Broadband Quantum Memory In Room-temperature Atoms

Quantum technologies,for example,quantum communication and quantum computation,promise spectacular quantum enhanced advantages beyond what can be done classically.However,quantum states,as the element of quantum technologies,are very fragile and easily get lost to the environment,and meanwhile,their generation and quantum operations are mostly probabilistic.These problems make it exponentially hard to build a long-distance quantum channel for quantum communications and large quantum systems for quantum computing unless one have some effective quantum memories.An effective quantum memory allows quantum states to be stored and retrieved in a programmable fashion,therefore providing an elegant solution to the probabilistic nature and associated limitation by coordinating asynchronous events.Enormous advances in quantum memory have been made by developing various photon storage protocols and their physical implementations.In order to have quantum memory practicable for efficient synchronisation and physical scalability,considerable efforts have been dedicated to meet key features known as high efficiency,low noise level,large time bandwidth product(lifetime divided by pulse duration)and operating at room temperature.It has proven very difficult to satisfy all the requirements simultaneously.Compared with the cold-atombased quantum memory,the room-temperature quantum memory doesn't need a complicated cooling system and is more practical.Similarly,superconductive materials which can work at room temperature will be more straightforward for practical applications.However,in the regime of large bandwidth and room temperature,noise and/or decoherence become dominant,therefore memories cannot work in quantum regime or only work in short time.At room temperature,electromagnetically induced transparency and near off-resonance Raman memory have collision-induced fluorescence noises which can not be filtered out because of being identical with the desired signal photons.By applying larger detuning,far off-resonance Raman is found to be able to well eliminate the fluorescence noises and posses a large storage bandwidth.Unfortunately,many experiments,including ours,have demonstrated that there are a lot of four wave mixing noises rising from spontaneous Raman scattering process.Several research groups use optical phonons in diamond as the storage state,or use the excited state of hot atoms as the storage state,but the lifetime is too short.This dissertation focuses on the realization of FORD(far off-resonance Duan-Lukin-CiracZoller)quantum memory protocol concieved by my advisor.No external photon sources are needed in FORD protocol.A weak or far off-resonance write pulse interacting with an atomic ensemble can excite a Stokes photon,and meanwhile generates a collective excitation in the atomic ensemble.Similar to any DLCZ-type memory,it is the collective excitation that the FORD memroy stores.On the other hand,in a far off-resonance protocol,the frequency difference between the fluorescence noises and the desired photons is several GHz,hence the fluorescence noises can be filtered out.With the above analysis,we can infer that the FORD protocol should work well in quantum regime.After years of hard work,we have realized the FORD quantum memory.It is worth mentioning that there are still some improvements that should be made for the FORD memory.Furthermore,there are many potential applications deserved to be investigated.I will present the achievements we obtained and the problems we ever encountered in sequence.This dissertation consists of 6 chapters.Chapter 1: Introduction.The necessary and research status of quantum memory.In addition,the principles of quantum communication and multiphoton synchronization based on DLCZ protocol are introduced.Chapter 2: The construction and optimization of the experimental setup based on roomtemperature caesium atoms.With many details about our experiments,readers can obtain a comprehensive understanding about our work.Chapter 3: Room-temperature broadband DLCZ quantum memory.This chapter will present the work principle,figures of merit and performance of our room-temperature broadband FORD quantum memroy.Chapter 4: Direct observation of broadband nonclassical states in a room-temperature lightmatter interface.Broadband photon pairs with nonclassical correlations can be obtained from the interference.Chapter 5: Light-matter hybrid Hanbury Brown-Twiss interferometer built in room-temperature quantum memory.The interference and beam splitting of single photon are realized inside a quantum memory.This is a new HBT interferometer built inside matter.Chapter 6: Conclusion,attempt and prospect.Firstly,I make a conclusion about the dissertation,then list two unsuccessful attempts: Long-lifetime room-temperature broadband quantum memory based on anti-relaxation coating;Room-temperature broadband Raman memory.Some qualitative results are presented.The last part is about the prospect of the quantum memory and quantum technologies based on quantum memory.