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arXiv:1705.10965 · Physical Review A

Continuous Faraday measurement of spin precession without light shifts

M. Jasperse, Michael J. Kewming, S. N. Fischer, Prasanna Pakkiam, R. P. Anderson, Lincoln D. Turner

Published Dec 1, 2017• 21 citations
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Abstract

We describe a dispersive Faraday optical probe of atomic spin which performs a weak measurement of spin projection of a quantum gas continuously for more than one second. To date, focusing bright far-off-resonance probes onto quantum gases has proved invasive due to strong scalar and vector light shifts exerting dipole and Stern-Gerlach forces. We show that tuning the probe near the magic-zero wavelength at 790 nm between the fine-structure doublet of $^{87}\mathrm{Rb}$ cancels the scalar light shift, and careful control of polarization eliminates the vector light shift. Faraday rotations due to each fine-structure line reinforce at this wavelength, enhancing the signal-to-noise ratio for a fixed rate of probe-induced decoherence. Using this minimally invasive spin probe, we perform microscale atomic magnetometry at high temporal resolution. Spectrogram analysis of the Larmor precession signal of a single spinor Bose-Einstein condensate measures a time-varying magnetic field strength with 1 $\ensuremath{\mu}\mathrm{G}$ accuracy every 5 ms; or, equivalently, makes more than 200 successive measurements each at $10\phantom{\rule{0.16em}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}$ sensitivity.

DOI
10.1103/physreva.96.063402

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We describe a dispersive Faraday optical probe of atomic spin which performs a weak measurement of spin projection of a quantum gas continuously for more than one second. To date, focusing bright far-off-resonance probes onto quantum gases has proved invasive due to strong scalar and vector light shifts exerting dipole and Stern-Gerlach forces. We show that tuning the probe near the magic-zero wavelength at 790 nm between the fine-structure doublet of 87Rb^{87}\mathrm{Rb}87Rb cancels the scalar light shift, and careful control of polarization eliminates the vector light shift. Faraday rotations due to each fine-structure line reinforce at this wavelength, enhancing the signal-to-noise ratio for a fixed rate of probe-induced decoherence. Using this minimally invasive spin probe, we perform microscale atomic magnetometry at high temporal resolution. Spectrogram analysis of the Larmor precession signal of a single spinor Bose-Einstein condensate measures a time-varying magnetic field strength with 1 \ensuremathμG\ensuremath{\mu}\mathrm{G}\ensuremathμG accuracy every 5 ms; or, equivalently, makes more than 200 successive measurements each at 10pT/Hz10\phantom{\rule{0.16em}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}10pT/Hz​ sensitivity.
We report a continuous measurement of the spin state of a spinor BEC. A far off-resonant linearly polarised probe beam illuminates a sodium spinor condensate in F=1. The beam is Faraday rotated in proportion to the spin component along the beam propagation direction, and the rotation measured at the shot-noise limit with an autobalanced polarimeter. The measurement is minimally-destructive and continuous. As the condensate spin Larmor precesses in a weak magnetic bias field, the Faraday signal oscillates at the Larmor frequency, typically of order 50 kHz. We investigate this novel measurement for monitoring spin-mixing and decoherence processes in real time.
We describe a compact, robust and versatile system for studying the macroscopic spin dynamics in a spinor Bose–Einstein condensate. Condensates of are produced by all-optical evaporation in a 1560 nm optical dipole trap, using a non-standard loading sequence that employs an ancillary 1529 nm beam for partial compensation of the strong differential light-shift induced by the dipole trap itself. We use near-resonant Faraday rotation probing to non-destructively track the condensate magnetization, and demonstrate few-Larmor-cycle tracking with no detectable degradation of the spin polarization. In the ferromagnetic F = 1 ground state, we observe the spin orientation between atoms in the condensate is preserved, such that they precess all together like one large spin in the presence of a magnetic field. We characterize this dynamics in terms of the single-shot magnetic coherence times and , and observe them to be of several seconds, limited only by the residence time of the atoms in the trap. At the densities used, this residence is restricted only by one-body losses set by the vacuum conditions.
This thesis presents a novel magnetic resonance imaging (MRI) technique for spinor Bose-Einstein condensates (BEC) with the paramagnetic Faraday effect. This quantum-photon interface couples the spin of the BEC to the polarisation of an off-resonant laser, which in a magnetic field gradient allows a 1D density profile of the BEC to be reconstructed. As opposed to conventional imaging techniques that are destructive or diffraction limited, this technique is minimally destructive to the BEC and not diffraction limited. Multishot in situ imaging of a single condensate is therefore possible, paving the way for time-resolved studies and 2D/3D reconstructions. A theoretical model for Faraday imaging is developed using the tensor polarisability formalism for the atom-light interaction, and the signal-to-noise ratio is derived. Resolution limits induced by Stern-Gerlach separation are considered, encouraging rapid measurements with strong magnetic field gradients. Analysing the trapping potential reveals that the `magic-zero' wavelength 790nm produces no dipole force and enables a tightly focused probe laser to be used without perturbing the trap. Polarisation-maintaining fibers were seen to impart large fluctuations depending on the incident polarisation, which were minimised. The broad diode laser emission background is seen to reduce the lifetime of the BEC, which was corrected with an interference filter. Short-time Fourier transforms are used to process the photodetected signal, showing the evolution of the polarisation rotation frequency and amplitude. Frequency-modulation is observed at the power line frequency, corresponding to magnetic field fluctuations induced by nearby equipment. Birefringence of optical elements results in elliptical polarisation of the probe and an effective magnetic field, which is cancelled using a quarter-waveplate. The quadratic Zeeman effect causes rapid amplitude modulation, and is eliminated by applying a microwave dressing field. Radiative spin echo is achieved in a magnetic field gradient, proving that dephasing is coherent, and that multishot imaging possible. Magnetic resonance imaging (MRI) is performed on a partially-evaporated atom cloud, distinguishing the thermal and condensed fractions. A split dipole trap is used to create two spatially separated BECs, which are individually resolved during MRI. The resolution is only limited by the strength of the applied magnetic field gradient, and is not subject to the diffraction limit. Custom coils will enable MRI of condensates at the sub-micron scale.
We demonstrate a sensitive spatially resolved magnetometer using a spinor Bose-Einstein condensate (BEC) of 87Rb^{87}\mathrm{Rb}87Rb atoms with increased probe fluence. By performing a probe parameter search using a two-polarization phase contrast imaging technique, we find that the probe-induced atom loss, a major constraint in optical magnetometry with high-density gas, can be reduced. We attain a sensitivity of 5.4(6) mrad for the phase measurement of Larmor precession using a measurement area of $1.4\ifmmode\times\else\texttimes\fi{}{10}^{2}\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$ with a probe fluence of $4.1\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}2}$. A dc magnetic field sensitivity of 5.0(4)pT/Hz5.0(4)\phantom{\rule{3.33333pt}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}5.0(4)pT/Hz​ is attained with a probe fluence of $3.9\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{\ensuremath{-}2}$. The results should contribute to further enhancement of the sensitivity of a BEC magnetometer.
The linear Faraday effect is used to implement a continuous measurement of the spin of a sample of laser-cooled atoms trapped in an optical lattice. One of the optical lattice beams serves also as a probe beam, thereby allowing one to monitor the atomic dynamics in real time and with minimal perturbation. A simple theory is developed to predict the measurement sensitivity and associated cost in terms of decoherence caused by the scattering of probe photons. Calculated signal-to-noise ratios in measurements of Larmor precession are found to agree with experimental data for a wide range of lattice intensity and detuning. Finally, quantum back-action is estimated by comparing the measurement sensitivity to spin projection noise, and shown to be insignificant in the current experiment. A continuous quantum measurement based on Faraday spectroscopy in optical lattices may open up new possibilities for the study of quantum feedback and classically chaotic quantum systems.
We have implemented a continuous measurement of the mean magnetic moment of an ensemble of atoms trapped in a far-off-resonance optical lattice, by detecting the Faraday rotation of one of the lattice beams after it has passed through the atom cloud. In a first demonstration experiment we have observed Larmor precession with high signal-to-noise ratio, and compared the performance of the measurement with a simple theory. Faraday spectroscopy offers an ideal method to monitor the atomic dynamics and will be applied to the study of quantum chaos in magneto-optical lattices. In principle the measurement sensitivity can be increased to the point where quantum backaction becomes significant, thereby opening the door to studies of quantum feedback, spin squeezing and the role played by quantum measurement in quantum/classical correspondence.