Radio-frequency dressed detection of atomic clock states

Thomas Fernholz

Our group currently investigates a new method to dispersively measure population and population difference of alkali atoms prepared in their two clock states (Fz=0). The Voigt effect, i.e. linear birefringence of the atomic medium, allows atom number detection via polarisation homodyning. To make full use of this common path interferometry and to achieve low technical noise levels, we perform sideband detection after adiabatically transforming the atomic states via radio-frequency dressing, see, e.g., (Lesanovsky2006). The balanced homodyne signal then oscillates at twice the dressing frequency, independent of field fluctuations, thus allowing for robust, phase-locked detection that circumvents low-frequency noise. Using probe pulses of two optical frequencies consecutively, we can detect atoms in both groundstate hyperfine manifolds separately and obtain population difference as well as total atom number in a single experimental cycle. Initial experiments with ensembles of 10^8 rubidium-87 atoms allowed us to measure Rabi oscillations with a technical noise level of only 0.15 percent, currently limited by laser fluctuations. While this corresponds to a noise power of ~23dB above the atomic shot noise limit, single-laser two-colour operation should enable us to reach quantum noise limited performance, potentially allowing for the preparation of spin squeezed states via quantum non-demolition measurements (Hammerer2010). Our detection method can be used in atomic clocks and atom interferometric measurements.

Lesanovsky2006 

I. Lesanovsky et al., "Adiabatic radio-frequency potentials for the coherent manipulation of matter waves", Phys. Rev. A 73, 033619 (2006).
Hammerer2010

K. Hammerer, A. S. Sorensen and E. S. Polzik, "Quantum interface between light and atomic ensembles", Rev. Mod. Phys. 82, 1041 (2010).