Towards quantum memory based single photon gates
Creators: name: Leung, Anthony
Publisher: name: The Australian National University
Year: 2023

description: Quantum information technologies hold the key to future advances in a wide range of technologies from secure communications to drug design. The innate parallelism of quantum computers promises exponentially more processing power than modern computers in certain problems while quantum cryptography guarantees secure communications. Optics is a promising platform for transmitting and processing quantum information with its robustness from environmental disturbance stemming from photons' weakly interacting nature. This is an advantage for communication but can be problematic for computation purposes. One of the biggest challenges to scaling up optical quantum computation has been the lack of resource-efficient methods to make two photons strongly interact. Currently, linear optics is the most popular method for achieving this two-photon interaction but its results are probabilistic and remain resource intensive to scale up. In this thesis, we explore an alternate method for facilitating strong photon-photon interactions using quantum memories as a platform. Strong optical nonlinearities are a relatively unexplored avenue for creating deterministic two photon interactions. Even though photons interact weakly with each other, they interact strongly with some physical systems such as atomic ensembles. It is then a natural extension to use these ensembles to facilitate strong interactions between two photons. Quantum memories are already an integral device for most quantum information applications, including computation, where they act as essential timing buffers required to synchronize between the probabilistic processes that make up a quantum circuit or network. To be able to efficiently interface between light and atoms, quantum memories have a necessarily strong atom-light interaction strength which makes them a promising platform for hosting strong optical nonlinearities. The most basic form of a quantum memory based two-photon gate is simply storing one photon in the quantum memory while another photon is propagated through the memory to impart a phase shift on the stored photon. Despite its conceptual simplicity, a number of technical challenges need to be tackled, some of which have been detailed in this thesis. The first milestone presented in this thesis was the demonstration of the storage of a single photon. Our demonstration was the first to achieve up to 84% high storage efficiency in a room temperature system which was also free from the low duty cycle operation that hampered the real world practicality of previous cold ensemble demonstrations. This was also the first single photon storage demonstration of the gradient echo memory scheme which proved the protocol's capability to store quantum states of light. The second milestone was the quantification of the nonlinearity strength attainable between two optical fields when mediated by a quantum memory. The initial measurement on a cold atomic ensemble was a promising baseline for which a number of improvement strategies could be applied. One of which, the tight focusing of the optical beams, was attempted on a Bose-Einstein condensate.