Long-lived Coherent Optical Memory

The loss of photons limits the distance of quantum communication based on direct transmission in optical fibers to the order of hundreds of kilometers.In order to extend the communication distance to more than 1000 kilometers and even to the global scale,the quantum repeater scheme has been long-expected and studied a lot in the past two decades.In the quantum repeater scheme,the channel is divided into several elementary links with shorter distances,and the links are connected by repeater station.The elementary links are connected to longer links through step-by-step entanglement swapping operations,and finally the quantum entanglement is established over the entire channel.However,due to the high complexity of quantum repeater system,there is no experimental demonstration that can surpass the performance of that relying on the direct transmission in optical fibers.As the losses in optical fibers approach theoretical limits,scientists have been ex-ploring alternative solutions.In recent years,quantum communication based on satellite and free-space transmission has made a series of important achievements in the fields of entanglement distribution,key distribution,teleportation,etc.Since the loss of the free-space channel between the satellite and the ground comes only from the 10-km atmosphere,this communication scheme can effectively overcome the problem of loss of the channel.In 2020,satellite-based quantum key distribution over 1,000 kilometers on the Earth's surface has been realized.However,in order to extend the range of communication to the global scale,we still need to apply the quantum repeater scheme to the satellites.The system difficulties of quantum repeaters remain great challenges.Quantum memory is the core device of quantum repeater.It is one of the greatest challenges to develop high-quality quantum memories towards practical quantum repeaters.Quantum memory refers to a device that can reversibly map the quantum information carried by photons into the quantum state of a material and read it on demand.In the quantum repeater scheme,the quantum memory can store the photons and synchronize the entanglement swapping between the links.Practical quantum repeaters require high storage efficiency and long lifetime of the quantum memories.In 2015,an Australian group has observed up to six hours of spin coherence in a europium-doped yttrium orthosilicate crystal.This long-lived spin coherence has inspired an alternative idea for long-distance key distribution,i.e.using high-speed transportation of a quantum memory.This scheme relaxes the requirements for the efficiency,wavelength and bandwidth of the quantum memories,but requires storage times on the order of hours.However,due to the unknown excited-state energy level structure of europium-doped yttrium orthosilicate in magnetic fields,narrow energy level spac-ings,complicated structure and weak optical depth,there is no optical memory developed that is able to take full advantage of this long-lived spin coherence,until recently.In order to carry out long-lived optical storage in europium-doped yttrium orthosilicate,I have finished a series of research studies during the Ph.D.period.Result 1:In order to determine the excited-state energy level structure of europium-doped yttrium orthosilicate,we constructed a Raman-heterodyne-detected spectrometer to characterize the Hamiltonian of the optically-excited state of europium-doped yttrium orthosilicate.Then we developed pulsed Raman heterodyne detection method that was able to precisely measure the energy level structure of the excited state.Result 2:Based on the experimentally determined energy level structure,we realized the first experimental demonstration of optical storage that combined zero-first-order Zeeman technique and atomic frequency comb protocol.Optical pulses were suc-cessfully written into the long-lived spin state of europium-doped yttrium orthosilicate.In the spin-wave storage step,we used dynamical decoupling sequences to further extend the spin coherence times.After an hour of storage,we successfully read out the optical pulses from the memory,and verified that the readout pulses had a high fidelity of 96.4±2.5%.This result pushes the record of coherent optical storage time from 1 minute to 1 hour.Our results show that europium-doped yttrium orthosilicate is an promising candidate for the development of long-lived quantum memory,which is helpful for the realization of global quantum key distribution based on satellite-based or transportable-memory-based schemes.