Experimental and theoretical comparison of different optical excitation schemes for a compact coherent population trapping Rb vapor clock

Abstract

We have investigated, theoretically as well as experimentally, the relative merits and demerits of using three different optical configurations for a compact coherent population trapping (CPT) vapor clock using 87 Rb. These correspond to the following choices of polarizations for the two Raman beams: ∥ lin lin, ( σ σ , ), and push-pull optical pumping (PPOP), applied on the D1 manifold. We have used a multi-level atomic model to study the dependence of the CPT spectrum on axial as well as transverse magnetic fields for these three schemes. Corresponding experimental studies have been performed using a laboratory scale CPT clock employing a two cm long, isotopically pure rubidium cell, loaded with a buffer gas. We observed a CPT contrast close to 20% with a sub-kilohertz linewidth by adopting the PPOP scheme. We discuss the strengths and weaknesses of each of the three optical excitation schemes, and present frequency-stability measurement data for the prototype clock.

Cold-atom clock based on a diffractive optic

“…An additional AOM (not shown in figure) is used to offset the laser lock, which allows us to control the one-photon detuning. We use the F = 1 & 2 → F = 2 transitions in order to minimize excitation of the weakly magnetic ∆m = 2 CPT resonances which are formed between |F = 1, m F = ±1 and |F = 2, m F = ∓1 [44][45][46]. This concentrates the atomic population in the |F = 1 & 2, m F = 0 ground states and minimises the magnetic sensitivity of the clock signal amplitude.…”

Low-Drift Coherent Population Trapping Clock Based on Laser-Cooled Atoms and High-Coherence Excitation Fields

“…Thus far, most of the cold-atom CPT studies performed with the system described here have been done with the lin ∥ lin polarization scheme, but cold-atom clocks can benefit from other polarization schemes in the form of simplified spectra and reduced Zeeman shifts (for a review, see Ref. [48]). Light shifts from off-resonant hyperfine transitions are currently under investigation.…”

Raman-Ramsey CPT with a grating magneto-optical trap

“…When F = 2, they are suppressed by the coupling to the |F = 2, m F = ±2 states as illustrated by the grey lines in Fig. 2 (b) [18]. However, it is possible to produce a coherence associated with the ∆m F = 2 dark resonances even when F >1 [16]- [18].…”

“…With Raman-Ramsey interrogation, this can manifest as oscillations in the fringe amplitude as a function of the magnetic field used to define the quantisation axis [10], [16], [17]. In the case of the D 1 line of 87 Rb, these interference effects depend strongly on the excited state chosen to drive the CPT resonance [18]. With F = 1, three dark resonances are expected to be present leading to large interference in the Ramsey fringe amplitude.…”

“…With F = 1, three dark resonances are expected to be present leading to large interference in the Ramsey fringe amplitude. With F = 2, two of these dark resonances are suppressed by couplings to the |F = 2, m F = ±2 states and so the magnetic sensitivity of the Ramsey fringe amplitude will also be reduced [18]. These dark state interference effects can degrade the performance of the CPT resonance as a frequency standard, and so it is important to characterise them experimentally [16], [17].…”