Cavity QED with high-Qwhispering gallery modes

Abstract

We report measurements of cavity-QED effects for the radiative coupling of atoms in a dilute vapor to the external evanescent field of a whispering-gallery mode ͑WGM͒ in a fused silica microsphere. The high Q (5ϫ10 7 ), small mode volume ͑10 Ϫ8 cm 3 ), and unusual symmetry of the microcavity evanescent field enable velocity-selective interactions between fields with photon number of order unity in the WGM and N T ϳ1 atoms in the surrounding vapor. ͓S1050-2947͑98͒50904-3͔PACS number͑s͒: 42.50.CtCavity QED has proven to be a fertile arena in which to study coherent interactions between single atoms and photons ͓1͔. In the optical domain, the cavities employed to achieve strong coupling have been Fabry-Perot microresonators with finesse Fϳ10 5 , as in the initial work of Ref.͓2͔ and continuing to the recent demonstrations of real-time cavity QED with individual atoms ͓3,4͔. By contrast, the whispering gallery modes ͑WGMs͒ of quartz microspheres ͓5͔ offer an alternative avenue to the regime of strong coupling with the potential to surpass Fabry-Perot cavities with respect to certain key parameters in cavity QED. For example, such resonators have the capability of achieving extremely long photon storage lifetimes while maintaining a strong dipole coupling to an atomic ͓6͔, ionic ͓7͔, or molecular ͓8͔ species via the small volume of a single mode, leading to the potential for ratios of coherent coupling to loss mechanisms in excess of 10 3 ͓6͔ . Indeed, quality factors QϷ8ϫ10 9 have been reported for wavelengths 633 nm рр852 nm, corresponding to finesse Fϳ2.2ϫ10 6 ͓9,10͔, which is the highest value on record for an optical resonator.Motivated by these prospects, in this Rapid Communication we report measurements of the interaction of atoms with the external evanescent field of a whispering gallery mode in a domain in which cavity-QED effects become important. More specifically, we study the modifications of cavity transmission due to the coupling of N T ϳ1 cesium atoms in a thermal gas with a single resonant WGM at the level of a few photons in the mode. The possibility for sensitivity to N T ϳ1 atoms in the microsphere's evanescent field in the face of Doppler broadening of roughly 100 times the natural linewidth is a consequence of the features of the microspheres used here, namely, small size ͑with radius aϽ60 m and mode volume V m ϳ10 Ϫ8 cm 3 ) and narrow linewidth ͑with Q 1 ϭ5ϫ10 5 рQрQ 2 ϭ5ϫ10 7 ). Although it is clearly desirable to reduce the Doppler broadening by coupling to cold atoms ͓3,4͔, our current experiments are an exciting initial step towards realizing the potential of WGMs for long-lived coherent dynamics in cavity QED.The actual setup is depicted in Fig. 1 and consists of a grating stabilized diode laser of a few hundred kHz linewidth coupled into a microsphere via frustrated total internal reflection from a prism ͓5͔. The microspheres are fabricated from low-OH fused silica of index nϭ1.452 using an oxygenhydrogen microtorch ͓11͔ and then mounted inside the vacuum system. The range Q 1 →Q 2 is ac...

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