Efficient Radiofrequency Sensing with Fluorescence Encoding

TL;DR

The paper addresses the coherence-time bottleneck in AC magnetic sensing by introducing fluorescence-encoding, an incoherent approach that transduces time-varying fields into modulated fluorescence to realize broadband DC-to-MHz sensing with shot-noise-limited sensitivity. By monitoring spin-dependent fluorescence under continuous microwave drive, it converts field-induced frequency shifts into observable fluorescence fluctuations, enabling complete spectral information from a single measurement. Key contributions include a closed-form sensitivity expression, bandwidth tunability via optical power, and capability for simultaneous multi-frequency and phase-sensitive spectrum recovery, demonstrated on NV centers and extendable to other optically-active spin qubits. This approach yields atomic-scale spectrum analyzers with broad bandwidth, high spectral resolution, and applicability to low-frequency RF communications, zero-field NMR, and bioelectronic sensing, complementing coherent techniques and expanding the operational sensing parameter space.

Abstract

Optically-active spin qubits have emerged as powerful quantum sensors capable of nanoscale magnetometry, yet conventional coherent sensing approaches are ultimately limited by the coherence time of the sensor, typically precluding detection in the sub-MHz regime. We present a broadly applicable fluorescence-encoding method that circumvents coherence-time constraints by transducing time-varying magnetic fields directly into modulated fluorescence signals. Using nitrogen-vacancy centers in diamond as a model system, we demonstrate shot-noise-limited sensitivity for AC magnetic fields spanning near-DC to MHz frequencies, with detection bandwidth tunable via optical excitation power. The technique captures complete spectral information in a single measurement, eliminating the need for point-by-point frequency scanning, and allows phase-sensitive multi-frequency detection with Hz-level resolution. This approach transforms quantum sensors into atomic-scale spectrum analyzers, with immediate applications for low-frequency RF communication, zero-field NMR, and bioelectronic sensing. Our approach is broadly applicable to the expanding class of optically-active spin qubits, including molecular systems and fluorescent proteins, opening new sensing regimes previously inaccessible to coherent techniques

Figures (5)

• Figure 1: (a) Detection of RF signals in the DC-MHz band is essential for a broad range of applications, from bioelectronics to radio communication. (b) The concept of the sensing approach uses the time-dependent frequency shift induced by a target signal to change the fluorescence of a defect such as the nitrogen vacancy center in diamond. Fourier transforming the photon count time series recovers the target signal. (c) Example signals, demonstrating both excellent resolution and bandwidth.
• Figure 2: (a) Theoretical frequency-dependent sensitivities for different sensing techniques. Below 1MHz the fluorescence encoding approach is more sensitive than coherence-based approaches. (b) Scaling of the empirical signal-to-noise ratio with averaging time for the FE method, showing the $\sqrt{t}$ scaling expected for a shot-noise limited process.
• Figure 3: (a) Frequency response of the NV center as a function of excitation laser power. Curves are offset for visual clarity (b) Bandwidth as a function of laser power, showing the increase with laser power. (c) Simulated response curves as a function of saturation parameter from low (blue) to high (red). Solid lines are fits to the empirical function described in the text. Inset shows the evolution of the exponent as a function of saturation parameter, consistent with the experimental data.
• Figure 4: Multifrequency detection: (a) the fluorescence encoding approach can simultaneously detect signals close or far in frequency space. (b) With appropriate reference signal, phase sensitive detection can be realized. Here, a phase modulated signal is recovered and compared against the expected analytical form.
• Figure 5: Detection and characterization of telegraph noise with the incoherent fluorescence encoding approach. Top row shows short segments of the magnetic field trace applied to the sample. Bottom row shows the measured and expected PSD responses.