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Atomic magnetometers are highly sensitive detectors of magnetic fields that monitor the evolution of the macroscopic magnetic moment of atomic vapors, and opening new applications in biological, physical, and chemical science. However, the performance of atomic magnetometers is often limited by hidden systematic effects that may cause misdiagnosis for a variety of applications, e.g., in NMR and in biomagnetism. In this work, we uncover a hitherto unexplained interference effect in atomic magnetometers, which causes an important systematic effect to greatly deteriorate the accuracy of measuring magnetic fields. We present a standard approach to detecting and characterizing the interference effect in, but not limited to, atomic magnetometers. As applications of our work, we consider the effect of the interference in NMR structural determination and locating the brain electrophysiological symptom, and show that it will help to improve the measurement accuracy by taking interference effects into account. Through our experiments, we indeed find good agreement between our prediction and the asymmetric amplitudes of resonant lines in ultralow-field NMR spectra -- an effect that has not been understood so far. We anticipate that our work will stimulate interesting new researches for magnetic interference phenomena in a wide range of magnetometers and their applications.
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Continuously monitored atomic spin-ensembles allow, in principle, for real-time sensing of external magnetic fields beyond classical limits. Within the linear-Gaussian regime, thanks to the phenomenon of measurement-induced spin-squeezing, they attain a quantum-enhanced scaling of sensitivity both as a function of time, $t$, and the number of atoms involved, $N$. In our work, we rigorously study how such conclusions based on Kalman filtering methods change when inevitable imperfections are taken into account: in the form of collective noise, as well as stochastic fluctuations of the field in time. We prove that even an infinitesimal amount of noise disallows the error to be arbitrarily diminished by simply increasing $N$, and forces it to eventually follow a classical-like behaviour in $t$. However, we also demonstrate that, thanks to the presence of noise, in most regimes the model based on a homodyne-like continuous measurement actually achieves the ultimate sensitivity allowed by the decoherence, yielding then the optimal quantum-enhancement. We are able to do so by constructing a noise-induced lower bound on the error that stems from a general method of classically simulating a noisy quantum evolution, during which the stochastic parameter to be estimated -- here, the magnetic field -- is encoded. The method naturally extends to schemes beyond the linear-Gaussian regime, in particular, also to ones involving feedback or active control.
We present an experimental and theoretical study of phase-dependent interference effects in multi-photon excitation under bichromatic radio-frequency (rf) field. Using an intense rf pulse, we study the interference between the three-photon and one-photon transition between the Zeeman sub-levels of the ground state of $^{87}$Rb that allows us to determine the carrier-envelope phase of the fields even for long pulses.
We report an optical inelastic-wave-mixing-enhanced atomic magnetometry technique that results in nT-level magnetic field detection at temperatures compatible with the human body without magnetic shielding, zero-field compensation, or high-frequency modulated phase-locking spectroscopy. Using Gaussian magnetic pulses that mimic the transient magnetic field produced by an action potential on a frogs nerve, we demonstrate more than 300,000-fold (550-fold) enhancement of magneto-optical rotation signal power spectral-density (power amplitude) over the conventional single-beam $Lambda-$scheme atomic magnetometry method. This new technique may bring possibilities for extremely sensitive magnetic field imaging of biological systems accessible via an optical fiber in clinical environments.
We demonstrate a magnetometry technique using nitrogen-vacancy centres in diamond which makes use of coherent two-photon transitions. We find that the sensitivity to magnetic fields can be significantly improved in isotopically purified diamond. Furthermore, the long-term stability of magnetic field measurements is significantly enhanced, thereby reducing the minimum detectable long-term field variations for both quasi-static and periodic fields. The method is useful both for sensing applications and as a spin qubit manipulation technique.
Our 2005 Physical Review Letter entitled Suppression of Spin-Projection Noise in Broadband Atomic Magnetometry (volume 94, 203002) relied heavily in its claims of experimental quantum-limited performance on the results of a prior publication from our group [1]. In subsequent work we have determined that the results of [1] were incorrect and must therefore retract this Physical Review Letter as well. The authors would like to emphasize that the broadband magnetometry approach taken in our work remains valid, as described in the theoretical paper [2], but we have lost confidence in the calibration procedures employed at the time to establish sensitivity relative to the spin-projection noise level. [1] JM Geremia, John K. Stockton and Hideo Mabuchi, Real-Time Quantum Feedback Control of Atomic Spin-Squeezing, Science 304, 270, (2004). [2] John K. Stockton, JM Geremia, Andrew C. Doherty and Hideo Mabuchi, Robust quantum parameter estimation: Coherent magnetometry with feedback, Phys. Rev. A 69, 032109, (2004).
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