Quantum information technology applications with squeezing: quantum key distribution and entanglement

Abstract

The focus of this thesis is to investigate the effects of quantum squeezing implementation toward the performance in two particular quantum information technology applications: quantum key distribution and quantum entanglement. First, we derive the general analytical solution for multi-photon interference in the Hong-Ou-Mandel interferometer setup with Gaussian spectral distribution. The results are then applied to the implementation of amplitude-squeezed coherent state in measurement-deviceindependent quantum key distribution. We find that with optimized squeezing parameter, the secret key rate is enhanced over the key rate using the coherent state. Second, we investigate entanglement of an experimental system of two nitrogenvacancy-center ensembles which are initially squeezed under the one-axis twisting Hamiltonian. Although evidence in the literature on idealized coupled oscillator systems and coupled quantum kicked tops suggests that initial squeezing can enhance entanglement, we find that, in the realistic system studied here, initial squeezing can improve entanglement overall when the field mode interacts in a particular manner with the two spin ensembles. Our analysis using the Holstein-Primakoff transformation and Wigner characteristic function in the rotating frame of reference shows that the entanglement enhancement is a subtle consequence of the way in which the dissipative decoherence rotates the state of the collective spin ensemble such that enhancement depends on the time-evolved rotated states between the presence and absence of initial squeezing. Third, we investigate the effect of initial squeezing on tripartite continuous-variable Hamiltonians based on real physical systems. The Hamiltonians are based on the three-mode magnons excited by an external field, the one-dimensional interconnected tripartite spring-like system, and two spin ensembles coupled to a cavity resonator in the non-dispersive regime in the continuous-variable limit. We find that for the three-mode magnons and the spring-like system, implementation of initial squeezing invariably enhances the two-way and three-way entanglement, while for the spin-resonator system it depends on the alignment of the spin ensembles relative to the resonator field. i To my Mother and Father I also thank my friends and colleagues, Dr.