*Important notice: This news reports on an unedited version of an accepted paper and is awaiting final editing. Therefore, the paper should not be regarded as conclusive or treated as established information.
Researchers have combined synchronized pulsed-probe readout with RF phase cycling and variable-delay window subtraction to separate probe-induced effects from atomic spin fluctuations. This combined novel approach reduced correlated noise by 3.25 ± 0.18 fT/√Hz and enabled the study of temporal spin correlations in warm-vapor optical quantum sensors. Their findings were published in Scientific Reports.
Study: High-density optical quantum sensors with pulsed probe read-out for correlated spin-noise reduction. Image Credit: Natali _ Mis/Shutterstock.com
Why Quantum Noise Limits Sensor Precision
Optical quantum sensors based on alkali-metal vapor enable magnetic-field measurements for biomagnetic imaging and fundamental physics experiments. These sensors utilize a circularly polarized pump laser to align atomic electron spins, while a linearly polarized probe laser measures their precession in response to an external magnetic field.
Once technical disturbances, including laser instability and environmental noise, are minimized, atomic spin-projection noise becomes a fundamental limit on measurement sensitivity. Quantum non-demolition (QND) measurements and spin-squeezing protocols offer pathways to manage quantum uncertainty while preserving useful spin information.
However, applying these approaches to warm-vapor systems faces challenges from spin relaxation, atomic diffusion, and probe-induced back-action.
In RF (radio-frequency) optical quantum sensors, sensitivity depends on the transverse spin-relaxation time, measurement volume, and atomic spin density. Additionally, probe intensity requires careful optimization: increasing it reduces photon shot noise but can also accelerate spin relaxation.
Using Pulsed Lasers to Track Atomic Spin Fluctuations
To investigate temporal spin correlations, researchers developed a radio-frequency optical quantum sensor using a potassium vapor cell measuring 2 × 2 × 2 cm3. The cell was enclosed in a cylindrical
ferrite magnetic shield and filled with helium and nitrogen.
Non-magnetic tungsten heating wires maintained the cell at 168 °C, with the heating current periodically switched off during data acquisition to reduce magnetic interference.
The optical system used a circularly polarized pump laser delivering about 20 mW near the potassium D1 resonance line to sustain atomic spin polarization. A linearly polarized probe beam, supplied by a tapered amplifier laser at a vacuum wavelength of approximately 769.60 nm, was oriented perpendicular to the pump beam and detuned 257 GHz from the resonance center.
An acousto-optic modulator converted the continuous probe light into pulses with an optimized 20% duty cycle to mitigate probe-induced light shifts.
The pulses were synchronized to twice the Larmor precession frequency, allowing successive measurements to sample consistent transverse spin projections. The transmitted probe signal passed through a polarizing beam splitter and was measured using balanced photodetectors. Custom transimpedance amplifiers converted the detector outputs into voltage signals, which a 24-bit data acquisition system digitized under LabVIEW control.
To separate coherent probe-induced effects from atomic spin fluctuations, the study combined RF phase cycling with a variable-delay window. This allowed the study to cancel reproducible probe artifacts and analyze temporal spin correlations within each cycle. Researchers identified an optimal probe-induced relaxation rate of 39 s-1 at 168 °C.
How Pulsed Probing Revealed Reduced Measurement Noise
At the operating temperature of 168 °C, independent calibration established a potassium vapor density of 2.5 × 1013 atoms/cm3. The instantaneous probe-induced spin-destruction rate reached 193 s-1, while the 20% duty cycle reduced the time-averaged rate to 39 s-1.
The effective transverse relaxation time was about 1 ms, corresponding to a fundamental Lorentzian resonance linewidth of 320 Hz. Measurements of magnetic-field amplitude spectral density at different delays between acquisition windows showed that noise increased with delay as temporal spin correlations decayed.
An exponential fit yielded an asymptotic noise level of 19.20 ± 0.17 fT/√Hz and a correlation-decay time of 0.62 ± 0.10 ms. At short delays near 1 ms, the measured noise decreased to nearly 15.95 fT/√Hz, compared with an asymptotic value of 19.20 ± 0.17 fT/√Hz at longer delays.
Download a free copy of this page here!
Noise calibration established background contributions. With the pump beam blocked, probe photon shot noise measured approximately 17.0 fT/√Hz, while intrinsic atomic spin noise was estimated at 6.4–8.6 fT/√Hz. Combining these contributions yields a predicted uncorrelated noise level of 18.16–19.05 fT/√Hz, consistent with the measurement.
The noise reduction at short delays indicates positive temporal correlations between successive measurements and is consistent with a conditional quantum non-demolition (QND) mechanism. However, residual probe-induced effects, atomic diffusion, and technical correlations cannot be excluded, and the experiment did not independently establish metrological spin squeezing below the standard quantum limit.
Implications for Future Research
The correlated-noise suppression method could support high-precision measurements in various fields, including biomagnetic imaging, where weak physiological magnetic fields must be distinguished from background noise. This approach could also benefit searches for low-mass axions and other dark matter candidates, where sensitive magnetometers detect weak RF magnetic signals.
The same techniques may also support precision experiments investigating exotic spin-dependent interactions and fundamental symmetries. Because the protocol avoids complex optical cavity resonators, it offers a relatively simple approach to studying correlated spin noise in high-density, warm-vapor sensors.
Toward More Precise Quantum Magnetic Sensing
This novel framework effectively investigated correlated spin noise and suppressed probe-induced artifacts in warm-vapor RF optical quantum sensors. By combining synchronized pulsed probing, RF phase cycling, and variable-delay window subtraction, the study measured delay-dependent spin correlations and achieved a correlated-noise reduction of 3.25 ± 0.18 fT/√Hz. Future work could establish independently calibrated projection-noise references to directly assess quantum enhancement.
Journal Reference
Savukov, I., and Kim, Y.J. (2026). High-density optical quantum sensors with pulsed probe read-out for correlated spin-noise reduction. Scientific Reports. DOI: 10.1038/s41598-026-73781-4, https://www.nature.com/articles/s41598-026-73781-4.
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.