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Chinese Physics Letters

Coherent Population Trapping-Ramsey Interference in Cold Atoms

Chen Xi, Yang Guo-Qing, Wang Jin, Mingsheng Zhan

Published Nov 1, 2010• 35 citations
Publisher

Abstract

We demonstrate an experimental observation of coherent population trapping-Ramsey interference in cold 87 Rb atoms by employing the time-domain separated oscillatory fields' method. The interference fringe with line width of 80Hz is obtained. We propose a novel method to measure the cold atom number. The measurement is insensitive to the pump beam intensity, the single photon detuning and even the initial state population. We use this method to normalize the interference signal and to improve the signal-to-noise ratio significantly.

DOI
10.1088/0256-307x/27/11/113201

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We demonstrate an experimental observation of coherent population trapping-Ramsey interference in cold 87 Rb atoms by employing the time-domain separated oscillatory fields' method. The interference fringe with line width of 80Hz is obtained. We propose a novel method to measure the cold atom number. The measurement is insensitive to the pump beam intensity, the single photon detuning and even the initial state population. We use this method to normalize the interference signal and to improve the signal-to-noise ratio significantly.
We present results from an atomic clock that employs a beam of cold 87Rb atoms and spatially separated (Ramsey) coherent population trapping interrogation of the hyperfine clock transition at 6.834 GHz. The cold atomic beam is generated through the use of a 2D+-magneto-optical trap. The interrogation is performed on the D2 line of 87Rb, and the optical fields use a counter-propagating σ+–σ− probing scheme. The use of cold atoms allows for relatively narrow Ramsey fringes even for a small spatial separation between the two interrogation zones (4.6 cm). The resulting clock has a short-term stability of 3×10−11/τ.
Ramsey spectroscopy via coherent population trapping (CPT) is essential in precision measurements. The conventional CPT-Ramsey fringes contain numbers of almost identical oscillations and so that it is difficult to identify the central fringe. Here, we experimentally demonstrate a temporal spinwave Fabry-Pérot interferometry via double-ΛΛΛ CPT of laser-cooled 87^{87}87Rb atoms. Due to the constructive interference of temporal spinwaves, the transmission spectrum appears as a comb of equidistant peaks in frequency domain and thus the central Ramsey fringe can be easily identified. From the optical Bloch equations for our five-level double-ΛΛΛ system, the transmission spectrum is analytically explained by the Fabry-Pérot interferometry of temporal spinwaves. Due to small amplitude difference between the two Landé factors, each peak splits into two when the external magnetic field is not too weak. This peak splitting can be employed to measure an unknown magnetic field without involving magneto-sensitive transitions.
We examine experimentally and theoretically the Ramsey effects on the coherent population trapping (CPT) resonance in a paraffin-coated Rb vapor cell. The observed CPT spectrum in the Hanle configuration of the 5S 1/2(F = 2)−5P 1/2(F′ = 1) transition of 87Rb atoms consists of a narrow structure with a spectral width of 0.10 mG due to the wall-induced Ramsey effect and a broad structure with a spectral width of 23 mG due to the transit effect. Considering the velocity distribution of atoms and the number of wall-atom collisions, we numerically calculated the transmittance of the laser beam as a function of the longitudinal magnetic field. The experimental results of the CPT Ramsey interference were in good agreement with the theoretical results, and the dependences of the CPT Ramsey effects on the atom-laser interaction time were investigated in detail.
We calculate the coherent population trapping (CPT) resonance in slow beam of rubidium 87 atoms caused by their interaction with laser bichromatic electromagnetic field in two separated spatial domains. We study influence of the finite width of the atomic beam, its angular divergence and residual beam velocity spread on the parameters of CPT-Ramsey resonance. We analyze properties of the CPT resonance depending on intensity of electromagnetic field, applied magnetic field strength and on width of the laser light beams.
We report an experimental study of magnetic-field-sensitive multi-wave interference, realized in a three-wave RF-atom system. In the F = 1 hyperfine level of the 87Rb 52S1/2 ground state, Ramsey fringes were observed via the spin-selective Raman detection. A decrease in the fringe contrast was observed with increasing free evolution time. The maximum evolution time for observable fringe contrasts was investigated at different atom temperatures, under free-falling and trapped conditions. As the main interest of the Ramsey method, the improvement in magnetic field resolution is observed with an increase of evolution time T up to 3 ms and with the measurement resolution reaching 0.85 nT. This study paves the way for precision magnetic field measurements based on cold atoms.