Picotesla magnetometry of microwave fields with diamond sensors

Magnetic sensitivity enhancement via polarimetric excitation and detection of an ensemble of NV centers

The negatively charged nitrogen-vacancy center (NV) presents remarkable spin-dependent optical properties that make it an interesting tool for magnetic field sensing. In this paper we exploit the polarization properties of the NV center absorption and emission processes to improve the magnetic sensitivity of an ensemble of NV centers. By simply equipping the experimental set-up of a half-wave plate in the excitation path and a polarizer in the detection path we demonstrate an improvement larger than a factor of two on the NV center magnetic sensitivity.

Long-baseline quantum sensor network as dark matter haloscope

Ultralight dark photons constitute a well-motivated candidate for dark matter. A coherent electromagnetic wave is expected to be induced by dark photons when coupled with Standard-Model photons through kinetic mixing mechanism, and should be spatially correlated within the de Broglie wavelength of dark photons. Here we report the first search for correlated dark-photon signals using a long-baseline network of 15 atomic magnetometers, which are situated in two separated meter-scale shield rooms with a distance of about 1700 km. Both the network's multiple sensors and the shields large size significantly enhance the expected dark-photon electromagnetic signals, and long-baseline measurements confidently reduce many local noise sources. Using this network, we constrain the kinetic mixing coefficient of dark photon dark matter over the mass range 4.1 feV-2.1 peV, which represents the most stringent constraints derived from any terrestrial experiments operating over the aforementioned mass range. Our prospect indicates that future data releases may go beyond the astrophysical constraints from the cosmic microwave background and the plasma heating.

Four-Order Power Reduction in Nanoscale Electron-Nuclear Double Resonance with a Nitrogen-Vacancy Center in Diamonds

Detecting nuclear spins using single nitrogen-vacancy (NV) centers is of particular importance in nanoscale science and engineering but often suffers from the heating effect of microwave fields for spin manipulation, especially under high magnetic fields. Here, we realize an energy-efficient nanoscale nuclear-spin detection using a phase-modulation electron-nuclear double resonance scheme. The microwave field can be reduced to 1/250 of the previous requirements, and the corresponding power is over four orders lower. Meanwhile, the microwave-induced broadening to the line-width of the spectroscopy is significantly canceled, and we achieve a nuclear-spin spectrum with a resolution down to 2.1 kHz under a magnetic field at 1840 Gs. The spectral resolution can be further improved by upgrading the experimental control precision. This scheme can also be used in sensing microwave fields and can be extended to a wide range of applications in the future.

High precision microwave measurement based on nitrogen-vacancy color center and application in velocity detection

Wide-range high-precision velocity detection with nitrogen-vacancy (NV) color center has been realized. By treating the NV color center as a mixer, the high-precision microwave measurement is realized. Through optimization of acquisition time, the microwave frequency resolution is improved to the mHz level. Combined with the frequency-velocity conversion model, velocity detection is realized in the range of 0-100 cm/s, and the velocity resolution is up to 0.012 cm/s. The maximum deviation in repeated measurements does not exceed 1/1000. Finally, combined with the multiplexed microwave reference technique, the range of velocity can be extended to 7.4 × 105 m/s. All of the results provide reference for high-precision velocity detection and play a significant role in various domains of quantum precision measurement. This study provides a crucial technical foundation for the development of high-dynamic-range velocity detectors and novel quantum precision velocity measurement technologies.

Simultaneous detection of multi-channel signals in MHz bandwidth using nitrogen-vacancy centers in a diamond

In this paper, we propose a method for simultaneously recovering multiple radio wave signals based on nitrogen-vacancy (NV) centers in diamond combining optically detected magnetic resonance (ODMR) spectrum. A controlled magnetic field gradient applied to the laser excitation area on the surface of diamond widens the detectable ODMR bandwidth to 200 MHz. Three different frequency-modulated (FM) signals with distinct carrier frequencies falling within the resonance frequency range are received and demodulated in real-time. Subsequently, the FM signal reception capability of this system is further investigated by measuring baseband signal frequencies ranging from 0.1 Hz to 200 Hz and adjusting the carrier power within a dynamic range from -10 dBm to 30 dBm. This proposal, which accomplishes multi-channel demodulation using a compact and single device, has potential applications in fields such as wireless communication, radar and navigation.

Compact multipass-laser-beam antenna for NV sensor sensitivity enhancement

Large-area, highly uniform microwave field radiation and efficient excitation of fluorescence are the key to achieving high sensitivity sensing of the NV (nitrogen-vacancy) magnetometer. In this paper, we report a compact multipass-laser-beam antenna for NV ensemble color centers sensing. The antenna not only provides a tridimensional uniform magnetic field, but also can be used for efficient excitation of the NV fluorescence. The optimal size of the antenna and the angle of laser incidence are determined by the multi-physics field simulation software COMSOL. For an equivalent excitation power, the designed structure increases the path length of the excitation beam by up to three orders of magnitude, up to the level of m, compared to the conventional direct beam mode. Finally, this method increased the sensitivity by a factor of 60 realized a magnetic field sensitivity of 2.8 nT/√Hz in the range of 10-100 Hz. This work provides an experimental method for the design of integrated NV magnetometers.

Nuclear quadrupole resonance spectroscopy with a femtotesla diamond magnetometer

Radio frequency (RF) magnetometers based on nitrogen vacancy centers in diamond are predicted to offer femtotesla sensitivity, but previous experiments were limited to the picotesla level. We demonstrate a femtotesla RF magnetometer using a diamond membrane inserted between ferrite flux concentrators. The device provides ~300-fold amplitude enhancement for RF magnetic fields from 70 kHz to 3.6 MHz, and the sensitivity reaches ~70 fT√s at 0.35 MHz. The sensor detected the 3.6-MHz nuclear quadrupole resonance (NQR) of room-temperature sodium nitrite powder. The sensor's recovery time after an RF pulse is ~35 μs, limited by the excitation coil's ring-down time. The sodium-nitrite NQR frequency shifts with temperature as -1.00±0.02 kHz/K, the magnetization dephasing time is T2*=887±51 μs, and multipulse sequences extend the signal lifetime to 332±23 ms, all consistent with coil-based studies. Our results expand the sensitivity frontier of diamond magnetometers to the femtotesla range, with potential applications in security, medical imaging, and materials science.

Robustness improvement of a nitrogen-vacancy magnetometer by a double driving method

The nitrogen vacancy (NV) color center in diamonds is an electron spin that can measure magnetic fields with high sensitivity and resolution. Furthermore, the robustness of an NV-based quantum system should be improved for further application in other sensing methods and in the exploration of basic physics. In this work, the robustness of an NV magnetometer is improved by the double driving method. The sensitivity of the NV magnetometer was improved 2.1 times by strengthening the pumping power from 100 to 600 mW. In this process, thermal drift was introduced, which affects the measurement accuracy. The temperature drift of a diamond matrix was measured using an infrared camera, and the temperature change of a diamond host drifted to ∼80 K under high laser and microwave power. To address the drift of temperature owing to sensitivity improvement by pumping enhancement, the double driving method was introduced, to suppress the drift of the resonance frequency, to improve the robustness of a continuous-wave NV magnetometer. The magnetic noise density was improved from 10 to 1.2 nT/Hz1/2. This study checked the source of temperature noise in the process of measuring with the NV color centers and proposes a double driving measurement method to track the resonant frequency change due to environmental temperature drift and improve sensitivity. The findings of this study are useful in applying complex pulse protocols in high-level sensing applications based on solid-state spin.

Spin Defects in hBN assisted by Metallic Nanotrenches for Quantum Sensing

The omnipresence of hexagonal boron nitride (hBN) in devices embedding two-dimensional materials has prompted it as the most sought after platform to implement quantum sensing due to its testing while operating capability. The negatively charged boron vacancy (V_B^-) in hBN plays a prominent role, as it can be easily generated while its spin population can be initialized and read out by optical means at room-temperature. But the lower quantum yield hinders its widespread use as an integrated quantum sensor. Here, we demonstrate an emission enhancement amounting to 400 by nanotrench arrays compatible with coplanar waveguide (CPW) electrodes employed for spin-state detection. By monitoring the reflectance spectrum of the resonators as additional layers of hBN are transferred, we have optimized the overall hBN/nanotrench optical response, maximizing thereby the luminescence enhancement. Based on these finely tuned heterostructures, we achieved an enhanced DC magnetic field sensitivity as high as 6 × 10^-5 T/Hz^1/2.

Quantum sensing of radio-frequency signal with NV centers in SiC

Silicon carbide is an emerging platform for quantum technologies that provides wafer scale and low-cost industrial fabrication. The material also hosts high-quality defects with long coherence times that can be used for quantum computation and sensing applications. Using an ensemble of nitrogen-vacancy centers and an XY8-2 correlation spectroscopy approach, we demonstrate a room-temperature quantum sensing of an artificial AC field centered at ~900 kHz with a spectral resolution of 10 kHz. Implementing the synchronized readout technique, we further extend the frequency resolution of our sensor to 0.01 kHz. These results pave the first steps for silicon carbide quantum sensors toward low-cost nuclear magnetic resonance spectrometers with a wide range of practical applications in medical, chemical, and biological analysis.

CSRR Structure Design for NV Spin Manipulation with Microwave Strength and Fluorescence Collection Synchronous Enhancement

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.

Bayesian-Based Hybrid Method for Rapid Optimization of NV Center Sensors

NV centers are among the most promising platforms in the field of quantum sensing. Magnetometry based on NV centers, especially, has achieved concrete development in areas of biomedicine and medical diagnostics. Improving the sensitivity of NV center sensors under wide inhomogeneous broadening and fieldamplitude drift is a crucial issue of continuous concern that relies on the coherent control of NV centers with high average fidelity. Quantum optimal control (QOC) methods provide access to this target; nevertheless, the high time consumption of current methods due to the large number of needful sample points as well as the complexity of the parameter space has hindered their usability. In this paper, we propose the Bayesian estimation phase-modulated (B-PM) method to tackle this problem. In the case of the state transforming of an NV center ensemble, the B-PM method reduced the time consumption by more than 90% compared with the conventional standard Fourier basis (SFB) method while increasing the average fidelity from 0.894 to 0.905. In the AC magnetometry scenario, the optimized control pulse obtained with the B-PM method achieved an eight-fold extension of coherence time T2 compared with the rectangular π pulse. Similar application can be made in other sensing situations. As a general algorithm, the B-PM method can be further extended to the open- and closed-loop optimization of complex systems based on a variety of quantum platforms.

Quantum enhanced radio detection and ranging with solid spins

The accurate radio frequency (RF) ranging and localizing of objects has benefited the researches including autonomous driving, the Internet of Things, and manufacturing. Quantum receivers have been proposed to detect the radio signal with ability that can outperform conventional measurement. As one of the most promising candidates, solid spin shows superior robustness, high spatial resolution and miniaturization. However, challenges arise from the moderate response to a high frequency RF signal. Here, by exploiting the coherent interaction between quantum sensor and RF field, we demonstrate quantum enhanced radio detection and ranging. The RF magnetic sensitivity is improved by three orders to 21 [Formula: see text], based on nanoscale quantum sensing and RF focusing. Further enhancing the response of spins to the target's position through multi-photon excitation, a ranging accuracy of 16 μm is realized with a GHz RF signal. The results pave the way for exploring quantum enhanced radar and communications with solid spins.

Millihertz magnetic resonance spectroscopy combining the heterodyne readout based on solid-spin sensors

The sensitivities of quantum sensing in metrology and spectroscopy are drastically influenced by the resolution of the frequency spectrum. However, the resolution is hindered by the decoherence effect between the sensor and the environment. Along these lines, the continue-wave optically detected magnetic resonance (CWODMR) method combined with the heterodyne readout was proposed to break the limitation of the sensor's coherence time. The frequency of the magnetic field was swept to match the unknown signal, and the signal can be transformed to a real-time frequency-domain curve via the heterodyne readout, with a frequency resolution of 4.7 millihertz. Using the nitrogen-vacancy (NV) center ensemble in a diamond as the solid-spin sensors, it was demonstrated that the frequency resolution and precision could be improved proportionally to the low-pass filter parameters of T_c ^-1 and T_c ^-1.5, respectively. Furthermore, the introduced method performed the sensing of arbitrary audio signals with a sensitivity of 7.32 nT·Hz^-1/2@10 kHz. Our generic approach can be extended to several fields, such as molecular structure determination and biomagnetic field detection, where high-fidelity detection properties across multiple frequency bands are required within small sensing volumes (∼ mm³).