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arXiv:2309.11130 · IEEE Sensors Journal

Modified Split-Ring Resonators for Efficient and Homogeneous Microwave Control of Large Volume Spin Ensembles

Yachel Ben-Shalom, Amir Hen, Nir Bar‐Gill

Published May 20, 2024• 2 citations
arXivHTMLPDF

Abstract

Quantum sensing using local defects in solid-state systems has gained significant attention over the past several years, with impressive results demonstrated both in academia and in industry. Specifically, using large volume and high-density ensembles for beyond state-of-the-art sensitives is of clear interest. A major obstacle for achieving such record sensitivities is associated with the need to realize strong, homogeneous driving of the sensor defects. Here, we focus on high-frequency microwave sensing using nitrogen-vacancy (NV) centers in diamond and develop a modified split-ring resonator design to address this issue. We demonstrate enhanced drive strengths and homogeneities over large volumes compared with previous results, with prospects for enabling the desired sensitivities. We reach Rabi frequencies of up to 18-MHz with an efficiency ratio of$2~\text {Gauss}/\sqrt {\text {Watt}}$, along with an inhomogeneity of < 0.7% in a volume of 0.1 mm3. This structure also has a narrow form factor that allows for efficient optical coupling and fluorescence collection.

DOI
10.1109/jsen.2024.3401049

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Quantum sensing using local defects in solid-state systems has gained significant attention over the past several years, with impressive results demonstrated both in academia and in industry. Specifically, using large volume and high-density ensembles for beyond state-of-the-art sensitives is of clear interest. A major obstacle for achieving such record sensitivities is associated with the need to realize strong, homogeneous driving of the sensor defects. Here, we focus on high-frequency microwave sensing using nitrogen-vacancy (NV) centers in diamond and develop a modified split-ring resonator design to address this issue. We demonstrate enhanced drive strengths and homogeneities over large volumes compared with previous results, with prospects for enabling the desired sensitivities. We reach Rabi frequencies of up to 18-MHz with an efficiency ratio of2 Gauss/Watt2~\text {Gauss}/\sqrt {\text {Watt}}2 Gauss/Watt​, along with an inhomogeneity of < 0.7% in a volume of 0.1 mm3. This structure also has a narrow form factor that allows for efficient optical coupling and fluorescence collection.
In this work, we designed, simulated, and tested a complementary split ring resonator (CSRR) for the purpose of applying a strong and uniform microwave field for the manipulation of nitrogen vacancy (NV) ensembles. This structure was fabricated by etching two concentric rings on a flat metal film that was deposited on a printed circuit board. A metal transmission on the back plane was used as the feed line. The fluorescence collection efficiency was improved by about 2.5 times with the CSRR structure compared to that without CSRR. Furthermore, the maximum Rabi frequency could reach 11.3 MHz, and the Rabi frequency variation was smaller than 2.8% in an area of 250 × 75 μm. This could pave the way to achieving high-efficiency control of the quantum state for spin-based sensor applications.
Developing high performance portable sensors is of significant importance for both terrestrial and satellite communications, with quantum sensors offering potential advantages in sensitivity and bandwidth. The nitrogen vacancy center in diamond is a popular quantum sensing platform for its relative simplicity and wide range of acceptable operating conditions, but suffers a limited instantaneous bandwidth due to the narrow linewidth of the spin resonance. In this work, we first present a magnetic quantum sensing scheme utilizing heterodyne mixing with strong optical and microwave driving, achieving MHz-scale instantaneous bandwidth with a sensitivity of 548  pT/Hz548 \; \text{pT/}\sqrt{\text{Hz}}548pT/Hz​ in an active diamond volume of $ 2.5\times 10^{-4} \; \text{mm}^3$ with 2×10112 \times 10^{11}2×1011 NV centers. We then demonstrate expansion of the sensing bandwidth by employing a strong magnetic gradient and many individually modulated microwave control fields, expanding the single resonance bandwidth to 48 MHz with a peak sensitivity of 8.5  nT/Hz8.5 \; \text{nT/}\sqrt{\text{Hz}}8.5nT/Hz​.
For quantum sensing based on nitrogen-vacancies (NV) ensembles, microwave antennas can couple the microwave field to the NV center, which leads it to becoming the core of spin manipulation and can directly affect the sensitivity of quantum sensing. The double split-ring resonator is a widely used microwave device for NV ensembles due to the advantages of high radiation efficiency and uniform magnetic field in millimeter-scale areas. But the bandwidth (30 MHz) is quite narrow which limits the application in quantum sensing with NV ensembles. Here, we experimentally achieve continuous tuning of the resonant frequency of the double split-ring resonator by changing the copper sheet position on the edge of the outer ring. The frequency tuning range can reach 80 MHz, up to 2-3 times the bandwidth, which can cover the transition of the electron spin under different magnetic field conditions. The performance of the tunable antenna in the quantum operation of NV centers is verified by optically detected magnetic resonance and Rabi oscillation. This tunable antenna is promising in the fabrication of integrated and arrayed quantum sensors based on NV ensembles.
Nitrogen-Vacancy (NV) centers in diamond are attractive tools for sensing and quantum information. Realization of this potential requires effective tools for controlling the spin degree of freedom by microwave (mw) magnetic fields. In this work, we present a planar microwave resonator optimized for microwave-optical double resonance experiments on single NV centers in diamond. It consists of a piece of wide microstrip line, which is symmetrically connected to two 50 Ω microstrip feed lines. In the center of the resonator, an Ω-shaped loop focuses the current and the mw magnetic field. It generates a relatively homogeneous magnetic field over a volume of 0.07 × 0.1 mm3. It can be operated at 2.9 GHz in both transmission and reflection modes with bandwidths of 1000 and 400 MHz, respectively. The high power-to-magnetic field conversion efficiency allows us to produce π-pulses with a duration of 50 ns with only about 200 and 50 mW microwave power in transmission and reflection, respectively. The transmission mode also offers capability for efficient radio frequency excitation. The resonance frequency can be tuned between 1.3 and 6 GHz by adjusting the length of the resonator. This will be useful for experiments on NV-centers at higher external magnetic fields and on different types of optically active spin centers.
The homogeneity of the microwave magnetic field is essential in controlling a large volume of ensemble spins, for example, in the case of sensitive magnetometry with nitrogen-vacancy (NV) centers in diamond. This is particularly important for pulsed measurement, where the fidelity of control pulses plays a crucial role in its sensitivity. So far, several magnetic field-forming systems have been proposed, but no detailed comparison has been made. Here, we numerically study the homogeneity of five different systems, including a planar antenna, a dielectric resonator, a cylindrical inductor, a barrel-shaped coil, and a nested barrel-shaped coil. The results of the simulation allowed us to optimize the design parameters of the barrel-shaped field-forming system, which led to significantly improved magnetic field uniformity. To measure this effect, we experimentally compared the homogeneity of a field-forming system having a barrel shape with that of a planar field-forming system by measuring Rabi oscillations of an ensemble of NV centers with them. Significant improvements in inhomogeneity were confirmed in the barrel-shaped coil.
For quantum sensing based on nitrogen-vacancy (NV) centers in diamond, microwave irradiation with uniform power density and large detection volume is necessary to achieve ultrahigh sensitivity. We demonstrate the fabrication of a double magnetic ring stereoscopic antenna used for the tridimensional uniform control of NV spin ensembles with a 4π mm 2 × 2 mm large volume. Compared with those of traditional microwave antennas (copper wire and single-loop antenna), the sensitivity of the sensor can be improved 4.93 and 9.98 times, and the linearity of the lock-in signal can be increased by 2.04 and 1.11%, respectively. Moreover, the proposed antenna can suppress the thermal noise generated by itself and is amenable to three-dimensional (3D) integration.