Experiments towards a quantum information network with squeezed light and entanglement

Quantum information science is a new and promising field of research that combines the techniques developed in quantum mechanics with those of information science. One primary experimental goal of the field is to implement a quantum information network consisting of nodes of atoms at which quantum computational algorithms can be performed connected by optical links. This thesis presents the development of techniques applicable to such a network in the continuous variable regime of quantum mechanics. We develop a pair of optical parametric amplifiers, each producing a stable strongly squeezed output field at 1064 nm. These optical parametric amplifiers form the work-horse utilised for the majority of the experiments performed in this thesis. The output fields are either amplitude or phase squeezed depending on our control system, with 4.4 dB of squeezing below the quantum noise limit consistently observed. A quadrature entangled state is generated by interfering the squeezed fields on a 50/50 beam splitter. The signature of this form of entanglement is strong correlations between the amplitude and phase quadratures of a pair of fields. We demonstrate that the correlations exhibited in our case are non-classical using a criterion for wave function inseparability, and through observation of the EPR paradox. Since the efficacy of quantum information protocols depends strongly on the properties of the entanglement used, we perform a complete characterisation of our entanglement, introducing a convenient physically relevant characterisation technique. The entanglement is utilised in a quantum teleportation protocol, demonstrating that the quantum statistics of a field can be transferred to another field via detection and classical communication. We investigate in some detail methods that can be used to characterise continuous variable quantum teleportation, and propose a new method that has a direct correspondence to entanglement swapping. We introduce a scheme to perform quantum secret sharing using our entangled state and quantum electro-optical feed-forward. In quantum secret sharing an encoded secret state is distributed securely to a group of players. In ideal circumstances, even if some of the players are malicious, or some distribution channels faulty, the secret can be reconstructed perfectly. We consider techniques to implement the transfer of quantum states from optical to atomic media. In the continuous variable regime such a transfer has been achieved for polarisation states. We investigate continuous variable polarisation states, demonstrating the transformation of our quadrature squeezed states into a variety of polarisation squeezed states; with one such state exhibiting squeezing of three of the four Stokes operators. Utilising our quadrature entanglement, we then generate continuous variable polarisation entanglement between two Stokes operators of a pair of fields. Interacting this entanglement with a pair of atomic ensembles could provide a macroscopic, spatially separated, source of atomic entanglement. Finally, we investigate the interaction between optical Raman pulses and a cold Cesium atomic ensemble. We manipulate such an ensemble with a sequence of optical pulses, producing non-classically correlated photon pairs. The non-classicality of the correlation is verified through a violation of the Cauchy-Schwarz inequality. The technology developed during this investigation has been shown to enable a feasible quantum information network, quantum memory, and ‘on demand’ single photon sources.