Quantum memories and monolithic resonators

Abstract

This thesis presents progress towards two complementary goals. The first part discusses our aim of building high-efficiency quantum memories for quantum optical states and exploring the potential of the memory for performing operations as well as for passive storage. The second part presents our work towards developing a compact source of quantum light by fabricating optical resonators from non-linear optical crystals. Our high-efficiency memories operate by storing the information carried by a weak laser field in the coherence between the hyperfine-split electronic energy levels in a vapour of atomic rubidium. We analyse the performance of a three-level gradient echo memory protocol that is used to store and retrieve optical information from the atomic coherence using an off-resonant Raman transition. Using a warm rubidium vapour, we find that the memory preserves the quantum state of the input light better than any classical means of storage could achieve. We also present results showing that trapped, cold atomic vapours provide an increased storage lifetime and analyse the contribution that four-wave-mixing makes to the retrieved light. Both the warm- and cold-atom implementations of the memory are shown to store and recall light with over 80% efficiency. We also present a number of extensions to the basic memory protocol. We demonstrate that the light-atom coupling can be controlled by the bright coupling field that is used to complete the Raman transition. This control is used to perform a beam-splitting operations between two optical pulses that are separated in time. Using frequency-multiplexed states, we show that beam-splitting operations are also possible between optical modes that are separated in frequency. This operation can be extended to perform arbitrary operations by using a series of memories. Finally, we present a high-fidelity memory for dual-rail q-bits encoded across two frequency modes. The resonators that were fabricated to explore compact sources of quantum light demonstrated many of the properties that are required to obtain the stable operation of a quantum source. We demonstrated that non-linear optical crystals can be machined into monolithic resonators that use total internal reflections to confine light. Two geometries were tested, whispering gallery resonators and square travelling wave cavities. Because both types of resonators use total internal reflection instead of dielectric coatings, they feature very low loss rates. Lithium niobate resonators of both types exhibited cavity decay rates that were dominated by bulk absorption in the crystal. The resonators were shown to be frequency tunable using an applied voltage and could be coupled to free-space modes with a tunable coupling rate. We present preliminary results of non-linear frequency conversion in the resonators. We also investigated calcium fluoride and magnesium fluoride resonators for use as high-finesse reference cavities. We report a finesse of 5000 in a calcium fluoride whispering gallery resonator. A square calcium fluoride monolithic resonator was also produce to have a low insertion loss when coupled to a Gaussian free-space mode. We demonstrate a power coupling efficiency of 94% to that resonator. We also present a proposal to use a square lithium niobate resonator as a tunable coupler for a high-finesse calcium fluoride whispering gallery resonator. The coupled resonator system could be used as a compact high-efficiency optical quantum memory.