Real-time, broadband spectral analysis of microwave (MW) signals is crucial for various applications, including communication, medicine, and navigation. Current methods, primarily based on electronic spectrum analyzers using fast Fourier transforms (FFTs), are limited in bandwidth by analog-to-digital converters. Photonic approaches offer broader bandwidth but often require cryogenic temperatures. This research explores an alternative using the quantum spin properties of nitrogen-vacancy (NV) centers in diamond, offering the potential for room-temperature operation, low power consumption, and compact design – highly desirable features for onboard device integration. Previous NV-based MW spectral analysis demonstrated limited frequency ranges due to magnetic architecture constraints. This work addresses these limitations by introducing a novel architecture that allows for broader frequency coverage while maintaining the alignment of the magnetic field with the NV center axis, enabling a substantial improvement in spectral analysis capabilities.

Existing real-time spectral analysis techniques predominantly rely on electronic spectrum analyzers employing fast Fourier transforms (FFTs). While effective, these methods face bandwidth limitations imposed by analog-to-digital converter sampling rates and power consumption. Typically, a 500 MHz bandwidth results in a frequency resolution of several hundred kHz and a time resolution of microseconds. Photonic techniques, transposing MW signals into the optical domain, show promise for wider bandwidth, but often require cryogenic temperatures for optimal performance. Techniques like spectral hole burning in ion-doped crystals, while achieving tens of GHz bandwidths and high dynamic ranges, still operate at cryogenic temperatures. The use of NV centers in diamond provides a pathway towards room-temperature operation, but previous implementations suffered from limited frequency ranges due to magnetic field alignment issues.

The proposed Quantum Diamond Signal Analyzer (Q-DISA) exploits the spin-dependent optical properties of an ensemble of NV centers in diamond. These centers act as "quantum tuning forks," with resonance frequencies spatially encoded using an external static magnetic field gradient. A specific diamond crystallographic cut ({110} facet) and a simplified magnetic arrangement maintain magnetic field alignment along the NV center axis during frequency tuning. The MW signal is applied to the diamond via a loop antenna, and a camera captures the photoluminescence (PL) emitted by the NV centers. A decrease in PL intensity at specific pixels indicates resonant spectral components. The calibration procedure involves associating each pixel with a corresponding NV resonance frequency. The system operates in continuous-wave (CW) mode for continuous spin readout and instantaneous signal detection, eliminating dead time except for camera refresh. The signal-to-noise ratio (SNR) is improved by summing the PL signals from pixels resonating at the same frequency. The relationship between bandwidth, magnetic field strength, and frequency resolution is investigated. The dynamic range is determined by measuring the ODMR contrast variation with MW power, demonstrating a linear response at low power and saturation at higher powers. The temporal resolution is evaluated by adjusting camera exposure time to achieve a SNR near 1, revealing a frequency-dependent temporal resolution due to contrast variation.

The Q-DISA demonstrates real-time spectral analysis with the following key performance characteristics: a tunable frequency range from 10 MHz to 25 GHz; a bandwidth adaptable up to 4 GHz; a frequency resolution of down to 1 MHz at lower frequencies and up to 50 MHz at higher frequencies; a dynamic range of 40 dB; and a temporal resolution ranging from 2 ms at 1.8 GHz to 600 ms at 23 GHz. The frequency resolution is limited by the NV center intrinsic linewidth, power broadening, and inhomogeneous broadening from the magnetic field gradient. The lower frequency limit is set by the ground state level anti-crossing (GSLAC). The upper frequency limit is determined by the maximum magnetic field strength of the permanent magnet. The temporal resolution shows strong frequency dependence, primarily due to contrast variation and not intrinsic NV center photodynamics. Frequency ambiguities, arising from multiple resonance frequencies associated with the same pixel, are addressed by using diamonds with preferentially oriented NV centers and employing MW filtering techniques to select a specific NV transition. The experiment also demonstrates measuring the nonlinearity of a MW generator using Q-DISA and comparing it to a commercial spectrum analyzer.

The Q-DISA significantly advances broadband RF signal analysis by offering room-temperature operation, low power consumption, and compact design. The achieved MHz frequency resolution and ms temporal resolution exceed the capabilities of conventional electronic and photonic approaches for real-time spectral analysis across such a wide frequency range. The frequency-dependent resolution and temporal resolution are inherent to the single-magnet architecture and represent areas for future improvement. The achieved dynamic range is substantial, making Q-DISA suitable for a wide range of signal strengths. The method demonstrates the potential for real-time spectral monitoring in diverse applications where compact and low-power consumption are critical.

This work demonstrates a novel quantum-based RF signal analyzer (Q-DISA) utilizing NV centers in diamond. The system offers significant improvements in real-time broadband spectral analysis capabilities. Future research should focus on optimizing the system architecture for even better frequency resolution, wider bandwidth and improved temporal resolution across the entire frequency range through improved optical collection, magnetic field architecture, and diamond sample characteristics. Furthermore, exploring heterodyne techniques could extend the frequency range significantly.

The current Q-DISA architecture exhibits a frequency-dependent resolution and temporal resolution. The lower frequency limit is constrained by the GSLAC effect, and the upper limit is governed by the permanent magnet's field strength. The temporal resolution varies significantly with frequency due to contrast differences. While frequency ambiguities are mitigated, they are not entirely eliminated. Improving these aspects could enhance the overall performance and broaden the applicability of the Q-DISA.