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.