The use of light, especially of laser light, is in many cases the most sensitive way to perform measurements. However, the highest sensitivity that can be achieved with laser light as probe is bounded by the standard quantum limit (SQL). As many instruments are approaching this fundamental limit, it becomes crucial to explore ways to overcome the SQL. Quantum metrology offers the possibilities to increase the sensitivities of the most accurate measurements beyond the SQL by using photonic quantum states of light as a tool. Two well-known classes of quantum states that provide a metrological advantage and break the SQL are squeezed states and a certain class of entangled states, called NOON states. While it is of special interest to apply these quantum states to atomic systems, such as atomic vapors, this requires quantum states of the highest quality in terms of purity, fidelity, brightness, and indistinguishability. Most importantly, for the probing of atomic systems, the quantum states need to be extremely narrowband to match the atomic linewidths. As NOON states are usually generated in a broadband spontaneous parametric down-conversion (SPDC) process, they are not compatible with narrowband atomic resonances.

The goal of this thesis was the generation of suitable narrowband entangled and squeezed quantum states of light and their application to atomic systems. To increase the rate of atom-resonant SPDC photons by orders of magnitude, we used a cavity-enhanced setup. Polarization-squeezed states and polarization-entangled NOON states were created. The spectral brightness of the generated NOON states is one of the highest of pairs of indistinguishable photons reported so far. The photon pairs were carefully characterized by full quantum state tomography showing high fidelities with a perfect NOON state. After filtering the photon source output by a novel filter based on the “interaction-free measurement” scheme, a cross-correlation measurement demonstrated its potential as a narrowband heralded single-photon source, needed for example in quantum information. To apply these states in a quantum metrology scheme and to show the metrological advantage, we chose an atomic magnetometer as a model system. The assembled shot-noise-limited magnetometer is based on the Faraday effect in a vapor of hot rubidium atoms. It could be demonstrated that both quantum states perform better in the magnetometer application than any classical state, i.e., they break the SQL. In the case of NOON states, this is the first use of multi-photon coherence in an atomic experiment. In addition to applications in quantum metrology, the presented techniques of quantum-light generation and filtering are also directly applicable to quantum information tasks, especially to the use in quantum memories.


Friday June 29, 11:00. ICFO Auditorium

Thesis Advisor: Prof. Morgan W. Mitchell