We study the use of squeezed probe light and evasion of measurement backaction to enhance the sensitivity and measurement bandwidth of an optically pumped magnetometer (OPM) at sensitivity-optimal atom number density. By experimental observation, and in agreement with quantum noise modeling, a spin-exchange-limited OPM probed with off-resonance laser light is shown to have an optimal sensitivity determined by density-dependent quantum noise contributions. Application of squeezed probe light boosts the OPM sensitivity beyond this laser-light optimum, allowing the OPM to achieve sensitivities that it cannot reach with coherent-state probing at any density. The observed quantum sensitivity enhancement at optimal number density is enabled by measurement backaction evasion.
We study the use of squeezed probe light and evasion of measurement backaction to enhance the sensitivity and measurement bandwidth of an optically pumped magnetometer (OPM) at sensitivity-optimal atom number density. By experimental observation, and in agreement with quantum noise modeling, a spin-exchange-limited OPM probed with off-resonance laser light is shown to have an optimal sensitivity determined by density-dependent quantum noise contributions. Application of squeezed probe light boosts the OPM sensitivity beyond this laser-light optimum, allowing the OPM to achieve sensitivities that it cannot reach with coherent-state probing at any density. The observed quantum sensitivity enhancement at optimal number density is enabled by measurement backaction evasion.
We study the effect of optical polarization squeezing on the performance of a sensitive, quantum-noise-limited optically pumped magnetometer. We use Bell-Bloom (BB) optical pumping to excite a ^{87}Rb vapor containing 8.2×10^{12} atoms/cm^{3} and Faraday rotation to detect spin precession. The sub-pT/sqrt[Hz] sensitivity is limited by spin projection noise (photon shot noise) at low (high) frequencies. Probe polarization squeezing both improves high-frequency sensitivity and increases measurement bandwidth, with no loss of sensitivity at any frequency, a direct demonstration of the evasion of measurement backaction noise. We provide a model for the quantum noise dynamics of the BB magnetometer, including spin projection noise, probe polarization noise, and measurement backaction effects. The theory shows how polarization squeezing reduces optical noise, while measurement backaction due to the accompanying ellipticity antisqueezing is shunted into the unmeasured spin component. The method is compatible with high-density and multipass techniques that reach extreme sensitivity.
We study the effect of optical polarization squeezing on the performance of a sensitive, quantum-noise-limited optically pumped magnetometer. We use Bell-Bloom (BB) optical pumping to excite a Rb87 vapor containing 8.2×1012 atoms/cm3 and Faraday rotation to detect spin precession. The sub-pT/Hz sensitivity is limited by spin projection noise (photon shot noise) at low (high) frequencies. Probe polarization squeezing both improves high-frequency sensitivity and increases measurement bandwidth, with no loss of sensitivity at any frequency, a direct demonstration of the evasion of measurement backaction noise. We provide a model for the quantum noise dynamics of the BB magnetometer, including spin projection noise, probe polarization noise, and measurement backaction effects. The theory shows how polarization squeezing reduces optical noise, while measurement backaction due to the accompanying ellipticity antisqueezing is shunted into the unmeasured spin component. The method is compatible with high-density and multipass techniques that reach extreme sensitivity.
(English) This thesis describes experiments that employ squeezed light to improve the performance of a sensitive optically-pumped magnetometer (OPM). The squeezed light source employs parametric amplification of vacuum fluctuations to produce squeezed vacuum and polarization-squeezed light tunable around the Rb D1 line. The OPM employs Bell-Bloom optical pumping of a high density vapor (with atom number density 10^{13}) and paramagnetic Faraday rotation, also on the Rb D1 line. The setup allows convenient switching from probing with laser light to probing with polarization-squeezed light, to study the use of the latter in atomic magnetometry. The magnetometer shows sub-pT/Hz^{1/2} sensitivity, limited by quantum noise; spin projection noise at low frequencies (<100Hz) and photon shot noise at high frequencies. Probing with polarization squeezed light suppresses the photon shot noise by 2dB, limited by the available squeezing and optical losses in passing through the vapor. This shot-noise suppression improves the high-frequency sensitivity and increases the measurement bandwidth, with no observed loss of sensitivity at any frequency. This result confirms experimentally the expected evasion of measurement back-action noise in the Bell-Bloom magnetometer. The thesis also develops a physical model to explain the observed spin dynamics of the Bell-Bloom magnetometer. The model describes the combined spin and optical polarization dynamics using Bloch equations with stochastic drive and detection noise terms. A perturbative approach and Fourier methods are then used to obtain analytic expressions for the magnetometer's frequency response, spin projection noise and photon shot noise. The role of measurement back-action emerges from a study of this model. As polarization squeezing reduces optical noise in the detected Stokes parameter, the accompanying ellipticity anti-squeezing is shunted into the unmeasured spin component. The thesis also reports a study of squeezed-light-enhanced magnetometry at a range of atomic densities, from 2.18 10^{12} atoms/cm3 to 1.13 10^{13} atoms/cm3 . Operating with fixed conditions of optical pumping, the signal amplitude, instrument noise spectrum and magnetic resonance width are measured as a function of atomic number density, for both laser- and squeezed-light probing. The equivalent magnetic noise spectra are then calculated. In the photon-shot-noise-limited portion of the spectrum, the squeezed light probing improves the magnetometer's sensitivity and measurement bandwidth for the full range of atomic density values. In particular, the laser-probed magnetometer shows a sensitivity optimum at n ~ 6 10 ^{12} atoms/cm3, and the squeezed-light-probed magnetometer surpasses this sensitivity. The thesis concludes with a discussion of the potential of stronger optical squeezing to enhance the instrument's sensitivity in different portions of the spectrum. Using the theory model we estimate the enhancement of the equivalent magnetic noise spectrum for 2 dB , 5.6 dB and perfect squeezing (zero noise in the detected polarization component) at the input to the atomic medium. (Català) Aqueta tesi descriu la millora d’un magnetòmetre de bombeig òptic (OPM) mitjançant l’ús d’estats de llum amb incertesa comprimida (squeezed states). S’usa amplificació paramètrica per a comprimir la incertesa de la font de llum. En concret, es comprimeix la incertesa de l’estat de buit quàntic, com també de la polarització òptica, amb la possibilitar d’ajustar la longitud d’ona al voltant de la transició atòmica D1 de 87Rb. L’OPM usa bombeig òptic Bell-Bloom de vapors d’alta densitat (amb densitats atòmiques properes a 1013) i rotació de Faraday, també al voltant de la transició atòmica D1 de 87Rb. L’aparell experimental permet canviar de mostreig amb llum coherent làser a mostreig amb llum de polarització comprimida, amb la finalitat d’avaluar el seu impacte en la sensitivitat del magnetòmetre. El magnetòmetre té una sensitivitat de sub-pT{ ?Hz , principalment limitada per soroll quàntic; soroll de projecció de spin a baixes freqüències (À 100 Hz) i soroll de quantització fotònica a altes freqüències. L’ús d’estats de llum amb polarització comprimida permet reduir el soroll fotònic en „ 2 dB, limitat per la compressió disponible i les pèrdues en travessar el vapor atòmic. La supressió del soroll fotònic augmenta l’amplada de banda del sistema amb l’avantatge de no perdre sensitivitat a cap banda de freqüència. Els resultats experimentals confirmen l’esperada supressió de retroalimentació de soroll en magnetòmetres de Bell-Bloom. La tesi també estudia el model teòric darrere les dinàmiques de spin en un magnetòmetre de tipus Bell-Bloom. El model descriu la combinació de les dinàmiques de spin i de la polarització òptica mitjançant equacions de Bloch forcades estocàsticament i amb termes de soroll de detecció. Es treballa en el límit pertorbatiu on mitjançant mètodes de Fourier s’obtenen expressions analítiques de la resposta en freqüència del magnetòmetre, dels sorolls de projecció de spin i del soroll de quantització fotònica. El rol de la retroalimentació de soroll també s’extrau d’aquest model. En concret, s’observa que la compressió en polarització redueix el soroll en els paràmetres de Stokes detectats, mentre els paràmetres de spin no mesurats experimenten una expansió de la seva incertesa (anti-squeezing). La tesi estudia magnetòmetres òptics de llum amb incertesa comprimida per a densitats entre 2.18 ˆ 1012 atoms{cm3 i 1.13 ˆ 1013 atoms{cm3. Es mesuren l’amplitud de senyal, l’espectre de soroll i l’amplada de la ressonància magnètica en funció de la densitat atòmica, per a un bombeig òptic constant i per a ambdós tipus de mostreig òptic (llum coherent i llum de polarització comprimida). A continuació, es calculen els espectres de soroll equivalents. En la part d’espectre on domina el soroll de quantització fotònica, s’observa que l’ús de llum de polarització comprimida millora la sensitivitat del magnetòmetre al llarg de tot el rang de densitats atòmics. En concret, la sensitivitat del magnetòmetre amb mostreig coherent és òptima per a n « 6ˆ1012 atoms{cm3 i es demostra una millora amb l’ús de mostreig amb llum comprimida. Es conclou amb una discussió sobre l’efecte de compressions més severes en la sensitivitat del magnetòmetre. Mitjançant el model teòric s’estima la millora en la sensitivitat per a compressions de 2 dB, 5.6 dB i “compressió perfecta” a l’entrada del medi atòmic
We study the effect of optical squeezing on the performance of a sensitive, quantum-noise-limited optically-pumped magnetometer. We use Bell-Bloom optical pumping to excite a 87Rb vapor and Faraday rotation to detect spin precession. The sub-pT/Hz sensitivity is limited by spin projection noise (photon shot noise) at low (high) frequencies. Probe polarization squeezing both improves high-frequency sensitivity and increases signal bandwidth. The accompanying polarization anti-squeezing perturbs only an unmeasured spin component, so there is no loss of sensitivity at any frequency. The method is compatible with high-density and multi-pass techniques that reach extreme sensitivity.
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.
The sensitivity of miniaturized optically pumped magnetometers (OPMs) is limited by weak atom-light coupling, which an optical cavity can enhance. In this work, we model the photon-shot-noise-limited sensitivity of cavity-enhanced OPMs in the regime of strongly collisionally broadened optical transitions, characteristic of buffer-gas-filled miniaturized vapor cells. The cavity enhancement is benchmarked against a single-pass free-induction-decay OPM employing Faraday-rotation readout, with the probe power and detuning jointly optimized using the Cramér-Rao lower bound as a figure of merit. For a Fabry-Pérot cavity, we compare side-of-fringe, homodyne, Pound-Drever-Hall, and Faraday-rotation readout. All four yield an optimal sensitivity enhancement scaling as α2F/π, where F is the cavity finesse and 0.5≤α≤1 is a readout-dependent prefactor. The enhancement is maximized at critical coupling, and we quantify its degradation away from this point. We further show that, despite spin-dependent absorption associated with the ensemble's vector polarizability, near-critical coupling can be maintained throughout spin precession at arbitrary finesse by exceeding a derived probe-power threshold and increasing the atomic detuning with finesse. We also establish a limit to the maximum cavity enhancement set by vector light-shift noise.