Demonstrating multilevel entanglement and optimal quantum measurements

Abstract

Optimal generalised quantum measurements are important for quantum information applications in both photonic and solid state systems. However, until now, the implementations of such measurements have been optical. Entanglement is also a very important resource in quantum communication and information processing. However, highdimensional entangled states and corresponding Bell-inequality violations are challenging to detect and demonstrate experimentally. This thesis focuses on these two aspects of signal detection. A cavity quantum electrodynamics (QED) scheme to realise an optimised quantum measurement demonstrating the superadditivity of quantum channel capacity is proposed and analysed. The measurement is shown to be feasible using atoms in a cavity QED setup even in the presence of rather high levels of experimental errors. This is interesting because cavity QED realisations could potentially be more easily scaled to increase quantum coding gain. Experimental unambiguous discrimination between non-orthogonal states is also carried out for the first time in the solid state using the nuclear spin of a nitrogen atom associated with a defect in bulk diamond—an important step for implementations of solid-state quantum computing. This thesis presents a method for verifying entanglement dimension using only Bell inequality test measurements. It also shows experimental results demonstrating genuine eleven-dimensional two-photon orbital angular momentum (OAM) entanglement and violations of generalised Bell inequalities up to dimension twelve. The demonstrated highdimensional entanglement is potentially useful for closing the detection loophole in Belltest experiments and for real-world large-alphabet quantum-cryptography applications.