Abstract The principal limitations imposed by spin (or atomic) quantum projection noise on the sensitivity of optical quantum sensors on thermal atoms, which include frequency standards, magnetometers, and gyroscopes that use optical detection of electron paramagnetic resonance, have been theoretically and experimentally studied. The effect of increasing the rms amplitude of the projection noise in a magnetometric scheme under the influence of a strong optical pumping has been demonstrated. Its explanation has been proposed and experimentally confirmed: it has been shown that this effect is explained by the invariance of the integral power of projection noise in relation to the magnetic resonance linewidth in a wide range of pumping intensities An experimental study of the parameters of projection noise in a magnetometric quantum sensor has been conducted and recommendations for optimizing the sensor parameters have been given.
Abstract The principal limitations imposed by spin (or atomic) quantum projection noise on the sensitivity of optical quantum sensors on thermal atoms, which include frequency standards, magnetometers, and gyroscopes that use optical detection of electron paramagnetic resonance, have been theoretically and experimentally studied. The effect of increasing the rms amplitude of the projection noise in a magnetometric scheme under the influence of a strong optical pumping has been demonstrated. Its explanation has been proposed and experimentally confirmed: it has been shown that this effect is explained by the invariance of the integral power of projection noise in relation to the magnetic resonance linewidth in a wide range of pumping intensities An experimental study of the parameters of projection noise in a magnetometric quantum sensor has been conducted and recommendations for optimizing the sensor parameters have been given.
The paper theoretically and experimentally investigates the fundamental limitations imposed by spin (or atomic) quantum projection noise on the sensitivity of optical quantum sensors based on thermal atoms (this class of devices includes frequency standards, magnetometers, and gyroscopes using optical detection of electron paramagnetic resonance). The effect of increasing the rms amplitude of the projection noise in the magnetometric scheme under the influence of strong optical pumping is demonstrated, its explanation is proposed and experimentally tested - it is shown that in a wide range of pump intensities this effect is explained by the invariance of the projection noise integral power with respect to the magnetic resonance line width. An experimental study of the parameters of projection noise in a magnetometric quantum sensor was carried out, recommendations were given for optimizing the sensor parameters.
Present protocols for obtaining the ultimate magnetic sensitivity of optically pumped magnetometers (OPMs) utilizing alkali-metal ensembles rely on uncorrelated atoms in stretched states. A new approach for calculating the spin projection noise (SPN)-limited signal to noise ratio (SNR) and the magnetic sensitivity of OPMs is proposed. Our model is based solely on the mean-field density matrix dynamics and in contrast to previous models, it applies to both low and high field regimes, it takes into account the degree of spin polarization, the intra- and interhyperfine correlations, the decoherence processes, the atom-light coupling and the effects of the spin dynamics on the spin-noise spectra. Fine tuning of the probe frequency allow us to explore different hyperfine states and ground-state correlations. Especially in the spin-exchange-relaxation-free (SERF) regime, alongside the magnetic resonance narrowing and the increased number density, hallmarks of SERF magnetometers, we report on a new SERF feature; the reduction of spin-projection noise at the spin precession frequency as a consequence of strongly-correlated hyperfine spins that attenuate and redistribute SPN when properly probed.
Quantum manipulation technology and photoelectric detection technology have jointly facilitated the rapid development of ultra-sensitive atomic spin magnetometers. To improve the output signal and sensitivity of the spin-exchange-relaxation-free (SERF) atomic spin magnetometer, the noises influencing on the output signal and the sensitivity were analyzed, and the corresponding noise suppression methods were presented. The magnetic field noises, including the residual magnetic field noise and the light shift noise, were reduced to approximately zero by employing the magnetic field compensation method and by adjusting the frequency of the pump beam, respectively. With respect to the operation temperature, the simulation results showed that the temperature of the potassium atomic spin magnetometer realizing the spin-exchange relaxation-free regime was 180 °C. Moreover, the fluctuation noises of the frequency and the power were suppressed by using the frequency and the power stable systems. The experimental power stability results showed that the light intensity stability was enhanced 10%. Contrast experiments on the sensitivity were carried out to demonstrate the validity of the suppression methods. Finally, a sensitivity of 13 fT/Hz(1/2) was successfully achieved by suppressing noises and optimizing parameters.
We study quantum enhancement of sensitivity using squeezed light in a multiparameter quantum sensor, the hybrid dc and rf optically pumped magnetometer (hOPM) [M. Lipka , Multiparameter quantum sensing and magnetic communication with a hybrid dc and rf optically pumped magnetometer, , 034054 (2024)]. Using a single spin ensemble, the hOPM acquires both the dc field strength (scalar magnetometry) and resonantly detects one quadrature of the ac magnetic field at a chosen frequency (rf magnetometry). In contrast to the Bell-Bloom scalar magnetometer [C. Troullinou , Squeezed-light enhancement and backaction evasion in a high sensitivity optically pumped magnetometer, , 193601 (2021)], the backaction evasion in the hOPM is incomplete, leading to a nontrivial interplay of the three quantum noise sources in this system: photon shot noise, spin projection noise, and measurement backaction noise. We observe these interactions using squeezed light as a tool to control the distribution of optical quantum noise between S 2 and S 3 polarization Stokes components, and the resulting effect on readout quantum noise and measurement backaction. These results demonstrate quantum-enhanced sensitivity in a continuously operating multiparameter sensor and reveal fundamental trade-offs between sensitivity, backaction, and bandwidth.
We study quantum enhancement of sensitivity using squeezed light in a multi-parameter quantum sensor, the hybrid dc-rf optically pumped magnetometer (hOPM) [Phys. Rev. Applied 21, 034054, (2024)]. Using a single spin ensemble, the hOPM acquires both the dc field strength (scalar magnetometry), and resonantly detects one quadrature of the ac magnetic field at a chosen frequency (rf magnetometry). In contrast to the Bell-Bloom scalar magnetometer [Phys. Rev. Lett. 127, 193601 (2021)], the back-action evasion in the hOPM is incomplete, leading to a nontrivial interplay of the three quantum noise sources in this system: photon shot noise, spin projection noise, and measurement back-action noise. We observe these interactions using squeezed light as a tool to control the distribution of optical quantum noise between S2 and S3 polarization Stokes components, and the resulting effect on readout quantum noise and measurement back-action. These results demonstrate quantum-enhanced sensitivity in a continuously operating multi-parameter sensor and reveal fundamental trade-offs between sensitivity, back-action, and bandwidth.
For spin-polarized ensembles of alkali-metal atoms with hyperfine structure and spin greater than 1/2, the interplay between spin dynamics and quantum noise mechanisms remains largely unexplored, especially in the spin-exchange-relaxation-free (SERF) regime. In particular, the theoretical understanding of the dynamics for the mean values of the spin operators was established shortly after the advent of optical pumping. In contrast, a corresponding theory for spin fluctuations has not yet been developed, and is addressed in this work. Here, we develop such a first-principles theory of spin fluctuations in alkalimetal vapors. The nonlinearity bottleneck of spin-exchange collisions, which dominate the physics of such vapors, is overcome by linearizing the dynamics around the equilibrium density matrix. The result is a precise calculation of the absolute magnetic sensing limit of optically pumped atomic magnetometers, which can precisely benchmark quantum limited and quantum enhanced protocols, in contrast to the traditional approach shown to be adequate only within a factor of 2. Further, we elucidate how signal and noise in atomic magnetometers are distributed among the spin eigenmodes defined under continuous optical pumping and probing via the paramagnetic Faraday rotation. Notably, in the SERF regime, alongside the magnetic resonance narrowing, we find a reduction of spin noise at the spin-precession frequency as a consequence of strong hyperfine correlations redistributing spin noise, when both hyperfine manifolds are simultaneously probed. These insights can lead to improved quantum devices utilizing alkali-metal atoms or similar spin-based sensing platforms.