🔬 physics

Noise suppression via pulsed all-optical magnetometry with nitrogen-vacancy ensembles

This paper introduces a pulsed all-optical protocol using nitrogen-vacancy ensembles in diamond that suppresses common-mode optical noise to achieve a 10-fold improvement in low-frequency sensitivity over continuous-wave techniques, while also characterizing the dependence of contrast on readout timing and dark time for optimizing diverse NV-diamond samples.

Original authors: Xiechen Zheng, Jeyson Támara-Isaza, John Blanchard, Ronald Walsworth

Published 2026-07-15

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CC BY 4.0

Original authors: Xiechen Zheng, Jeyson Támara-Isaza, John Blanchard, Ronald Walsworth

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to hear a whisper in a room where the air conditioner is constantly humming and the lights are flickering. That's what scientists face when they try to measure tiny magnetic fields using special diamonds. Inside these diamonds are tiny defects called Nitrogen-Vacancy (NV) centers. Think of them as microscopic, glowing fireflies that change their brightness depending on the magnetic field around them.

For a long time, scientists tried to listen to these fireflies using a "continuous" method, like keeping a light switch flipped on and just watching the glow. But this approach had a big problem: if the light bulb flickered or the power surged, it looked like the magnetic field changed, even when it didn't. It was like trying to hear a whisper while someone kept slamming the door.

The New Trick: A Stroboscope for Fireflies

In this paper, the researchers, led by Xiechen Zheng and colleagues, introduced a clever new way to listen: a "pulsed" method. Instead of keeping the light on, they flash it on and off very quickly. But here is the magic part: they don't just look once. They take two snapshots of the fireflies' brightness during a single flash of light.

Imagine you are taking a photo of a bouncing ball. If you take one picture at the very start of the bounce and another picture right at the end, and then you divide the first picture's brightness by the second, you cancel out the effect of the camera shaking or the sun getting brighter. You are left with just the ball's movement.

The team did exactly this with the diamond fireflies. They measured the light intensity near the beginning of a laser pulse (the "signal") and again near the end (the "reference"). By comparing these two, they could cancel out the "flickering" noise caused by the laser itself. This allowed them to hear the magnetic whisper much more clearly.

What They Found

When they tested this new "stroboscope" method on three different diamond samples (labeled D1, D2, and D3), they found some exciting things:

Quieter Listening: At very low magnetic fields (near zero), their new method reduced the background noise by about 10 times compared to the old continuous method. It's like turning down the volume on the air conditioner so you can finally hear the whisper.
Timing is Everything: They discovered that the "dark time" (the pause between laser flashes) matters a lot. If they waited too long or too short, the signal got fuzzy. The perfect pause depended on how many fireflies were in the diamond.
- In the sample with the most fireflies (D1, about 3.8 ppm), the best pause was around 2.5 ms.
- In the sample with fewer fireflies (D2, about 2 ppm), they had to wait longer, around 7 ms, to get the best result.
The "Cross-Talk" Effect: The reason the signal changes is due to a phenomenon called "NV-NV cross-relaxation." Imagine the fireflies are neighbors who can talk to each other. When they are all in sync (near zero magnetic field), they start "talking" (interacting) and lose their individual glow. The new method is sensitive enough to see exactly when this neighborly chat happens.

What They Didn't Do (and What They Ruled Out)

The paper is very clear about what this method is not.

It is not a method that uses microwaves. Many other ways to read these diamonds require blasting them with microwaves, which is complicated and can disturb sensitive samples. This new method uses only light, making it simpler and safer for delicate materials.
It does not require wiggling the magnetic field to make the signal visible. Some older methods had to shake the magnetic field back and forth (amplitude modulation) to use a special lock-in detector. The new pulsed method works without this shaking, which is great because shaking the field might disturb the very thing you are trying to measure (like a superconducting material).
The paper does not claim this solves all noise problems. They found that while the new method is great at low frequencies (below 10 Hz), the old continuous method was actually better at higher frequencies (above 10 Hz) because it could take measurements much faster (100 kS/s vs 400 S/s). The new method is currently slower, so it misses some of the fast, high-pitched "whispers."

How Sure Are They?

The authors didn't just guess or simulate this on a computer; they built the experiment and measured it.

They measured the noise reduction directly, showing a ~10 dB improvement (which translates to that 10x noise reduction) in the low-frequency range.
They tested this with real laser power settings of 275 mW and 57 mW, and the results held up.
They even added fake noise (modulating the light intensity by 0.2%) to see if their method could handle it, and it did, staying 4-5 dB quieter than the old method even with the extra noise.

The Bottom Line

This paper shows that by flashing a laser and taking two quick snapshots of the light, scientists can cancel out the noise and hear the magnetic signals from diamond fireflies much better than before. It's a simpler, quieter way to listen to the magnetic world, especially for low-frequency signals. However, it's not a magic wand for everything; it's currently slower than the old way, so for very fast signals, the old continuous method might still be the champion. But for those quiet, steady whispers near zero magnetic field, this new pulsed approach is a game-changer.

Technical Summary: Noise Suppression via Pulsed All-Optical Magnetometry with Nitrogen-Vacancy Ensembles

Problem Statement
All-optical (AO), microwave-free magnetometry using nitrogen-vacancy (NV) centers in diamond offers a simplified experimental design and broader sample compatibility compared to conventional microwave-driven schemes. However, continuous-wave (cw) detection of AO photoluminescence (PL) changes is inherently susceptible to systematic fluctuations, particularly optical intensity noise and mechanical instability. While lock-in detection using amplitude-modulated magnetic fields can improve the signal-to-noise ratio (SNR) in cw AO measurements, this approach introduces design complexity and may perturb sensitive samples, such as superconducting materials. Existing microwave-based protocols effectively suppress low-frequency common-mode noise through normalized on/off-resonance measurements, but a comparable noise-suppression strategy for cw AO magnetometry without modulated fields has been lacking.

Methodology
The authors introduce a pulsed AO protocol, termed the "single-τ sequence," designed to resolve AO-PL contrast features at near-zero magnetic fields while suppressing common-mode noise. The experimental setup utilizes a 532 nm laser modulated by an acousto-optic modulator (AOM) to create optical pulses incident on CVD-grown diamond samples containing NV ensembles with varying concentrations (approx. 0.3 to 3.8 ppm).

The core of the methodology involves performing two PL readouts within a single optical pulse:

Signal Readout ( $I_{sig}$ ): Measured at a variable time $t_r$ shortly after the beginning of the optical pulse.
Reference Readout ( $I_{ref}$ ): Measured during the final 1 µs of the optical pulse.

The normalized AO-PL intensity is calculated as $n = I_{sig}/I_{ref}$ . This normalization suppresses common-mode noise (e.g., slow optical intensity variations) that equally affects both measurements. The pulsed AO-PL contrast is defined as $C = 1 - n/n'$ , where $n'$ is the normalized intensity measured at a reference off-resonant magnetic field ( $B_0 = 0.45$ mT) where NV-NV cross-relaxation effects are minimal.

The study investigates the dependence of this contrast on:

The timing of the signal readout ( $t_r$ ).
The dark time duration ( $\tau$ ) between optical pulses.
NV concentration (comparing samples D1, D2, and D3).
Laser power (57 mW vs. 275 mW).

Key Contributions and Results

Noise Suppression: The pulsed AO protocol demonstrates a significant improvement in low-frequency noise performance compared to conventional cw AO techniques. Without the use of lock-in detection or amplitude-modulated magnetic fields, the pulsed method achieves up to a 10× (approx. 10 dB) improvement in the noise floor in the near-DC frequency range. This advantage persists even when additional optical intensity noise (0.2% amplitude modulation depth) is introduced.
Contrast Dynamics: The study resolves AO-PL contrast features arising from NV-NV cross-relaxation near zero magnetic fields. The contrast is shown to be dependent on the readout timing $t_r$ , with higher contrast observed when $I_{sig}$ is measured shortly after optical excitation and $I_{ref}$ is measured near steady-state.
Optimization of Dark Time ( $\tau$ ): The pulsed AO-PL contrast $C(\tau)$ $C (τ)$ exhibits a non-monotonic dependence on the dark time $\tau$ $τ$ , initially increasing, reaching a maximum, and then decreasing. This behavior is quantitatively described by a bi-exponential function involving two characteristic timescales: $T_{cr}$ $T_{cr}$ (associated with NV-NV cross-relaxation) and $T_{in}$ $T_{in}$ (associated with intrinsic depolarization).
- The optimal $\tau$ for maximum contrast is found to be NV-concentration dependent. Higher NV concentrations (e.g., ~3.8 ppm) result in stronger dipolar interactions, shorter timescales ( $T_{cr}$ and $T_{in}$ ), and a shorter optimal $\tau$ (approx. 2.5 ms) compared to lower concentrations.
- The characteristic timescales were found to be largely independent of laser power (57 mW vs. 275 mW), suggesting they are primarily determined by sample properties.
Trade-offs: While higher NV concentrations improve contrast and allow for shorter $\tau$ (potentially increasing duty cycle), the authors note that millisecond-scale $\tau$ values currently result in low duty cycles for the pulsed sequence, limiting the sampling rate to 400 S/s. This makes the system susceptible to aliasing at frequencies above 10 Hz, where cw methods (with higher sampling rates) currently exhibit lower noise floors.

Significance and Claims
The paper claims that the proposed pulsed AO protocol provides a robust method for effective common-mode noise rejection in NV ensemble magnetometry without the need for amplitude-modulated magnetic fields. This is particularly significant for sensitive DC magnetometry in challenging environments where sample perturbation must be minimized.

The findings offer new insights for optimizing NV-diamond samples and AO protocols for low size, weight, and power (low-SWaP) quantum sensors. Specifically, the work highlights that selecting samples with higher nitrogen/NV concentrations can enhance AO-PL contrast and reduce the required dark time, though this introduces trade-offs regarding optical absorption and linewidth broadening that require further evaluation. The authors suggest that while the current implementation is limited by duty cycle, the protocol is promising for future applications in diverse quantum sensing scenarios, including extreme temperature, pressure, and radiation conditions.

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