Experimental Study Of EIT-Based Quantum Memory

Quantum memory plays an important role in construct quantum network, including quantum computation, communication and metrology. In recent years, many physical processes, such as Electromagnetically Induced Transparency (EIT), Spontaneous Raman Emission (SRE), Atomic-Frequency Combs (AFC), Off-resonant Raman Interaction, and so on have been exploited to store quantum states of light. Among them, electromagnetic induction transparent effect in the atomic ensemble is a effective method.In this dissertation the EIT-based PPQ memory, coherence manipulation of spin wave, etc. have been experimental studied. The main contents include:(1). Experimental Investigation of quantum interference between spin-waves constructed by dual-channel quantum memory. A circular polarization signal pulse was stored in a cold atomic ensemble by two orthogonal polarization components of "writing control light". By changeing the relative phase between two components of "reading control light", we observed the readout signal as the change of the relative phase between two " reading control light". When relative phase between two " reading control light" was zero, the dual-channel quantum memory shown constructive interference, the readout signal reached the maximum; When relative phase between two " reading control light " was π, dual-channel quantum memory shown destructive interference, the readout signal was almost zero, quantum memory remained in atomic medium, when read after a while, quantum memory remain in medium were read out. The results show that the interference we observed is the quantum interference of two transition channels which belong to a single Tripod atomic system. Based on this, the readout signal with the change of the relative phase between two "read control light" was observed, the result shows a sinusoidal interference pattern. Then, the readout signal with the change of the storage time was measured. The results show that the larmor precession of the atoms in magnetic field cause "constructive" and "destructive" interference of the dual-channel quantum memory. Quantum interference is the cause of the readout signal oscillation. By changing the relative phase between two readout light can eliminate the oscillation of readout signal.(2).Conherent manipulation of spin-wave excitation in atomic ensemble was studied. Conherent manipulation of single Qubit plays an important role on quantum information process. Conherent manipulation of single Qubit have been demonstrated in the trapped ions and quantum dot, but haven’t been demonstrate in atomic ensemble yet. We used Raman two-photon transition to manipulate the spin-wave in EIT-based quantum memory, coherent transition of atomic population have been realized, the trans efficiency reached 97%. And we realized spin-wave’s relative phase controllable precession by manipulating relative phase between two spin-waves with a magnetic pulse. The measured π/2 fidelity is 96% by the Ramsey measurement. The R(θ,φ) and Rz(φz) rotation of spin-wave can be used to construct single Qubit manipulate which is based on quantum memory. Then the multiple Raman coherent manipulation experiment was carried out, which provide a foundation of dynamic decoupling to prolong the coherence time of quantum memory.(3). 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 200μs,and 78.4% at 4.5ms.