2∗{T}_{2}^{*}T2∗​, the relevant sensing time for a dc field, is much shorter than the spin coherence time T2{T}_{2}T2​, which determines the sensitivity to ac fields. Here we demonstrate measurement of dc magnetic fields using a physically rotating ensemble of nitrogen-vacancy centers at a precision ultimately limited by T2{T}_{2}T2​ rather than T2∗{T}_{2}^{*}T2∗​. The rotation period of the diamond is comparable to T2{T}_{2}T2​ and the angle between the nitrogen-vacancy (NV) axis and the target magnetic field changes as a function of time, thus upconverting the static magnetic field to an oscillating field in the physically rotating frame. Using spin-echo interferometry of the rotating NV centers, we are able to perform measurements for over 100 times longer compared to a conventional Ramsey experiment. With modifications our scheme could realize dc sensitivities equivalent to demonstrated NV center ac magnetic field sensitivities of order $0.1\phantom{\rule{0.28em}{0ex}}\mathrm{nT}\phantom{\rule{0.16em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}$.
Microwave magnetometry is essential for the advancement of microwave technologies. We demonstrate a broadband microwave sensing protocol using the AC Zeeman effect with ensemble nitrogen-vacancy (NV) centers in diamond. A widefield microscope can visualize the frequency characteristics of the microwave resonator and the spatial distribution of off-resonant microwave amplitude. Furthermore, by combining this method with dynamical decoupling, we achieve the microwave amplitude sensitivity of 5.2 μT/Hz, which is 7.7 times better than 40.2 μT/Hz obtained using the protocol in previous research over a sensing volume of 2.77 μm×2.77 μm×30 nm. Our achievement is a concrete step in adapting ensemble NV centers for wideband and widefield microwave imaging.
Nitrogen-vacancy (NV) color centers in diamond are promising solid-state quantum sensors working at room temperature. The number of NV centers limits the magnetic sensitivity of the quantum magnetometer. Increasing the number of NV centers is a challenging task due to technical issues such as microwave power density, detection noise.Here, we developed the sensor system with a large detection volume which contains a lot of NV centers. The microwave circuit enhances microwave power density. Digital filter with frequency modulation of microwave allows us to detect the magnetic field with low noise. Consequently, we obtained a DC magnetic field sensitivity of 980 pT/√Hz.This work was supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118067395.
We demonstrate significant improvements of the spin coherence time of a dense ensemble of nitrogen-vacancy (NV) centers in diamond through optimized dynamical decoupling (DD). Cooling the sample down to 77 K suppresses longitudinal spin relaxation T1{T}_{1}T1​ effects and DD microwave pulses are used to increase the transverse coherence time T2{T}_{2}T2​ from $\ensuremath{\sim}0.7\phantom{\rule{0.28em}{0ex}}\mathrm{ms}$ up to $\ensuremath{\sim}30\phantom{\rule{0.28em}{0ex}}\mathrm{ms}$. We extend previous work of single-axis (Carr-Purcell-Meiboom-Gill) DD towards the preservation of arbitrary spin states. Following a theoretical and experimental characterization of pulse and detuning errors, we compare the performance of various DD protocols. We identify that the optimal control scheme for preserving an arbitrary spin state is a recursive protocol, the concatenated version of the XY8 pulse sequence. The improved spin coherence might have an immediate impact on improvements of the sensitivities of ac magnetometry. Moreover, the protocol can be used on denser diamond samples to increase coherence times up to NV-NV interaction time scales, a major step towards the creation of quantum collective NV spin states.
Portable quantum sensors are crucial for developing practical quantum sensing and metrology applications. Fiberized nitrogen-vacancy (NV) centers in diamonds have emerged as one of the most promising candidates for compact quantum sensors. Nevertheless, due to the difficulty of coherently controlling the ensemble spin and noise suppression in a large volume, it often faces problems such as reduced sensitivity and narrowed bandwidth in integrated lensless applications. Here, we propose a fluorescence signal treatment method for NV spin ensemble manipulation by the exponential fitting of spin polarization processes, instead of integrating the photon emission. This enables spin state readout with a high signal-to-noise ratio and applies to the pulse sensing protocols for large-volume NV spins. Based on this, we further developed a fiberized diamond-based AC magnetometer. With an XY8-N dynamical decoupling pulse sequence, we demonstrated a T2-limited sensitivity of 8 pT/Hz and T1-limited frequency resolution of 90 Hz over a wide frequency band from 100 kHz to 3 MHz. This integrated diamond sensor leverages quantum coherence to achieve enhanced sensitivity in detecting AC magnetic fields, making it suitable for implementation in a compact and portable endoscopic sensor.
The nitrogen-vacancy (NV) color center in diamond has found applications in magnetometry across various geometrical scales. In particular, vector magnetometry using hand-held devices with low power consumption is of interest for mobile and geological applications. However, such a device requires efficient driving of the four different orientations of the NV center, thus placing demands on a broadband microwave resonator that provides a uniform and highly insensitive microwave magnetic field at the resonance frequencies of the NV centers. Here, we propose a unique planar resonator design that enables efficient driving of all four orientations of NV color centers over a 100 MHz range with only 0.5 W of microwave input power and 250 mW of laser power, allowing vector magnetic field measurements. Using the planar resonator to drive NV centers in diamond, a compact vector magnetometer was fabricated and experimentally studied. The sensitivity of the device is about 1.2 nT/Hz  {1.2\text{ nT}}/{\sqrt{\text{Hz}}}\;1.2 nT/Hz​. While this is not a record sensitivity for diamond-based magnetometers, this device occupies its own niche as a compact hand-held magnetometer.