“It's high noon. It's a showdown. It's a question of who will flinch first.” -Richard Durbin

In quantum imaging and quantum metrology a class of path-entangled or polarization-entangled states achieves better performance compared to coherent states as input. These NOON states have proven to show super-resolution and to achieve the so-called 'Heisenberg limit' of phase sensitivity. So far, because of the broad emission spectrum of most down-conversion sources one interesting field of applications for NOON states has been out of reach: interaction with atomic systems. The challenge is to create narrowband NOON states at an atomic transition while maintaining at the same time a high count rate and a high quality. This task requires an ultra-bright source of highly indistinguishable photon pairs in combination with an ultra-narrow, but at the same time highly-transmissive filter.

In our lab we use a type-II cavity-enhanced down-conversion source. Due to the cavity design the emission of the SPDC process is enhanced into the spatial and spectral modes of the resonator by a factor of the finesse. Therefore this source shows an ultra-bright flux of down-conversion bandwidth. Moreover, the photons are emitted into the TEM00 mode of the cavity, which allows a high coupling efficiency into single-mode optical fibers.

From this cavity output a polarization NOON state in the H/V basis (HH + exp(iΦ) VV)/√2 is created. To select the atom resonant photons, we use a narrow-band filter that is based on a rubidium-vapor cell.

After filtering, the NOON state is narrowband, bright, rubidium-tuned, has a high fidelity and therefore represents an ideal tool for studying the interaction between indistinguishable photons and atomic ensembles. We use these NOON states for quantum metrology measurements on atomic systems.