A coherent microwave interface for manipulation of single optical photons

This thesis proposes a means of implementing quantum information processing using photonic qubits as information carriers. Electromagnetically-induced transparency (\textsmaller{EIT}) is used to store information encoded in photons into Rydberg excitations in a cloud of cold atoms, where strong dipole-dipole interactions induce interactions between qubits. After a storage time, information is mapped back into photons collectively emitted from the cloud again via \textsmaller{EIT}. A new experimental apparatus is built to implement non-linear Rydberg quantum optics. A high repetition rate is achieved owing to a \textsmaller{2D-MOT} atom source, and high optical resolution for trapping and probing microscopic atomic ensembles is achieved by the use of aspheric lenses inside the vacuum chamber. A new, high resolution computer control scheme is implemented. This thesis demonstrates that, during the holding time, multiple collective Rydberg excitations at a controlled separation interact with each other to imprint a non-uniform phase gradient resulting in anti-correlation of photon emission. Interactions are observed at up to $15$ times the wavelength of the photonic qubits. These long range interactions offer a promising approach to scaling all-optical quantum computing. Applying resonant microwave fields during the storage time is demonstrated to offer a competitive method of performing sensitive microwave electrometry. A sensitivity of $12 \pm 7 \uV \percm \perRHz$ is found at a frequency of $7.7\GHz$. The high sensitivity is shown to arise from remnant, Rydberg excitations providing an additional source of atom loss from the atomic ensemble, leading to a suppression of photon storage and retrieval efficiency. An additional stage of microwave driving to sanitise the atomic ensemble for recycling is shown to successfully suppress the atom-loss mechanism. The use of successive microwave pulses is shown to provide a feasible approach to interfacing microwave and optical quantum information processing systems. Information encoded as the presence or absence of a microwave field is translated into information encoded as early or late retrieval of single photons, demonstrating proof of principle for an approach to implementing a proposal for an all-optical controlled-$z$ gate. Externally driven microwave fields are used to provide rapid, low-loss modulation of the signal retrieved from an atomic ensemble, demonstrating the proof of principle of implementing a probabilistic single photon source that can intensity modulate with low-loss at frequencies of at least $27\MHz$, with evidence that modulation may be achievable at rates in excess of $500\MHz$