Quantum non-demolition measurement with an off-resonance polarized probe is a widely utilized technique for atomic sensors. Typically, QND instruments read out spin states through optical polarimetry using a linearly off-resonance probe in atomic media. This optical read-out process offers high accuracy, with potential applications in biomagnetism, inertial navigation and new physics exploration beyond the standard model. It was observed that previously unexplored spin-alignment polarization induced by the far-detuned off-resonance linear probe in an ultrasensitive optical polarimetry. The evolution of probe-generated multipole moments exacerbates probe noise due to strong magnetic couplings. A method was demonstrated to decouple spin-alignment from magnetic fields by manipulating the multipole moments in zero fields. A suppression of the probe noise by 8.2 dB at 1 Hz and 10.4 dB at 10 Hz with the proposed method, reaching the standard quantum limit, was achieved. The realized shot-noise-limited optical polarimetry shows significant potential for advancing the performance of atomic sensors, particularly in applications requiring ultra-high sensitivity, such as precision measurements and quantum metrology. These findings provide a pathway toward enhanced sensor precision in diverse scientific and technological applications.
Quantum non-demolition measurement with an off-resonance polarized probe is a widely utilized technique for atomic sensors. Typically, QND instruments read out spin states through optical polarimetry using a linearly off-resonance probe in atomic media. This optical read-out process offers high accuracy, with potential applications in biomagnetism, inertial navigation and new physics exploration beyond the standard model. It was observed that previously unexplored spin-alignment polarization induced by the far-detuned off-resonance linear probe in an ultrasensitive optical polarimetry. The evolution of probe-generated multipole moments exacerbates probe noise due to strong magnetic couplings. A method was demonstrated to decouple spin-alignment from magnetic fields by manipulating the multipole moments in zero fields. A suppression of the probe noise by 8.2 dB at 1 Hz and 10.4 dB at 10 Hz with the proposed method, reaching the standard quantum limit, was achieved. The realized shot-noise-limited optical polarimetry shows significant potential for advancing the performance of atomic sensors, particularly in applications requiring ultra-high sensitivity, such as precision measurements and quantum metrology. These findings provide a pathway toward enhanced sensor precision in diverse scientific and technological applications.
Quantum non-demolition measurement with an off-resonance polarized probe is a widely utilized technique for atomic sensors. We observe previously unexplored spin-alignment polarization induced by the far-detuned off-resonance linear probe in an ultrasensitive optical polarimetry. The evolution of probe-generated multipole moments exacerbates probe noise due to strong magnetic couplings. We demonstrate a method to decouple spin-alignment from magnetic fields by manipulating the multipole moments in zero-fields. The probe noise is suppressed by 10 dB with the proposed method, achieving a noise floor at the standard quantum limit. The realized shot-noise-limited optical polarimetry holds significant promise for atomic sensors aimed at ultra-sensitivity.
Optical rotation detection system (ORDS) utilizing quantum nondemolition (QND) measurement methods is widely applied in the field of quantum metrology and quantum information. However, the sensitivity of ORDS is limited by the uncertainty from optical-couple noise during the measurement of the atomic spin ensemble. In this study, we specifically analyze the mechanism of optical-couple noise caused by the fluctuations of probe light’s polarization in the modulated ORDS with a new model established to describe atomic spin precession in this particular condition. It is discovered that transverse electron-spin polarization errors are generated by the residual probe photon spin polarization in the ORDS, which results in extra coupling magnetic noise. In order to suppress this noise, a novel in situ method is proposed that the resultant electron-spin errors are reduced by a specifically designed closed-loop system. The results are verified through the ORDS in a co-magnetometer. After zeroing the extra probe photon spin polarization, an angular sensitivity better than1×10−8rad/Hz1/2is achieved for frequencies higher than 5 Hz, demonstrating a probe background noise of 0.26 fT/Hz1/2@14.5 Hz, approaching electronic noise and photon shot noise (PSN). With closed-loop control, the optical rotation bias instability is promoted by 4.2 times (from 107.9 to$25.7 \mu $rad/h), and the angular noise of the ORDS is reduced by 2.4 times at 1–100 Hz. The measurement uncertainty of the realized ORDS nears the standard quantum limit, paving the road for long-term ultrasensitive measurements for new physics explorations.
At the beginning of the 20th century measurement precision was related exclusively to the capabilities of the experimenter and the apparatus. Any imprecision was attributed to imperfect devices for the measurement. Later on, when the quantumness of objects like photons, electrons, or atoms, was recognized, it became clear that the measurement and the object to be measured could no longer be separated. Both follow the laws of quantum physics which impose limits on the accuracy of any measurement. In the last three decades, a lot of research has been dedicated to find strategies to overcome these limitations. This new field of quantum metrology is where this work is placed.
This thesis describes the generation of interparticle entanglement in form of a squeezed spin state in a cloud of cold 87Rb atoms by performing nondestructive spin measurements. Towards this goal, we have implemented a shot-noise limited polarization probing and detection system, atom counting by means of absorption imaging, spin state preparation via optical pumping and nondestructive spin state detection. The quantum noise limited spin readout with polarized light is characterized in two complementary ways in terms of sensitivity and all possible quantum and classical noise sources are measured.
To address the rich opportunities provided in the multilevel structure of 87Rb with dispersive probing, we have developed and implemented a frequency offsetlock which allows a large tuning range of several GHz for the dispersive probing.
Working with a magnetically sensitive atomic system triggered the development of a magnetic field imaging technique. This applies the elongated atomic sample and internal atomic states as a way of measuring magnetic fields over several millimeters with micrometer resolution and sensitivities down to a few tens of pT/ Hz.
We developed a new method for tomographic measurements of the atomic density matrix. Based on our ability of shot noise limited detection of polarization rotations we can in principle, characterize the whole atomic spin state.
First, measurements are shown and possible improvements are discussed.
An in depth characterization of the light-atom interaction between polarized pulses of light and collective atomic states are the basis for measurements at the quantum level. On one side, we apply classical measurements of spin polarized atoms to measure the strength of the lightatom interaction. On the other side, we use quantum measurements of unpolarized atomic states to characterize all noise sources both technical and quantum.
One of the major results of this thesis is the realization of quantum nondemolition measurements in a largespin system. We realized limitations to the performance of the QND measurement implied by higher order light shifts in multilevel atoms, which are often neglected for these kind of systems. We develop a technique which suppresses the detrimental effects of higher order light shifts and recovers an ideal QND measurement.
The thesis culminates in the demonstration of spin squeezing in the magnetically sensitive ground state of 87Rb. We achieve a quantum noise reduction by 3 dB, where metrological sensitivity is improved by 2 dB beyond the standard quantum limit of a coherent spin state with the same spin polarization.
We address the question how spatial and temporal inhomogeneities both in light and atoms influence the preparation of nonclassical states. An existing covariance matrix approach is extended and we apply it to study effects of detector time resolution, spatial inhomogeneities and atomic motion.
Atomic projection noise limits the ultimate precision of all atomic sensors, including clocks, inertial sensors, magnetometers, etc. The independent quantum collapse of N atoms into a definite state (for example spin up or down) leads to an uncertainty ΔθSQL= 1/√N in the estimate of the quantum phase accumulated during a Ramsey sequence or its many generalizations. This phase uncertainty is referred to as the standard quantum limit. Creating quantum entanglement between the N atoms can allow the atoms to partially cancel each other's quantum noise, leading to reduced noise in the phase estimate below the standard quantum limit. Recent experiments have demonstrated up to 10 dB of phase noise reduction relative to the SQL by making collective spin measurements. This is achieved by trapping laser-cooled Rb atoms in an optical cavity and precisely measuring the shift of the cavity resonance frequency by an amount that depends on the number of atoms in spin up. Detecting the probe light with high total efficiency reduces excess classical and quantum back-action of the probe. Here we discuss recent progress and a technique for reducing the relative frequency noise between the probe light and the optical cavity, a key requirement for further advances.
Ultrasensitive atomic comagnetometers have proven to be valuable tools for precise measurement and search for new physics. Besides, comagnetometers have potential for inertial navigation and quantum information. Despite their great potential, sensitivity improvement has been limited by different types of noise. To address this issue, the sources and magnitudes of noises are comprehensively analyzed and a suppression method is proposed for the most critical noise source, optical non-orthogonality. The optical non-orthogonality leads to critical transverse pumping effect, thus transverse light shift, and transverse effective magnetic noise. A direct correlation between the optical non-orthogonality and the electronic spin transverse relaxation rate is found, and a method for suppression of the optical non-orthogonality is proposed accordingly. Finally, the optical non-orthogonal noise is suppressed by 3.1 dB. Besides, an increase of 5.2 dB in the scale factor is achieved. With the combined effect of noise suppression and scale factor enhancement, the resulting inertial rotation sensitivity of 2.4×10-8rad/s/Hz1/2in the frequency range from 1 to 3 Hz is among the most sensitive comagnetometers, which opens up promising opportunities for new physics measurements, such as the search for ultralight dark matter and the fifth force.
Atomic spin sensors are essential for beyond-the-standard-model exploration, biomagnetic measurement, and quantum navigation. While the traditional DC mode spin-exchange relaxation-free (SERF) comagnetometer achieves ultrahigh sensitivity, further improvements require suppressing technical noise and surpassing standard quantum limit. In this work, we develop a K-Rb- 21 Ne SERF atomic comagnetometer that dynamically polarizes the electron and nuclear spins, shielding signals from direct interference by pump light. We establish a three-phase evolutionary model for hybrid spin ensemble dynamics, yielding a complete analytical solution, and analyze the responses to various spin perturbations. Additionally, we achieve an average 39% suppression of the polarization noise and identify the key factors that limit sensitivity improvements. The dynamically polarized comagnetometer exhibits effective suppression of technical noise and holds the potential to overcome quantum noise limit while offering promising applications in exploring new physics and precise magnetic field measurements.