Microwave (MW) magnetometry utilizing nitrogen-vacancy (NV) centers in diamond offers a broad, tunable detection bandwidth enabled by the magnetic-field-dependent electron spin resonances (ESR) of the NV centers. In this work, we present and experimentally demonstrate two NV-based protocols for spectral analysis of amplitude-modulated (MW) signals. The first protocol employs the NV center as a bandpass filter and reconstructs frequency-domain information of MW signals by sweeping the MW carrier frequency. The second protocol achieves optical imaging of the MW spectrum using an NV-based quantum spectrum analyzer, wherein a magnetic field gradient spatially maps the ESR frequencies of the NV centers, enabling parallel detection of multiple spectral components over a wide instantaneous bandwidth. Both protocols show results that agree well with reference measurements from a classical spectrum analyzer. Our approaches extend the application of spin-based quantum sensors to the field of modern communications and can be adapted for other spin-based quantum sensors.
Microwave (MW) magnetometry utilizing nitrogen-vacancy (NV) centers in diamond offers a broad, tunable detection bandwidth enabled by the magnetic-field-dependent electron spin resonances (ESR) of the NV centers. In this work, we present and experimentally demonstrate two NV-based protocols for spectral analysis of amplitude-modulated (MW) signals. The first protocol employs the NV center as a bandpass filter and reconstructs frequency-domain information of MW signals by sweeping the MW carrier frequency. The second protocol achieves optical imaging of the MW spectrum using an NV-based quantum spectrum analyzer, wherein a magnetic field gradient spatially maps the ESR frequencies of the NV centers, enabling parallel detection of multiple spectral components over a wide instantaneous bandwidth. Both protocols show results that agree well with reference measurements from a classical spectrum analyzer. Our approaches extend the application of spin-based quantum sensors to the field of modern communications and can be adapted for other spin-based quantum sensors.
We propose an original analog method to perform instantaneous and quantitative spectral analysis of microwave signals. An ensemble of nitrogen-vacancy (NV) centers held in a diamond plate is pumped by a 532 nm laser. Its photoluminescence is imaged through an optical microscope and monitored by a digital camera. An incoming microwave signal is converted into a microwave field in the area of the NV centers by a loop shaped antenna. The resonances induced by the magnetic component of that field are detected through a decrease of the NV centers photoluminescence. A magnetic field gradient induces a Zeeman shift of the resonances and transforms the frequency information into spatial information, which allows for the simultaneous analysis of the microwave signal in the entire frequency bandwidth of the device. The time dependent spectral analysis of an amplitude modulated microwave signal is demonstrated over a bandwidth of 600 MHz, associated to a frequency resolution of 7 MHz , and a refresh rate of 4 ms. With such integration time, a field of a few hundreds of μW can be detected. Since the optical properties of NV centers can be maintained even in high magnetic field, we estimate that an optimized device could allow frequency analysis in a range of 30 GHz, only limited by the amplitude of the magnetic field gradient. In addition, an increase of the NV centers quantity could lead both to an increase of the microwave sensitivity and to a decrease of the minimum refresh rate down to a few μs.
Spin-based quantum sensors have been applied to the quantum sensing of magnetic fields from DC to GHz with various schemes. The recently developed quantum spectrum analyzer (QSA) technique based on the nitrogen-vacancy (NV) centers in diamond shows an ultrawide frequency bandwidth in this application. Here, we improve this technique and develop a robust QSA via diamond engineering. This QSA exploits the electron spin resonance of NV centers in an engineered {111} industrial diamond, which cancels the complex process of magnetic arrangement and significantly enhances the robustness of QSA against spin mixing under high magnetic fields in practice. A tunable frequency bandwidth of up to 18 GHz with a maximum instantaneous bandwidth of 2 GHz is achieved as well as a frequency resolution of several MHz and a large dynamic range of 65 dB. Using this QSA, we realize the optical imaging of frequency-modulated microwaves. Our work provides a promising rapid-deployment architecture for QSA and extends its application to spectral analysis in modern communications.
High-resolution wide-spectrum measurement techniques have important applications in fields such as astronomy, wireless communication, and medical imaging. Nitrogen-vacancy (NV) center in diamond is well known for its high stability, high sensitivity, real-time monitoring, single-point detection, and suitability for long-term measurement, and has an outstanding choice for spectrum analyzers. Currently, spectrum analyzers based on NV centers as detectors can perform real-time spectrum analysis in the range of several tens of gigahertz, but their frequency resolution is limited to a MHz level. In this study, we construct a quantum diamond microwave spectrum imaging system by combining continuous wave-mixing techniques. According to the spin-related properties of the NV center in diamond, we implement optical pumping by 532 nm green laser light illuminating the diamond NV center. A spherical magnet is used to produce a magnetic field gradient along the direction of the diamond crystal. By adjusting the size and direction of the magnetic field gradient, spatial encoding of the resonance frequency of the NV center is achieved. The magnetic field gradient induces the Zeeman effect on the diamond surface at different positions, generating corresponding ODMR signals. Through accurate programming, we coordinate the frequency scanning step size of the microwave source with the camera exposure and image storage time, and synchronize them circularly according to the order of image acquisition. Ultimately, after algorithmic processing, we successfully obtain comprehensive spectrum data in a range from 900 MHz to 6.0 GHz. Within the measurable spectrum range, the system employs continuous wave-mixing, simultaneously applying resonant microwaves and slightly detuning auxiliary microwaves to effectively excite the NV center. This method triggers off microwave interference effects, disrupting the balance between laser-induced polarization and microwave-induced spontaneous relaxation. Specifically, microwave interference causes the phase and amplitude of the fluorescence signal to change, leading to the generation of alternating current fluorescence signals. This further enhances the response of the NV magnetometer to weak microwave signals. The method enables the system to achieve a frequency resolution of 1 Hz in the measurable spectrum range, and it can separately measure the frequency resolution of multiple frequency points with a frequency step size of 1 MHz. The research results indicate that the wide-spectrum measurement based on NV centers can achieve sub-hertz frequency resolution, providing robust technical support for future spectrum analysis and applications.
The nitrogen-vacancy center (NV) is a diamond spin defect consisting of a nitrogen atom and a carbon vacancy in two adjacent positions of the diamond lattice. Its remarkable spin-dependent optical properties make it a promising platform for quantum sensing and quantum information applications.In the present manuscript, two applications involving an ensemble of NV centers in diamond are investigated: a radio frequency (RF) spectrum analyser and a widefield imaging technique of the RF radiation emitted in near field by microwave devices.In the first chapter, the NV centers in diamond, their properties and the motivations behind the two NV centers based applications proposed in this manuscript are presented. The available solutions and the state-of-the-art techniques for both RF spectral analysis and near field imaging of RF radiation are reviewed and the potential of alternative solutions using NV centres are discussed.In the second chapter, a set-up for the widefield optical detection of the NV centers spin resonances is presented and used to investigate the main properties of this diamond defect. The NV center electron spin resonances are detected over a large frequency range (more than 10 GHz) and different phenomena are investigated: the behaviour of an ensemble of NV centers under a static magnetic field, the polarization properties of the NV center photoluminescence (PL) and how they can be exploited to recognise the NV centers orientations, the NV’s nitrogen nuclear spin polarization over a large frequency range (more than 2 GHz).The third chapter is devoted to the implementation of a real-time RF spectrum analyser based on an ensemble of NV centers. The main physical features of NV centers considered as MW detectors are investigated and an experimental architecture which allows a real time RF spectral analysis over a tuneable frequency range (10 MHz to 25 GHz), 4 GHz bandwidth, a MHz frequency resolution and a ms time resolution is demonstrated. The implementation of this device required an interesting analysis on the behaviour of an ensemble of NV centers under strong magnetic field gradient (tens of Tesla per meter) which is detailed in the chapter.The fourth and last chapter deals with the widefield imaging of RF field based on an ensemble of NV centers. Two techniques are proposed. The first is based on the RF power broadening of the NV center spin resonances in the continuous regime domain while the second is based on the widefield detection of NV centers Rabi oscillations in a pulsed regime. Both techniques allowed reconstructing the magnetic near field RF field emitted by a loop antenna. However, while the first technique requires a calibration to retrieve the amplitude of the RF field, the second allows a direct measurement of it. The widefield detection of the NV centers Rabi oscillations has thus been investigated both by means of experiments and simulations. Since the NV centers Rabi oscillations detected using our widefield experimental set-up showed an asymmetric shape, a model has been realized to figure out the NV centers behaviour during the pulse sequence employed to drive the Rabi oscillation. The results of the simulations, in agreement with the experimental results, enable to have a better insight of the NV center spin optical polarization and readout during the widefield detection of the Rabi oscillations.
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.
Magnetic field fluctuations on nanometer length scales manifest in a diverse range of phenomena -- electron and spin dynamics in materials and devices, quantum many-body systems, and molecular chemistry. Measuring these phenomena requires sensors with a challenging combination of broad spectral bandwidth, high sensitivity, and nanoscale spatial resolution. Nitrogen-vacancy (NV) centers, atom-like quantum sensors in diamond, possess the requisite sensitivity and nanoscale sensing volume, but are typically limited in bandwidth by the practical speed of the applied quantum control sequence. Here, we overcome this limitation by exposing the NV qubit to a microwave dressing field during a dynamical decoupling sequence, which both amplifies and frequency-mixes target signals at arbitrary frequencies into the detection band of the dynamical decoupling protocol. We demonstrate this approach by using NV centers to detect both coherent and noisy nanoscale spin wave dynamics in a magnetic yttrium-iron-garnet (YIG) thin film over a broad frequency range. Our technique generalizes to other qubit platforms, providing a versatile framework for nanoscale spectroscopy across diverse physical and chemical systems.