Quantum noise reduction through atomic coherence effects

Abstract

The interaction of light with atomic media, i. e., media describable as an ensemble of free particles, can be used to alter the quantum statistics of the interacting light field. Nonlinear effects present in such an interaction are rather weak unless the light is tuned close to resonance with a transition between two atomic energy eigenstates. While semi-classical optical effects are now enhanced, the desired changes to the quantum statistics of the interacting light are normally washed out by atomic noise from absorption and spontaneous emission. Tuning away from resonance turns off the nonlinearities faster than absorption. In this thesis, I will investigate the interaction of three-level atom systems with more than one light field. Driving the atoms with an external coherent field, the probe, can be used to establish coherence within the atomic system, thereby altering the optical response to the signal light field. The established coherence can lead to a significant reduction of quantum noise injected into the signal light field. I will demonstrate the utility of two atomic coherence effects in active as well as passive interaction schemes. Electromagnetically induced transparency makes the atoms transparent to the signal field due to an interference of probability amplitudes. At the same time the emissive properties of the atom are not reduced so that laser gain can be achieved without inversion. In second harmonic generation, such transparency prevents the second harmonic from being reabsorbed. As the signal field builds up it tends to influence the pump cycle in a way that can lead to a reduction of photon number fluctuations. Another way to reduce absorption while retaining nonlinear effects is given by cw self-induced transparency and Stark tuning. In A and ladder configuration three-level atoms, applying a strong signal field can create a ghost transition. The electron is optically pumped to the remaining third level and the medium becomes transparent. A probe applied to the other transition is tuned into the vicinity of the Autler-Townes doublet resulting from the dressing with the signal field. The cross-phase modulation effect can be used to carry out a quantum nondemolition measurement of the signal amplitude. Tuning the probe closer to resonance with the Rabi level can result in significant squeezing of the signal amplitude.