Quantum Memory And Manipulation Based On Cold Atomic Ensemble

With the explosion of the quantum information science,quantum network has attracted more and more attention by various research institutions all over the world.In the quantum nodes,the quantum storage and quantum manipulation of quantum information are needed.The cold atomic ensemble has realized long-lived and high efficiency quantum storage has become a promising quantum node.Due to the imperfect that we can’t isolate the storage system from environment completely,there is a big loss during the quantum storage and quantum manipulation.We need to design scheme to improve storage efficiency,storage lifetime and storage fidelity and other performance indicators of quantum memory.This article has carried on the experimental research of quantum storage and quantum manipulation based on the cold atomic ensemble.The main contents are the following aspects:(1)Extension the storage lifetime of photonic polarization qubit stored in the cold atomic ensemble by the quantum manipulation of dynamical decoupling.The coherence transfer between the two ground states can be achieved by the manipulating of Raman process.By using the multiple Raman π pulses of the Carr-Purcell-Meiboom-Gill(CPMG)sequences,the decoherence between spin-waves can be suppressed.Thus,the quantum process fidelity remains better than 80% for the storage time of 800μs,while it is under 80% for the storage time of 180μs without the CPMG sequences.This work can improve the success probability of quantum teleportation in long-distance quantum network and can be achieved more useful applications in the quantum information process in the future.(2)Experimental study of the dependences of retrieval efficiencies on the time delay(the delay between magneto-optical-trap being turned off and optical storage)and the dependences of retrieval efficiencies on the experiment repetition rate(the number of storage process in unit time): The results show that in the delay ranging from 0.015 ms to 3.5 ms,the retrieval efficiency for a zero-storage time(0.2 μs)can exceed 15%.When the delay is under 0.015 ms,the remaining magnetic field will cause significant influence of the quantum storage.When the delay exceed than 3.5ms,the increase of the cold atoms temperature will leads to the decrease of quantum storage.The measured results will provide important help for optimizing the storage of the polarized entanglement photons in cold atomic ensembles.(3)The long-lived quantum memories and quantum router of photonic polarization qubits: By applied a moderate magnetic field of 13.5G,the magnetic-field-sensitive spin waves are removed from the level structure.So the photonic polarization qubits can only be stored as two magnetic-field-insensitive coherences.By applied the phase matching conditions and electromagnetic induction transparency to change the propagating directions of the read laser beam,we controllably release the retrieved photonic polarization qubits into 7 different photonic output channels,respectively.In addition to the improving of filtering system in the quantum channels,the signal-to-noise ratio of the released optical signal increased.And we achieved long-lived storage and high-fidelity arbitrary polarization qubits in quantum router finally.At the storage time of 6 ms,the quantum-process fidelity is still beyond 78%.This work can be applied to building large quantum networks,long-distance quantum communications and distributed quantum computing,etc.(4)The arbitrary polarized light is studied in the annular cavity of the polarization maintaining: high fidelity is one of the important indexes of quantum storage,and it is difficult to ensure the fidelity of signal in a annular cavity.By using a special designed cavity and two half wave plates,the reflectivity difference between the two orthogonal polarization models can be compensated and the output peak value of two orthogonal polarization models can be equal.The phase difference between two orthogonal polarization models in optical cavity can be compensated to zero by using a l/4-l/2-l/4 wave plates sequence and two orthogonal polarization models can resonate with the cavity simultaneously.By perform amplitude compensation and phase compensation,the polarization of cavity input can be remained after through the cavity.This work provides an experimental foundation for high efficiency and fidelity quantum memory of arbitrary polarization qubits with cold atoms inside optical cavity.