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Journal of Physics B Atomic Molecular and Optical Physics

Magic frequency enabled by quantum interference for a dual atomic device

Swarupananda Pradhan, A. Kani, Harshawardhan Wanare, Satish Mishra, A K Das

Published Mar 17, 2015• 5 citations
Publisher

Abstract

Abstract We present a magic frequency associated with two-photon processes in room temperature atomic vapour at the coherent population trapping (CPT) condition for an elliptically polarized light. It is realized by combining the symmetries of the lineshape functions arising from the atomic coherence. In practice, the magic frequency is synthesized from a polarimetric measurement involving the reflected and transmitted CPT signal at the polarizing beam splitter, which provides an atomic frequency standard for near zero magnetic fields. We further develop a magnetometer, which concurrently measures the magnetic field and its direction from the enhanced reflected signal with significantly improved performance. Additionally, the magic frequency and its dependence on the polarization content provide a handle to precisely locate the single photon resonance.

DOI
10.1088/0953-4075/48/7/075502

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Abstract We present a magic frequency associated with two-photon processes in room temperature atomic vapour at the coherent population trapping (CPT) condition for an elliptically polarized light. It is realized by combining the symmetries of the lineshape functions arising from the atomic coherence. In practice, the magic frequency is synthesized from a polarimetric measurement involving the reflected and transmitted CPT signal at the polarizing beam splitter, which provides an atomic frequency standard for near zero magnetic fields. We further develop a magnetometer, which concurrently measures the magnetic field and its direction from the enhanced reflected signal with significantly improved performance. Additionally, the magic frequency and its dependence on the polarization content provide a handle to precisely locate the single photon resonance.
We present a new polarization-selective detection scheme that enables simultaneous dual-frequency laser stabilization and implementation of high-contrast coherent population trapping (CPT) resonance. The proposed configuration splits elliptically polarized light into two optical paths after atomic interaction. The component reflected from the polarizing beam splitter, containing Doppler-broadened spectral features, is employed for precise laser frequency locking. The reflected signal exhibits a frequency discrimination slope nine times steeper than that of the transmitted polarizing beam splitter component. Simultaneously, the transmitted component isolates the CPT resonance signals from the positive and negative first-order sidebands for microwave frequency stabilization. Experiment demonstrates excellent agreement with theoretical simulations derived from the Liouville equation. Compared with conventional circular polarization methods, this approach achieves a fourfold enhancement in short-term frequency stability. These advancements make the technique particularly promising for developing compact CPT atomic clocks with improved metrological performance.
The operation of a highly sensitive atomic magnetometer using elliptically polarized resonant light is demonstrated. It is based on measurement of zero magnetic field resonance in degenerate two level systems using polarimetric detection. The transmitted light through the polarimeter is used for laser frequency stabilization, whereas reflected light is used for magnetic field measurement. Thus, the experimental geometry allows autonomous frequency stabilization of the laser frequency leading to compact operation of the overall device and has a preliminary sensitivity of <10 pT/Hz(1/2) @ 1 Hz. Additionally, the dynamic range of the device is improved by feedback controlling the bias magnetic field without compromising on its sensitivity.
The polarization rotation and coherent population trapping signal arising due to two photon process using linearly polarized light are found to be significantly enhanced for a Zeeman degenerate system. The zero crossing of the dispersive profile is found to be shifting proportional to the applied magnetic field, albeit the absorptive profile position remains invariant for a slightly imbalanced orthogonal circular polarization component. It provides an alternative method for precise measurement of vector magnetic field without requirement of a bias field. The use of polarization rotation signal for magnetic field measurement offers added advantage due to improved signal to noise ratio.
The method of operation of an atomic magnetometer based on coherent population trapping (CPT) without any requirement of radio frequency scanning is demonstrated. Using a hybrid approach comprising of polarization rotation and tailored transmission due to CPT states, the simultaneous operation of the device as atomic frequency standard as well as atomic magnetometer is investigated. The magnetometer sensitivity can be further improved by utilizing the inherent option of magnetic field assisted synchronized radio frequency scanning.
Coherent population trapping (CPT) resonances on the degenerate two-level system of the (Fg=2→Fe=1) transition of the 87Rb D1 line were investigated in an uncoated vacuum cell by means of level-crossing type experiments-sweeping a magnetic field around zero value and using a single laser beam. The two-photon resonance manifests itself as fluorescence canceling when the system of levels is degenerate (at zero point of magnetic field). The influence of weak transverse magnetic fields on the amplitude and width of the resonance was investigated. The CPT resonances were observed with linear, circular and elliptical polarized laser light. Transverse magnetic field widens the resonances obtained by linear and circular polarization but splits the resonance created by slightly elliptically polarized laser light. The dependence of this splitting on the elliptical ratio and experimental geometry were investigated. Due to its narrow width and sensitivity to low magnetic fields (< 1 G) this resonance is very promising for small magnetic field measurement.
We have experimentally demonstrated that magnetic-field-insensitive coherent-population-trapping (CPT) resonances are generated between the ground hyperfine levels on the D1{D}_{1}D1​ line of 133Cs^{133}\mathrm{Cs}133Cs using a two-photon $\mathrm{\ensuremath{\Lambda}}$ scheme excited by lin||lin polarizations. The frequency shift of the CPT resonance is 0.04 Hz for the deviation of $1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ at a ``magic'' magnetic field of $139\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ and is 50 times smaller than that of the conventional clock transition at a typical bias magnetic field for the clock operation. The amplitude of the CPT spectrum excited by lin||lin polarizations is enhanced as the excitation intensity increases and is well explained by the three-level model without trap levels. Thus, the CPT resonance on the D1{D}_{1}D1​ line of 133Cs^{133}\mathrm{Cs}133Cs excited by a simple lin||lin scheme will be one of the best candidates for frequency reference of miniature atomic clocks.