Multimode Solid State Quantum Memory

Quantum memory is the interface between the photon and matter,allowing the storage and retrieval of single photons.Quantum memories are key elements in the quantum repeater,which provides a solution for the quantum channel loss problem in long-distance quantum communication.To reach useful data rate in a large-scale quan-tum network,highly multimode quantum memories are required to build a multiplexed quantum repeater.Multimode memories are capable of storing N photons simultaneously.This is thereby a N-fold increase in the success probability for entanglement creation per round-trip and the communication would be N times faster.This multiplexing can be imple-mented in various degrees of freedom such as time,frequency or space.In addition,multiplexing using multiple degrees of freedom can greatly increase the number of modes in a product manner.Furthermore the multiplexed quantum memory can sig-nificantly reduce the requirements on the storage time in a network.Rare-earth ion-doped crystal is an appealing candidate for practical quantum mem-ories due to the long coherence time and large storage bandwidth at liquid helium tem-perature.In this thesis,I investigated multimode quantum memory in a praseodymium-doped yttrium orthosilicate crystal(Pr3+:Y2SiO5)for potential use in quantum network-ing applications.As a first experiment,we demonstrated on-demand storage of orbital-angular-momentum states with weak coherent pulses at the single-photon level in a Pr3+:Y2SiO5 crystal.Furthermore,combining this spatial degree of freedom with temporal and spec-tral degrees of freedom,we create a multiple degrees of freedom memory with high multimode capacity.This result reveals highly multimode capacity of rare-earth ion-doped crystals.Based on multiple degrees of freedom memory,we have implemented quantum mode conversion and real-time arbitrary operation of quantum states.In addition,we built a high-brightness narrow-band photon-pair source which is compatible with the Pr3+:Y2SiO5 crystal.The photon-pair source is generated by spon-taneous parametric down conversion inside a bow-tie cavity with the signal photons at 606 nm and the idler photons at 1540 nm.Moreover,using the telecom C-band idler photons for heralding,we demonstrate the reversible transfer of orbital-angular-momentum qubit between the signal photons and the quantum memory based on a Pr3+:Y2SiO5 crystal.We obtain that the storage efficiency of is 12.6%with 8 ?s stor-age time by using atomic frequency comb memory scheme.The second-order cross-correlation between the retrieved signal photons and the idler photons is 11.4,which is much greater than the classical limit,demonstrating the quantum nature of the source and the memory.Then the 606-nm photons are encoded with orbital-angular-momentum qubit,and the visibility for the retrieved signal photons is measured to be 94%.These results presented in this thesis can possess some practical applications in optical quantum information processing in the near future.