Researchers at the University of Alberta predict a new quantum sensor could detect magnetic fields as weak as 24 aT/√Hz in just 10 minutes. This sensitivity, achieved with a 10 W ultraviolet laser, has the potential to surpass that of current cryogenic magnetometers, devices that require extremely low temperatures to operate.

The analysis reveals that a relative intensity noise of -100 dBc/Hz is important for optimal performance, guiding efforts to either increase laser power or minimize noise. The approach uses a noble gas, eliminating the need for alkali vapors and enabling hours-long spin relaxation at room temperature.

Ultraviolet Frequency Comb Laser Enables Noble Gas Detection

This level of noise control, combined with high laser power, dictates whether experimental efforts should focus on boosting laser output or refining noise reduction techniques. The research details how these factors influence the magnetometer’s sensitivity, pinpointing regimes where either fundamental or technical noise limits detection capabilities. Unlike conventional methods, this approach circumvents the need for alkali vapors during the sensing stage, enabling spin relaxation times lasting hours at room temperature.

This is made possible by employing an ultraviolet frequency comb laser for optical detection of nuclear spin precession in a noble gas, specifically xenon. The team’s calculations predict a record-breaking sensitivity of 24 aT/√Hz is achievable within 10 minutes using a 10 W ultraviolet laser operating at these optimized parameters. A Yb:fiber frequency comb laser system, capable of delivering over 100 Watts of average power, is proposed as the driving force behind this technology.

Frequency quadrupling through two second-harmonic generation processes converts the laser output to 256 nm ultraviolet light, facilitating two-photon excitation in xenon. The researchers anticipate an overall conversion efficiency of a few percent in this process. “No alkali vapour is present in the detection phase, even if it has previously been used for hyperpolarization,” the analysis notes, further highlighting the system’s simplified operational requirements and potential for long-duration measurements. The work demonstrates a pathway toward quantum magnetic sensing with a simplified setup and enhanced sensitivity, potentially opening new avenues for applications requiring precise magnetic field detection.

Noble Gas Spin Relaxation Time Exceeds 10 Milliseconds

Extended spin relaxation times in noble gases enable magnetometer designs that bypass limitations of alkali-based sensors, with measured durations now exceeding 10 milliseconds. This prolonged coherence is facilitated by the complete electron shell structure of noble gases, leaving nuclear spin as the sole source of angular momentum and minimizing disruptive interactions. The research details how this intrinsic property supports hours-long polarization, a significant advantage over atomic magnetometers reliant on alkali atoms where spin-exchange relaxation typically limits sensitivity.

The approach also addresses a key challenge in alkali atomic magnetometers: spin relaxation. It has been established that spin-exchange relaxation is a dominant limiting factor in these systems, but suppression requires precise zeroing of the magnetic field, often at the expense of other performance metrics. Noble gas-based sensors, however, inherently minimize this issue, offering a more stable and predictable operating environment.

The work demonstrates that the variance due to spin projection noise is independent of laser power, scaling instead with the number of xenon atoms and measurement time, providing another avenue for sensitivity enhancement. The paper states, outlining the factors influencing achievable precision.

SERF Regime Limitations in Alkali Atomic Magnetometers

Existing atomic magnetometers, while achieving sensitivities of 160 aT/√Hz, comparable to superconducting quantum interference devices (SQUIDs), are often hampered by millisecond-scale spin lifetimes, limiting their potential for detecting weaker magnetic fields. The most sensitive designs rely on the spin-exchange relaxation-free (SERF) regime, a state achieved with high alkali atom density at elevated temperatures and low magnetic fields. But even these systems face limitations imposed by spin-exchange relaxation.

Careful magnetic field zeroing is required to suppress this relaxation, often at the expense of minimizing other noise sources. The SERF regime’s reliance on specific alkali vapor conditions presents a challenge; maintaining the necessary density and temperature while simultaneously minimizing magnetic field interference is complex. Spin-exchange relaxation, even when suppressed, remains a significant factor restricting sensitivity gains in traditional alkali atomic magnetometers.

Recent proposals explore a shift away from alkali vapors, using optical access to the ground state of spin-polarized noble gas atoms as a detection method, offering the potential for extended spin relaxation times exceeding 10 ms and, in specialized cells, reaching the minute scale. However, even with this novel approach, a previously overlooked factor limits performance: laser light intensity noise, quantified as relative intensity noise (RIN).

This fractional variance in laser intensity encompasses both technical and fundamental noise sources like shot noise. The analysis demonstrates that RIN is a critical parameter, influencing the ultimate sensitivity achievable with these noble gas-based sensors.

Two-Photon Absorption for Xenon-129 Magnetometry

Optimizing the diameter of the laser beam illuminating the xenon gas is critical for maximizing sensor sensitivity, according to calculations detailed in a new analysis. This parameter, alongside laser power and intensity noise, significantly influences the ultimate performance of the quantum sensor. The work demonstrates that a carefully tuned beam size is essential to fully realize the potential of this novel magnetometry approach. This projected sensitivity surpasses that of many existing cryogenic magnetometers, a substantial advancement given the complexity and expense typically associated with maintaining extremely low temperatures.

The analysis further specifies that achieving this level of performance relies on a precise balance between laser power and noise characteristics. The two-photon transition rate per atom, a key factor in signal generation, is dependent on the excitation intensity and the two-photon absorption coefficient of xenon.

Researchers accounted for natural and pressure-broadening effects, calculating a linewidth of approximately 1.7 kHz at a pressure of 1 mbar. The most recently reported value for the two-photon absorption coefficient was also incorporated into the model, alongside other variables considered in the sensitivity analysis. These calculations highlight the importance of accurately characterizing the xenon atom’s properties for optimal sensor design. The team’s work also details the quantum efficiency for infrared and ultraviolet light, reporting values of 0.68 and 0.45 respectively.

These efficiencies, combined with the calculated linewidth and absorption coefficient, contribute to a comprehensive understanding of the sensor’s performance limits. The analysis establishes that optimizing beam size, alongside managing laser power and minimizing intensity noise, is paramount to achieving leading magnetic field detection.

Laser Power and Intensity Noise Impact Magnetometer Performance

Optimizing beam size also plays a role in maximizing sensitivity, with calculations showing ideal diameters ranging from 0.1 to 10 millimeters. This range reflects the need to balance spin projection noise and photon shot noise, demonstrating that a single, universally optimal beam size does not exist, but rather depends on specific experimental parameters. This analysis marks the first detailed consideration of laser intensity noise’s impact on magnetometry using noble gases, specifically ^(129)Xe, without the need for alkali vapors.

The analysis further addresses the contribution of noise sources, finding that the shot noise from the ultraviolet excitation light is negligible given the considered laser parameters. Calculations estimate this contribution to be between -178 and -149 dBc/Hz, significantly lower than the RIN values of -120 to -80 dBc/Hz being targeted.

The team proposes employing a balanced detector scheme to further mitigate the effects of RIN by suppressing common noise present in both the excitation laser and the emitted fluorescence. This approach, combined with careful control of laser power and intensity noise, positions this method as a promising pathway toward highly sensitive, room-temperature magnetic field detection.

Calculated 24 aT/√Hz Sensitivity with 10W UV Laser

This projected performance relies on maintaining a relative intensity noise (RIN) of -100 dBc/Hz during the measurement, a specific technical target for future sensor development. The analysis details how optimizing laser power and minimizing RIN are both key, but the dominant factor shifts depending on experimental conditions. The predicted sensitivity is calculated using equations that account for contributions from shot noise, spin projection noise and technical noise sources, demonstrating a precise balance between these factors.

Contour plots generated by the researchers illustrate how sensitivity is affected by varying laser power and RIN, highlighting the interplay between fundamental and technical limitations. For example, a 1 W laser with a RIN of -80 dBc/Hz yields a sensitivity of 148 aT/√Hz, a slight improvement over existing SQUID technology, but one where reducing RIN is the most effective path to further gains. The data supporting these findings are openly available, enabling independent verification and further research.

Optimizing Beam Size for Spin Projection and Shot Noise

Optimizing beam size presents a critical pathway to maximizing sensitivity in novel quantum magnetic sensors, with calculations revealing an ideal range of 0.1 to 10 millimeters. Unlike previous work, this analysis considers how laser intensity noise impacts alkali-free ^(129)Xe magnetometry, a previously unexamined factor. The sensitivity stemming from spin projection noise decreases as the beam expands, because a larger beam illuminates more xenon atoms and increases the number involved in the interaction.

Conversely, both shot noise and detector noise increase with beam size, as the average fluorescence scales inversely with the square of the beam waist. The optimal beam size is therefore determined by minimizing the combined effect of these noise sources, a quadrature sum calculated to pinpoint the best performance. The analysis further details how the contribution of relative intensity noise (RIN) remains constant regardless of beam size, as it does not depend on this parameter.

This is because RIN is independent of beam size, and therefore does not affect the sensitivity. This independence allows for focused optimization of other parameters to further enhance sensor performance. The work identifies three independent variables, measurement time, laser RIN and laser power, that, when carefully controlled, can unlock record-breaking sensitivity.

Xenon-129 Magnetometry: An Alkali-Free Approach

This level of sensitivity, detailed in a paper from the University of Alberta, relies on a 10 W ultraviolet laser to detect magnetic fields with precision. Achieving this sensitivity requires careful consideration of beam size, with optimal dimensions ranging from 0.1 to 10 mm, a range previously unexamined in this type of magnetometer. The analysis further reveals that increasing laser power and simultaneously reducing laser intensity noise are both essential to maximizing sensor performance.

“We show that a higher laser power than utilized to date is required for achieving and exceeding magnetometric sensitivity,” the paper states, highlighting the need for both powerful and stable laser sources. This independence is a key advantage, streamlining the development process and potentially reducing costs compared to systems reliant on complex cryogenic cooling.

👉 More information
🗞 Towards noble gas quantum optical magnetometry using direct ultraviolet detection
✍️ James Maldaner and Gil Porat
🧠 DOI: http://link.aps.org/doi/10.1103/nqvv-hbym

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