Generation And Memory Of Entanglement State Based On Cold Atomic Ensembles

Quantum repeaters play a key role in the quantum information science. In quantum communication network, fragile quantum states are transmitted between a sender and a receiver. Due to decoherence introduced during transmission, from losses or from environmental contamination, quantum communication is restricted to a range of 200 km. A quantum repeater can extend the range of quantum communication.Quantum repeaters consist entanglement swapping, quantum memory and entanglement purification. Entanglement swapping is used for connecting the neighboring entanglement sections, so as to avoid the losses of long transmission distance. Quantum memory is used to solve the probabilistic nature of the generation of the entanglement, so as to avoid the exponential cost of resources. Quantum purification is used to solve the decoherence during the memory process.This thesis is mainly focused on the quantum repeaters based on cold atomic ensembles, including the following three parts:(1) Generation of the narrow-band entanglement photon pairs. We simultaneously generated two spin-wave-photon (atom-photon) entangled states in a cold Rb ensemble via SRS. Based on joint Bell-state measurements on the two photons coming from the two atom-photon entangled sources, respectively, we project the two stored spin waves into a Bell state and then mapped the quantum memory into a polarization-entangled photon pair. Such polarization-entangled photon pair can be released on demand. Also, based on the two atom-photon entangled states, we generate a GHZ entangled state of three photons with a programmable delay.(2) We perform an experiment of long lifetime and high-fidelity quantum memory of photonic polarization qubit(PPQ). A moderate magnetic field (13.5G) was applied on the atomic ensemble to left the Zeeman degeneracy, so the PPQ can only be stored as two magnetic-field-insensitive coherences, the impact of magnetic-field-sensitive coherence was washed out. The measured quantum state fidelity is 98.6% at 200ms, and 78.4% at 4.5ms.(3) We experimentally demonstrate the PPQ's storages protected by Carr-Purcell-Meiboom-Gill (CPMG) dynamical decoupling pulse sequences. Based on EIT effect in a cold atomic ensemble, the PPQ is stored as two spin waves, which are associated with the magnetic-field-insensitive and magnetic-field-sensitive coherences, respectively. The CPMG sequences containing multiple Raman ? pulses are applied to suppress decoherence of the spin waves and thus the quantum process fidelity remains better than 0.8 for up to 800?s storage time.