Generating quantum resources through measurement and control

Abstract

The quantum resources of entanglement and single photons are key to a range of quantum applications. These resources are challenging to produce cleanly and efficiently. This thesis investigates and improves methods of producing entanglement and single photons using measurement and control. To robustly produce entanglement, this thesis extends work by Carvalho et al., who developed a method for creating an entangled state of two atoms. This is done by coupling the atoms to a damped cavity, and using measurement of the output of the cavity to trigger a feedback pulse on the atoms. The robustness of this scheme against imperfect localisation of the two atoms is tested in this thesis, and limits are placed on the size of a trap that will still allow an entangled state to be produced. Additionally, using three-level (Lambda configuration) atoms slows the rate at which the system evolves, allowing more time for measurement and feedback to be applied, though it does not counteract the influence of spontaneous emission. In extending this work to multiple atoms we found that the system rapidly becomes more complex, and we developed a strategy for choosing a feedback pulse that stabilises a specific, highly entangled steady state of multiple particles. In addition to this general strategy, we developed a local and separable feedback for generating entanglement in a four-partite system. This thesis also investigates the production of single photons through the rephased amplified spontaneous emission (RASE) protocol. This protocol offers the promise of a stream of precisely shaped single photons, though it is plagued by efficiency issues. We develop a model of RASE and show that it overcomes the trade-off between efficiency and photon spacing by using different optical depths for the amplified spontaneous emission (ASE) preparation of the ensemble, and the RASE emission of single photons. By tailoring the density profile of the ensemble to mode-match the RASE emission of a single photon, we also able to eliminate reabsorption of the emitted photon, which would otherwise have limited the efficiency to 70%. Show more