In the past few decades, optical magnetometry has experienced remarkable development and reached to an outstanding sensitivity. For magnetometry based on optical readout of atomic ensemble, the fundamental limitation of sensitivity is restricted by spin projection noise and photon shot noise. Meanwhile, in practical applications, ambient magnetic noise also greatly limits the sensitivity. To achieve the best sensitivity, it is essential to find an efficacious way to eliminate the noises from different sources, simultaneously. Here, we demonstrate a quantum magnetic gradiometer with sub-shot-noise sensitivity using entangled twin beams with differential detection. The quantum enhancement spans a frequency range from 7 Hz to 6 MHz with maximum squeezing of 5.5 dB below the quantum noise limit. The sensitivity of gradiometer reaches 18 fT/cm[Formula: see text] at 20 Hz. Our study inspires future possibilities to use quantum-enhanced technology in developing sensitive magnetometry for practical applications in noisy and physically demanding environments.
In the past few decades, optical magnetometry has experienced remarkable development and reached to an outstanding sensitivity. For magnetometry based on optical readout of atomic ensemble, the fundamental limitation of sensitivity is restricted by spin projection noise and photon shot noise. Meanwhile, in practical applications, ambient magnetic noise also greatly limits the sensitivity. To achieve the best sensitivity, it is essential to find an efficacious way to eliminate the noises from different sources, simultaneously. Here, we demonstrate a quantum magnetic gradiometer with sub-shot-noise sensitivity using entangled twin beams with differential detection. The quantum enhancement spans a frequency range from 7 Hz to 6 MHz with maximum squeezing of 5.5 dB below the quantum noise limit. The sensitivity of gradiometer reaches 18 fT/cm[Formula: see text] at 20 Hz. Our study inspires future possibilities to use quantum-enhanced technology in developing sensitive magnetometry for practical applications in noisy and physically demanding environments.
Abstract In the last few decades, the atomic magnetometry [1–7] has experienced remarkable development and reached to an outstanding sensitivity. This exciting technology is finding applications in various of fields [8–18]. The last two obstacles to further improve the sensitivity of magnetometry are quantum fluctuations and ambient magnetic field. For magnetometry based on optical readout of atomic ensemble’s spin precession, the fundamental limitation of sensitivity is restricted by spin projection noise (SPN) and photon shot noise (PSN). Meanwhile, in practical applications, ambient magnetic field noise which is regarded as a common-mode magnetic-field (CMM) noise is also limiting the sensitivity. Several techniques for reducing PSN [19–25], SPN [26, 27], and CMM noise [7, 28–31] have been proposed, respectively. To achieve the best sensitivity, it is essential to find an efficacious way to eliminate the noises from different sources, simultaneously. For this purpose, here we demonstrate a sub-shot-noise magnetic gradiometer utilizing entangled optical detection with quantum-correlated twin beams. This leads to the simultaneous suppression of PSN and CMM noise. The quantum enhancement spans a frequency range from 7 Hz to 6 MHz with maximum squeezing of 5.5 dB below the standard quantum limit (SQL). The sensitivity of gradient magnetic field reaches 18 fT/cm√Hz at 20 Hz with these sophisticated techniques. Our study opens up new possibilities to use quantum-enhanced technology in developing sensitive magnetometry for practical applications with noisy and physically demanding environments.
We present a quantum magnetic gradiometer that achieves sub-shot-noise sensitivity through the use of entangled twin beams and differential detection. Benefiting from large quantum enhancement, high common-mode rejection ratio, and best sensitivity ever observed in magnetometry with squeezed light, the developed quantum gradiometer opens up new possibilities for utilizing quantum-enhanced technology in the development of sensitive magnetometry for practical applications in noisy and physically demanding environments.
Abstract Tensor measurements play a vital role in both fundamental sciences and interdisciplinary applications such as navigation and remote sensing which require sensitive perception of the gradient of vector fields. Fundamentally, these measurements rely on the estimation of multiple parameters, which is a key focus in metrology. Quantum metrology has achieved remarkable success in surpassing the standard quantum limit for single-parameter estimation, offering extraordinary sensitivity. Quantum multiparameter estimation has also been demonstrated recently with few-particle quantum systems. Here, we develop tensor quantum metrology by advancing multiparameter quantum sensing to an unprecedented level with both high scalability and multidimensional performance, fulfilling the essential demands on sensitivity for tensor measurements. We generate a bright, multidimensional polarization-entangled light source from an atomic ensemble, with three simultaneously squeezed Stokes operators, enabling superior performance in multiparameter quantum sensing compared to conventional polarization-entangled light. By employing such a quantum light in a tensor magnetometry, we achieve a 6 dB quantum enhancement in both direction and amplitude measurements of magnetic field gradient, with a directional gradient sensitivity of 21μrad/cm/√Hz and an amplitude gradient sensitivity of 6 fT/cm/√Hz. As the first realization of high-sensitivity quantum-enhanced sensing of a tensor field, this work marks a milestone in the advancement of quantum metrology.
The ability to measure magnetic fields with high sensitivity is a key requirement in many physical, biological and medical applications. Currently the most sensitive magnetometers are optical magnetometers, in which the polarization of an atomic sample responds to the field and is read out by an optical measurement. These instruments are limited by two fundamental sources of quantum noise: projection noise of the atoms and shot noise of the light. For magnetometers designed to have a high statistical sensitivity, contributions from both sources are comparable. Here we show that the sensitivity can be improved by the application of non-classical light.
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.
Optical atomic magnetometry is a powerful tool for continuous sensing applications, yet, in the absence of quantum correlations, its sensitivity is limited by the standard quantum limit (SQL) stemming from a trade-off between optical probe imprecision and quantum measurement backaction. Beyond-SQL sensitivity requires quantum correlations that modify these measurement noise sources. Here we demonstrate such sensitivity by using entangled state of the probe light and by engineering correlations between measurement imprecision and backaction. Having first explored SQL in a broad range of frequencies, we demonstrate overcoming the limit by combining variational readout with coupling the magnetometer to one mode of a bipartite entangled light state and conditioning the results on the other entangled mode. Tuning the detected light quadratures and combining the signals from the two measurement channels, we achieve sensitivity beyond the SQL in a broad range of acoustic frequencies which has so far remained inaccessible to quantum-noise-limited optical magnetometry.