Engineering Optical Nonclassical States Based On Four-wave Mixing Process In Rubidium Atomic Ensembles

Optical nonclassical states (Such as entangled state, squeezed state, single photon state, and Schrodinger cat state et al.) are those that can’t be described by classical electromagnetism. They are important research topics in quantum optics as well as atomic and molecular physics. At the same time, they are also indispensable investigation tools in these fields. Entangled state and single photon state are necessary in exploring some fundamental quantum physics problems, such as the experimental verification of EPR (Einstein-Podolsky-Rosen) entanglement, quantum nonlocality, and Bell inequality. Besides, entangled state and single photon state constitute the basis of continuous-variable and discrete-variable quantum information technologies, respectively. So engineering ideal entangled state and single photon state has been a significant research field in quantum optics and quantum information technologies.This thesis includes the following four projects.1. Using diode-laser-pumped nondegenerate four-wave mixing (FWM) process in hot Rubidium vapour, we generated quantum correlated twin beams which show intensity-difference squeezing down to 8 kHz with a maximum squeezing of -7 dB. To our knowledge, this is the first demonstration of kilohertz-level intensity-difference squeezing using a semiconductor laser as the pump source.2. Multipartite entanglement and correlations are important for both fundamental science and the future development of quantum technologies. We theoretically proposed a method to generate multiple quantum correlated beams based on cascaded FWM processes. The main advantage of our method is that as the number of quantum modes increases, so does the total degree of quantum correlations. To verify this, we generated three quantum correlated beams using cascaded two-FWM configuration. The degree of intensity-difference squeezing of these three beams is larger than that of twin beams generated by single FWM process.3. Single photons and single photon qubits are among the foundations of discrete variable quantum optical information processing techniques such as quantum cryptography, quantum teleportation, quantum repeaters, and quantum computing. We generated single photons heralded from biphotons generated via spontaneous FWM in atomic vapour. We also generated arbitrary optical qubits based on collective spin excitations. Quantum homodyne was used to reconstruct the density matrices and Wigner functions of single photon state and single photon qubits.4. Possessing precise information about the temporal mode of single photon state is crucial for improving the efficiencies of quantum information processing techniques such as quantum cryptography and quantum computing. We proposed and experimentally demonstrated a method of determining the complete temporal mode function of a single photon, polychromatic optical heterodyne tomography. This method can obtain the temporal density matrix of signal photon state by measuring the autocorrelation function of the photocurrent from a balanced homodyne detector at multiple local oscillator frequencies, and therefore achieve its temporal mode function. We tested our method for three different settings and obtained almost excellent agreement with theoretical predictions.