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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Impact of static-magnetic-field-gradients on relaxation times in a Rb vapor cell

Mohammadreza Gharavipour, C. Affolderbach, Florian Gruet, G. Mileti, I. S. Radojičić, Aleksandar J. Krmpot, B. M. Jelenković

Published Jul 1, 2017• 0 citations
Publisher

Abstract

We apply an innovative method called Optically-Detected Spin-Echo to measure the intrinsic coherence relaxation time in the buffer-gas vapor cell of a Rb atomic clock [1]. This method suppresses the influence of static-magnetic-field-gradients across the cell which is a source of relaxation processes. Such studies are of interest for high-performance Rb atomic clocks, where both intrinsic population and coherence relaxation times (T1and T2, respectively) of the “clock transition” (52S1/2|Fg= 1, mF= 0〉 ↔ |Fg= 2, mF= 0〉) are relevant.

DOI
10.1109/fcs.2017.8088799

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We apply an innovative method called Optically-Detected Spin-Echo to measure the intrinsic coherence relaxation time in the buffer-gas vapor cell of a Rb atomic clock [1]. This method suppresses the influence of static-magnetic-field-gradients across the cell which is a source of relaxation processes. Such studies are of interest for high-performance Rb atomic clocks, where both intrinsic population and coherence relaxation times (T1and T2, respectively) of the “clock transition” (52S1/2|Fg= 1, mF= 0〉 ↔ |Fg= 2, mF= 0〉) are relevant.
We introduce and demonstrate an experimental method, optically-detected spin-echo (ODSE), to measure ground-state relaxation times of a rubidium (Rb) atomic vapor held in a glass cell with buffergas.The work is motivated by our studies on high-performance Rb atomic clocks, where both population and coherence relaxation times (T 1 and T 2 , respectively) of the 'clock transition' (5) are relevant.Our ODSE method is inspired by classical nuclear magnetic resonance spin-echo method, combined with optical detection.In contrast to other existing methods, like continuous-wave double-resonance (CW-DR) and Ramsey-DR, principles of the ODSE method allow suppression of decoherence arising from the inhomogeneity of the static magnetic field across the vapor cell, thus enabling measurements of intrinsic relaxation rates, as properties of the cell alone.Our experimental result for the coherence relaxation time, specific for the clock transition, measured with the ODSE method is in good agreement with the theoretical prediction, and the ODSE results are validated by comparison to those obtained with Franzen, CW-DR and Ramsey-DR methods.The method is of interest for a wide variety of quantum optics experiments with optical signal readout.
We use a Ramsey-type interaction scheme to measure spatially-resolved images of the static magnetic field (C-field) amplitude Bdcapplied across the Rb cell in the physics package of a high-performance vapor-cell atomic clock. Low field variations ofdcalso along the direction of laser propagation. Images of the T2relaxation time are obtained in a similar way, and show a distribution that correlates with the Bdcdistribution. This indicates inhomogeneous dephasing due to C-field gradients, which also results in spatial variation of the T2time for the clock transition.
In the atomic cell of the spin exchange optical pumping system, the optical pumping inevitably introduces a magnetic field gradient, which affects the measurement result of the nuclear spin relaxation time a lot. To address this issue, our method achieves the measurement while excluding the influence of this magnetic field gradient. In this method, the pump laser is turned off after the spins of 87Rb and 129Xe have been effectively polarized, which results in the spin polarization of the 87Rb atoms being provided only by the spin exchange with 129Xe. Ultimately, the transverse and longitudinal components of the polarization of 129Xe are detected by the in situ87Rb atom magnetometer to obtain the T1 and T2 of 129Xe under different conditions. Using our method, since the influence of inhomogeneity caused by the pump laser is eliminated, the relaxation characteristic of atomic cells can be measured more quickly, essentially, and conveniently, which helps compare the performance of different atomic cells and find the best cell fabrication process further.
Inside a spin-exchange relaxation-free (SERF) co-magnetometer with a high-pressure buffer gas atomic cell, the magnetic field gradient causes the decoherence of atomic spins to produce magnetic-field gradient relaxation. This paper presents a new method for the accurate measurement of magnetic field gradient relaxation of alkali metal atoms and inert atoms of strongly coupled spin systems under triaxial magnetic field gradients in the K-Rb-21Ne co-magnetometer. The magnetic field gradient relaxation of alkali metal atoms is measured using a step magnetic field modulation method, and the magnetic field gradient relaxation of inert atoms is measured using a combined free induction decay and spin growth method. The method does not require the use of large background magnetic fields and RF fields to maintain the atoms in the SERF state, does not require additional optics, and is not affected by the pumping or detecting of optical power. A kinetic model that considers a large electron-equivalent magnetic field was designed and a gradient relaxation model was developed. The quadratic coefficients of the experimentally measured gradient relaxation curves fit the theoretical model well over the range of the applied magnetic field gradients, confirming the validity of the proposed method.
In this work, we experimentally measure the broadened linewidth of magneto-optical resonance signal due to magnetic-field gradient with an alkali buffer-gas-free anti-relaxation-coated cell. We also study how the temperature of the atomic vapor cell affects the magneto-optical resonance signal linewidth under different magnetic-field gradients. Theoretical calculations show that the relaxation caused by magnetic-field gradient could be suppressed with increased temperature, which could be useful to optimize the working parameters for unshielded atomic magnetometer based on anti-relaxation-coated cells.
In this work, a sensitivity of the rate of relaxation of ground-state atomic coherences to magnetic-field inhomogeneities is studied. Such coherences give rise to many interesting phenomena in light-atom interactions, and their lifetimes are a limiting factor for achieving better sensitivity, resolution, or contrast in many applications. For atoms contained in a vapor cell, some of the coherence-relaxation mechanisms are related to magnetic-field inhomogeneities. We present a simple model describing relaxation due to such inhomogeneities in a buffer-gas-free antirelaxation-coated cell. A relation is given between relaxation rate and magnetic-field inhomogeneities including the dependence on cell size and atomic species. Experimental results, which confirm predictions of the model, are presented. Different regimes, in which the relaxation rate is equally sensitive to the gradients in any direction and in which it is insensitive to gradients transverse to the bias magnetic field, are predicted and demonstrated experimentally.