Observation of Motion-Dependent Nonlinear Dispersion with Narrow-Linewidth Atoms in an Optical Cavity

Abstract

As an alternative to state-of-the-art laser frequency stabilisation using ultra-stable cavities, it has been proposed to exploit the non-linear effects from coupling of atoms with a narrow transition to an optical cavity. Here we have constructed such a system and observed non-linear phase shifts of a narrow optical line by strong coupling of a sample of strontium-88 atoms to an optical cavity. The sample temperature of a few mK provides a domain where the Doppler energy scale is several orders of magnitude larger than the narrow linewidth of the optical transition. This makes the system sensitive to velocity dependent multi-photon scattering events (Dopplerons) that affect the cavity field transmission and phase. By varying the number of atoms and the intra-cavity power we systematically study this non-linear phase signature which displays roughly the same features as for much lower temperature samples. This demonstration in a relatively simple system opens new possibilities for alternative routes to laser stabilization at the sub 100 mHz level and superradiant laser sources involving narrow line atoms. The understanding of relevant motional effects obtained here has direct implications for other atomic clocks when used in relation with ultranarrow clock transitions.

Nonlinear spectroscopy of Sr atoms in an optical cavity for laser stabilization

“…In this limit the nonzero velocities of the atoms bring additional multi-photon resonance phenomena into play, so-called doppleron resonances, which modify the complex transmission coefficient of the cavity field close to the atomic resonance [19].…”

“…A sharp dispersion signal with a steep slope can be seen around resonance. The magnitude of this slope, as well as the signal-to-noise-ratio, ultimately determines the obtainable frequency stability [18,19]. The dots are data measured by cavity-enhanced FM spectroscopy.…”

“…Recent studies have proposed a new scheme for laser stabilization to a mK thermal sample of atoms placed in a low finesse optical cavity [16][17][18][19]. By placing the atoms inside the optical cavity atom-light interactions are enhanced by order of the finesse of the cavity and non-linear effects are brought into play, which enhances the spectral sensitivity of the detected photo current used for stabilization.…”

“…Interestingly, the first proposals considered atoms trapped in an optical lattice or systems with a sample temperature equivalent to zero degrees Kelvin [17]. Further studies have extended these results to also include the finite temperature of the sample [18,19]. At finite temperatures the bi-stability regime, present for zero-Kelvin systems, turns out to be effectively removed.…”

Strong coupling on a forbidden transition in strontium and nondestructive atom counting

“…We verify that this sensitivity is achieved with as few as 0.01 photon recoils imparted to each atom, a well defined criteria for nondestructiveness. These proof-of-principle measurements pave the way for the generation of entangled squeezed states on the 1 S 0 to 3 P 0 optical clock transition in Yb and Sr optical lattice clocks [15][16][17].Cavity-enhanced nonlinear spectroscopy [31] of the same transition has been performed in a MOT in [32], but inhomogeneous doppler broadening prevented the observation of a collective vacuum Rabi splitting. Nonfluorescence based atom-counting in 87 Sr [28,29] and 171 Yb [33] have been performed using the dipole-allowed transition 1 S 0 to 1 P 1 and no optical cavity.…”