Efficient, uniform, and large area microwave magnetic coupling to NV centers in diamond using double split-ring resonators

Compact and Fully Integrated LED Quantum Sensor Based on NV Centers in Diamond

Quantum magnetometry based on optically detected magnetic resonance (ODMR) of nitrogen vacancy centers in diamond nano or microcrystals is a promising technology for sensitive, integrated magnetic-field sensors. Currently, this technology is still cost-intensive and mainly found in research. Here we propose one of the smallest fully integrated quantum sensors to date based on nitrogen vacancy (NV) centers in diamond microcrystals. It is an extremely cost-effective device that integrates a pump light source, photodiode, microwave antenna, filtering and fluorescence detection. Thus, the sensor offers an all-electric interface without the need to adjust or connect optical components. A sensitivity of 28.32nT/Hz and a theoretical shot noise limited sensitivity of 2.87 nT/Hz is reached. Since only generally available parts were used, the sensor can be easily produced in a small series. The form factor of (6.9 × 3.9 × 15.9) mm3 combined with the integration level is the smallest fully integrated NV-based sensor proposed so far. With a power consumption of around 0.1W, this sensor becomes interesting for a wide range of stationary and handheld systems. This development paves the way for the wide usage of quantum magnetometers in non-laboratory environments and technical applications.

Large-scale optical characterization of solid-state quantum emitters

Solid-state quantum emitters have emerged as a leading quantum memory for quantum networking applications. However, standard optical characterization techniques are neither efficient nor repeatable at scale. Here we introduce and demonstrate spectroscopic techniques that enable large-scale, automated characterization of colour centres. We first demonstrate the ability to track colour centres by registering them to a fabricated machine-readable global coordinate system, enabling a systematic comparison of the same colour centre sites over many experiments. We then implement resonant photoluminescence excitation in a widefield cryogenic microscope to parallelize resonant spectroscopy, achieving two orders of magnitude speed-up over confocal microscopy. Finally, we demonstrate automated chip-scale characterization of colour centres and devices at room temperature, imaging thousands of microscope fields of view. These tools will enable the accelerated identification of useful quantum emitters at chip scale, enabling advances in scaling up colour centre platforms for quantum information applications, materials science and device design and characterization.

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.

New constraints on exotic spin-dependent interactions with an ensemble-NV-diamond magnetometer

Laboratory search of exotic interactions is crucial for exploring physics beyond the standard model. We report new experimental constraints on two exotic spin-dependent interactions at the micrometer scale based on ensembles of nitrogen-vacancy (NV) centers in diamond. A thin layer of NV electronic spin ensembles is synthesized as the solid-state spin quantum sensor, and a lead sphere is taken as the interacting nucleon source. Our result establishes new bounds for two types of exotic spin interactions at the micrometer scale. For an exotic parity-odd spin- and velocity-dependent interaction, improved bounds are set within the force range from 5 to 500 μm. The upper limit of the corresponding coupling constant [Formula: see text] at 330 μm is more than 1000-fold more stringent than the previous constraint. For the P, T-violating scalar-pseudoscalar nucleon-electron interaction, improved constraints are established within the force range from 6 to 45 μm. The limit of the corresponding coupling constant [Formula: see text] is improved by more than one order of magnitude at 30 μm. This work demonstrates that a solid-state NV ensemble can be a powerful platform for probing exotic spin-dependent interactions.

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.

Variable bandwidth, high efficiency microwave resonator for control of spin-qubits in nitrogen-vacancy centers

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.

Glass-patternable notch-shaped microwave architecture for on-chip spin detection in biological samples

We report a notch-shaped coplanar microwave waveguide antenna on a glass plate designed for on-chip detection of optically detected magnetic resonance (ODMR) of fluorescent nanodiamonds (NDs). A lithographically patterned thin wire at the center of the notch area in the coplanar waveguide realizes a millimeter-scale ODMR detection area (1.5 × 2.0 mm²) and gigahertz-broadband characteristics with low reflection (∼8%). The ODMR signal intensity in the detection area is quantitatively predictable by numerical simulation. Using this chip device, we demonstrate a uniform ODMR signal intensity over the detection area for cells, tissue, and worms. The present demonstration of a chip-based microwave architecture will enable scalable chip integration of ODMR-based quantum sensing technology into various bioassay platforms.

Design of a High-Bandwidth Uniform Radiation Antenna for Wide-Field Imaging with Ensemble NV Color Centers in Diamond

Radiation with high-efficiency, large-bandwidth, and uniform magnetic field radiation antennas in a large field of view are the key to achieving high-precision wide-field imaging. This paper presents a hollow Ω-type antenna design for diamond nitrogen-vacancy (NV) ensemble color center imaging. The uniformity of the antenna reaches 94% in a 4.4 × 4.4 mm2 area. Compared with a straight copper antenna, the radiation efficiency of the proposed antenna is 71.8% higher, and the bandwidth is improved by 11.82 times, demonstrating the effectiveness of the hollow Ω-type antenna.

Diamond quantum thermometry: from foundations to applications

Diamond quantum thermometry exploits the optical and electrical spin properties of colour defect centres in diamonds and, acts as a quantum sensing method exhibiting ultrahigh precision and robustness. Compared to the existing luminescent nanothermometry techniques, a diamond quantum thermometer can be operated over a wide temperature range and a sensor spatial scale ranging from nanometres to micrometres. Further, diamond quantum thermometry is employed in several applications, including electronics and biology, to explore these fields with nanoscale temperature measurements. This review covers the operational principles of diamond quantum thermometry for spin-based and all-optical methods, material development of diamonds with a focus on thermometry, and examples of applications in electrical and biological systems with demand-based technological requirements.

Optimized Planar Microwave Antenna for Nitrogen Vacancy Center Based Sensing Applications

Individual nitrogen vacancy (NV) color centers in diamond are versatile, spin-based quantum sensors. Coherently controlling the spin of NV centers using microwaves in a typical frequency range between 2.5 and 3.5 GHz is necessary for sensing applications. In this work, we present a stripline-based, planar, Ω-shaped microwave antenna that enables one to reliably manipulate NV spins. We found an optimal antenna design using finite integral simulations. We fabricated our antennas on low-cost, transparent glass substrate. We created highly uniform microwave fields in areas of roughly 400 × 400 μm² while realizing high Rabi frequencies of up to 10 MHz in an ensemble of NV centers.

Parallel optically detected magnetic resonance spectrometer for dozens of single nitrogen-vacancy centers using laser-spot lattice

We develop a parallel optically detected magnetic resonance (PODMR) spectrometer to address, manipulate, and read out an array of single nitrogen-vacancy (NV) centers in diamond in parallel. In this spectrometer, we use an array of micro-lenses to generate a 20 × 20 laser-spot lattice (LSL) on the objective focal plane and then align the LSL with an array of single NV centers. The quantum states of NV centers are manipulated by a uniform microwave field from a Ω-shape coplanar coil. As an experimental demonstration, we observe 80 NV centers in the field of view. Among them, magnetic resonance (MR) spectra and Rabi oscillations of 18 NV centers along the external magnetic field are measured in parallel. These results can be directly used to realize parallel quantum sensing and multiple times speedup compared with the confocal technique. Regarding the nanoscale MR technique, PODMR will be crucial for a high throughput single molecular MR spectrum and imaging.

A hybrid magnetometer towards femtotesla sensitivity under ambient conditions

Detecting magnetic field is of great importance for many applications, such as magnetoencephalography and underground prospecting. There have been many magnetometers being widely used since the age of Hall magnetometer. One of the magnetometers, the superconducting quantum interference device, is capable of measuring femtotesla magnetic fields at cryogenic temperature. However, a solid-state magnetometer with femtotesla sensitivity under ambient conditions remains elusive. Here we present a hybrid magnetometer based on the ensemble nitrogen-vacancy centers in diamond with the sensitivity of (195±60)fT/Hz^1/2 under ambient conditions, which can be further advanced to 11fT/Hz^1/2 at 100 Hz with cutting-edge fabrication technologies. Our method will find potential applications in biomagnetism and geomagnetism.

Imaging nano-defects of metal waveguides using the microwave cavity interference enhancement method

Here, we demonstrate a microwave (MW) cavity interference enhancement method to image nano-defects on the surface of metal waveguide. The MW cavity interference system mainly consisted of a MW coaxial resonant cavity with a nano-probe. The MW signals have been evenly divided into two channels. One was the reference signal inputted into the MW waveguide and coupled into the MW cavity via the probe. Also, the coupling strength depends on the distance between the probe and the MW waveguide. Another one was directly inputted the MW cavity to interfere with the reference signal, and was enhanced in the cavity. Then, the surface topography of the metal waveguide was mapped by calculating the enhanced signals. In our experiment, a weak signal of ∼1 pW coupled from the waveguide can be detected by a MW cavity with the quality factor of ∼209. As a proof of application, the topography of nano-defects on the surface of metal waveguide in an MW chip has been mapped with a resolution of ∼15 nm. We have proved that this is a high-resolution, easy-to-manufacture, low-cost, and real-time online monitoring approach for online assessment and screening chips. This potentially has broad applications in the fields of chip manufacturing, chip inspection, nano-structure detection, and so on.

System for the remote control and imaging of MW fields for spin manipulation in NV centers in diamond

Nitrogen-vacancy (NV) centers in diamond have been used as platforms for quantum information, magnetometry and imaging of microwave (MW) fields. The spatial distribution of the MW fields used to drive the electron spin of NV centers plays a key role for these applications. Here, we report a system for the control and characterization of MW magnetic fields used for the NV spin manipulation. The control of the MW field in the vicinity of a diamond surface is mediated by an exchangeable lumped resonator, coupled inductively to a MW planar ring antenna. The characterization of the MW fields in the near-field is performed by an FFT imaging of Rabi oscillations, by using an ensemble of NV centers. We have found that the Rabi frequency over a lumped resonator is enhanced 22 times compared to the Rabi frequency without the presence of the lumped resonator. Our system may find applications in quantum information and magnetometry where a precise and controlled spin manipulation is required, showing NV centers as good candidates for imaging MW fields and characterization of MW devices.

Circularly polarized microwave antenna for nitrogen vacancy centers in diamond

The sensing applications of nitrogen-vacancy color centers in a diamond require an efficient manipulation of the color center ground state over the whole volume of an ensemble. Thus, it is necessary to produce strong uniform magnetic fields of a well-defined circular polarization at microwave frequencies. In this paper, we develop a circularly polarized microwave antenna based on the excitation of hybrid electromagnetic modes in a high-permittivity dielectric resonator. The influence of the geometrical parameters of the antenna on the reflection coefficient and magnetic field magnitude is studied numerically and discussed. The Rabi frequencies and their inhomogeneity over the volume of a commercially available diamond sample are calculated. With respect to the numerical predictions, a Rabi frequency as high as 34 MHz with an inhomogeneity of 4% over a 1.2 mm × ∅2.5 mm (5.9 mm³ in volume) diamond sample can be achieved for 10 W of input power at room temperature. The antenna prototype is fabricated, and experimental investigations of its characteristics are performed in microwave and optical frequency domains. The circular polarization of the microwave magnetic field with an ellipticity of 0.94 is demonstrated experimentally. The Rabi oscillation frequency and its inhomogeneity are measured, and the results demonstrate a good agreement with the numerically predicted results.

Broadband loop gap resonator for nitrogen vacancy centers in diamond

We present an S-band tunable loop gap resonator (LGR), which provides strong, homogeneous, and directionally uniform broadband microwave (MW) drive for nitrogen-vacancy (NV) ensembles. With 42 dBm of input power, the composite device provides drive field amplitudes approaching 5 G over a circular area ≳50 mm² or cylindrical volume ≳250 mm³. The wide 80 MHz device bandwidth allows driving all NV Zeeman resonances for bias magnetic fields below 20 G. The device realizes percent-scale MW drive inhomogeneity; we measure a fractional root-mean-square inhomogeneity σ _rms = 1.6% and a peak-to-peak variation σ _pp = 3% over a circular area of 11 mm² and σ _rms = 3.2% and σ _pp = 10.5% over a larger 32 mm² circular area. We demonstrate incident MW power coupling to the LGR using two methodologies: a printed circuit board-fabricated exciter antenna for deployed compact bulk sensors and an inductive coupling coil suitable for microscope-style imaging. The inductive coupling coil allows for approximately 2π steradian combined optical access above and below the device, ideal for envisioned and existing NV imaging and bulk sensing applications.

Ultra-broadband coplanar waveguide for optically detected magnetic resonance of nitrogen-vacancy centers in diamond

We report on coplanar waveguides (CPWs) designed for optically detected magnetic resonance of nitrogen-vacancy (NV) centers in diamonds. A broad band up to 15.8 GHz has been realized, which ensures that the electron spins can be manipulated under external magnetic fields up to 5000 G. The conversion factor of CPW has been measured by Rabi nutation experiments, which ranges from 6.64 G W^-1/2 to 10.60 G W^-1/2 in the frequency band from 0.76 GHz to 17.3 GHz. Broadband CPWs also provide high quality control pulses due to the minimization of the distortion. These characteristics will find potential applications in NV-based quantum information processing and single spin magnetometry.

On the Possibility of Miniature Diamond-Based Magnetometers Using Waveguide Geometries

We propose the use of a diamond waveguide structure to enhance the sensitivity of magnetometers relying on the detection of the spin state of nitrogen-vacancy ensembles in diamond by infrared optical absorption. An optical waveguide structure allows for enhanced optical path-lengths avoiding the use of optical cavities and complicated setups. The presented design for diamond-based magnetometers enables miniaturization while maintaining high sensitivity and forms the basis for magnetic field sensors applicable in biomedical, industrial and space-related applications.

Broadband, large-area microwave antenna for optically detected magnetic resonance of nitrogen-vacancy centers in diamond

We report on a microwave planar ring antenna specifically designed for optically detected magnetic resonance (ODMR) of nitrogen-vacancy (NV) centers in diamond. It has the resonance frequency at around 2.87 GHz with the bandwidth of 400 MHz, ensuring that ODMR can be observed under external magnetic fields up to 100 G without the need of adjustment of the resonance frequency. It is also spatially uniform within the 1-mm-diameter center hole, enabling the magnetic-field imaging in the wide spatial range. These features facilitate the experiments on quantum sensing and imaging using NV centers at room temperature.

Selective microwave sensors exploiting the interaction of analytes with trap states in TiO2 nanotube arrays

Sensing of molecular analytes by probing the effects of their interaction with microwaves is emerging as a cheap, compact, label-free and highly sensitive detection and quantification technique. Microstrip ring-type resonators are particularly favored for this purpose due to their planar sensing geometry, electromagnetic field enhancements in the coupling gap and compatibility with established printed circuit board manufacturing. However, the lack of selectivity in what is essentially a permittivity-sensing method is an impediment to wider adoption and implementation of this sensing platform. By placing a polycrystalline anatase-phase TiO2 nanotube membrane in the coupling gap of a microwave resonator, we engineer selectivity for the detection and differentiation of methanol, ethanol and 2-propanol. The scavenging of reactive trapped holes by aliphatic alcohols adsorbed on TiO2 is responsible for the alcohol-specific detection while the different short chain alcohols are distinguished on the basis of differences in their microwave response. Electrodeless microwave sensors which allow spectral and time-dependent monitoring of the resonance frequency and quality factor provide a wealth of information in comparison with electrode-based resistive sensors for the detection of volatile organic compounds. A high dynamic range (400 ppm-10,000 ppm) is demonstrated for methanol detection.